dvm x1000 micro-volt (uv) pre-amplifier - distinti · dvm x1000 micro-volt (uv) pre-amplifier...

Rev 1.2 30 July 2003 SKU EE001

DVM x1000 micro-volt (uV) Pre-amplifier Extends the range of a standard Digital Volt Meter (DVM) or oscilloscope (See Note 5) into the micro-volt (uV) range. Please read this manual prior to using the device. The device is very sensitive and is capable of receiving unwanted interference and offsets. This manual shows how to identify and eliminate the unwanted effects.

Description The DVM x1000 Pre-Amp extends the dynamic range of test equipment into the micro-volt (uV) range. The DVMx1000 enables researchers to explore natural phenomenon using scaled-down experiments that can be constructed with inexpensive toy motors, magnets and batteries (some examples included in this data sheet – See Note 4 below). Naturally the same experiments can be created on a larger scale to develop higher emfs; however, to do this requires more elaborate fabrication skill, money and time. The larger experiments may be potentially dangerous. The DVMx1000 provides the researcher with an inexpensive and accurate means to measure micro-volt emfs developed by subtle natural phenomenon as well emfs developed by scaled-down experiments. This allows proof-of-concept experiments to be developed faster and instructive laboratory demonstrations to be less expensive and safer (see examples in this manual). Off the shelf micro-volt (uV) sensitive instrumentation costs between $400.00 and $2000.00. The x1000 pre-amp turns a $20.00 DVM or an Oscilloscope (See note 5 below) into a uV sensitive instrument.

Notes: 1) The DVM x1000 Pre amp is intended to be used with Digital Volt Meters (DVM), Digital Multi-

Meters (DMM) or any other voltage measuring device with 100K or greater input impedance and DC-300HZ Bandwidth.

2) The device has very sensitive –easily damaged—inputs, please read these instructions before use.3) The device can be used with an Oscilloscope: Please read the appropriate section prior to use. 4) A more complete list of experiments to be published in a separate document titled: New

Electromagnetism Experimenters Guide. 5) Soon to be available is the SCOPEx1000. This SCOPEx1000 is specifically designed for use

with Oscilloscopes. It has higher bandwidth, higher dynamic range, lower noise and improved shielding.

Features Auto-calibrating DC Gain accuracy 0.3% -3db @300HZ 1012 Input Impedance Typical Input Offset: 0.5 microvolts (uV) Max Input Offset: 2 microvolts

Requires single 9 volt battery. Runs 12 hours on alkaline battery.

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Rev 1.2 30 Dec 2003 SKU EE001

Table of Contents

1 Sample Applications/Demonstrations ________________________________________________ 3 1.1 The Thermoelectric Effect (Seebeck effect) ______________________________________________ 3 1.2 Magnetic Sensing____________________________________________________________________ 5 1.3 Homopolar Generator _______________________________________________________________ 7 1.4 Measuring Very Small Resistances _____________________________________________________ 8

2 Placing the DVMx1000 into operation ______________________________________________ 10 2.1 Measurements using a DVM _________________________________________________________ 10

2.1.1 DC measurements ____________________________________________________________________ 10 2.1.2 AC measurements ____________________________________________________________________ 11

2.2 Measurements using an Oscilloscope __________________________________________________ 11 2.2.1 DC measurements ____________________________________________________________________ 12 2.2.2 AC measurements ____________________________________________________________________ 12

3 Support Tests___________________________________________________________________ 13 3.1 Functional Check __________________________________________________________________ 13 3.2 Saturation Voltages (operational limits) ________________________________________________ 13 3.3 Measuring output range (Saturation Voltages) __________________________________________ 14

4 Improving measurement accuracy __________________________________________________ 17 4.1 Sources of DC error ________________________________________________________________ 17

4.1.1 Input Offset _________________________________________________________________________ 17 4.1.2 Test Lead Error ______________________________________________________________________ 18

4.2 Sources of AC error ________________________________________________________________ 20 4.2.1 Interference received at input____________________________________________________________ 20 4.2.2 Minimizing interference________________________________________________________________ 23 4.2.3 Input Noise (thermal noise) _____________________________________________________________ 24

5 Example of AC measurement______________________________________________________ 26

6 Miscellaneous __________________________________________________________________ 33 6.1 How to determine if saturation is occurring with just a DVM ______________________________ 33 6.2 Determining Battery Voltage without opening battery compartment ________________________ 33

7 Electrical Specifications __________________________________________________________ 35

8 Making Short Male-Male Jumpers _________________________________________________ 38

9 To make an Audio/Oscilloscope adaptor _____________________________________________ 39


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1 Sample Applications/Demonstrations

Any connection of dissimilar metals forms a thermoelectric junction producing an electric potential which varies with temperature (Seebeck effect). This phenomenon is used to create temperature sensors call thermocouples. Standard thermocouples are constructed from two metals that yield potentials (voltages) high enough to be read by standard DVMs. In experiments where microvolt (uV) effects are to be measured, these thermoelectric emfs (also called thermal emfs; this is not to be confused with thermal noise which is covered later) are a cause of measurement errors. The following experiment demonstrates the thermoelectric effect and how a slight temperature change on a copper/brass junction can cause unwanted mischief. In the experiment, copper wires are wrapped around a flat brass rod at two different points. The each copper wire is connected to the DVMx1000 which in turn is connected to a DVM. The system has been turned on and allowed to settle for about 5 minutes (allowing all temperatures to stabilize). Figure 1-1 show the system after 5 minutes. Since the temperature of the system is uniform (or close to uniform) the thermal emfs generated by each brass/copper junction are substantially the same and in opposing directions. Thus the total thermal emf of the system is in balance and there should be no emf reading on the meter. Note that the reading shows -1.9 uV offset (error), the source of this offset can be from many sources as explained in section 4.1. At least 0.5uV of this error is from the DVMx1000 itself; the remainder could be from the old worn-out DVM probes being used or even the difference in the amount of corrosion on the brass where the copper contacts are made. For now, we compensate further readings by adding 1.9 uV to meter output obtain the actual reading (If your meter has a REL function, you can press the REL button now and your meter will do this for you).


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Figure 1-1: “zero” output

By placing a finger over the lower copper/bass junction, the heat from the finger alone causes an imbalance in the thermal emfs between the two junctions. Normally the junctions are at the same temperature and therefore, the emfs oppose and cancel each other. Now that one junction is warmer than the other, its emf will overpower the other giving us a net of -12.6+1.9= -10.7uV

Figure 1-2

Next, remove the finger and let the system cool until the reading once again reads -1.9 (may take a few minutes. (Don’t blow on it! The humidity from your breath will cause other problems).


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Next, apply the finger to the upper junction as shown in the following photo.

Figure 1-3

This gives us a net +8.2+1.9 = 10.1uV The above experiment demonstrates the thermoelectric effect in a very simple manner. The brass/copper junction is one of the tamest junctions since brass contains copper. A standard set of test leads (like the ones used in the above experiment) may contain many different metal-to-metal junctions to include spring clips, plated connectors, and solder joints. A new set of well made DVM leads should have all thermal emfs in balance; however, as the plating wears off or tarnishes, or the solder junctions corrode or the copper becomes mechanically changed through bending and stretching, the leads may develop a thermal emf offset. Because of certain bright light conditions, the black lead may be higher in temperature than the red lead; this must be accounted for depending upon the magnitude of the effect. Measuring test lead thermal error is covered in a later section.

The following picture (Figure 1-4) shows a toy magnet suspended over a 10 turn precision coil (available at www.distinti.com) by a slinky. The 10 turn coil is connected to the DVMx1000 using the standard DVM clip-on lead set, and the DVMx1000 is connected to a scope using a scope adaptor (see section 9: To make an Audio/Oscilloscope adaptor). The period of oscillation is very slow (about two seconds), which means that the induced emf in the coil is going to be very low (see scope screen capture Figure 1-5). The emf measured on the scope is 592mV which means that it is 592uV at the input to the DVMx1000 (we are not worried about the few microvolts of input error in this case).


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Without the DVMx1000, the 10 turn coil would have to be 10,000 turns to obtain the same emf. This 10,000 turn coil would have 1000 times the resistance and 1,000,0000 times the self inductance of the 10 turn coil. This demonstrates how the DVMx1000 saves money by reducing experiment complexity.

Figure 1-4

Figure 1-5 Scope screen capture


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Homopolar generators/motors were invented by Faraday in the 1800s. They have many applications in science and technology such as nuclear submarine propulsion systems. Homopolar generators suffer from the fact that very powerful and expensive magnets along with very large disks and very high RPMs are required to generate useful amounts of power. Even to make a generator produce enough DC voltage to be measured by a DVM requires a large undertaking. The DVMx1000 enables the development of smaller scale models enabling proof of concept and scaled engineering to be quick and practical. The next photo shows a very simple prototype of a Homopolar generator used by www.Distinti.com to prove the incompleteness of the Theory of Relativity. It is a Homopolar generator where the magnet and the disk rotate together (see Faraday’s Final Riddle at www.distinti.com). According to classical electromagnetism and the Theory of Relativity, no power should be developed by such a device; yet it does develop power (444uV). Only the New Magnetic model (part of New Electromagnetism at www.Distinti.com) completely explains the phenomenon associated with the operation of this device.

Figure 1-6

The DVMx1000 enabled us to develop a very small scale model using only toy motors and hobby magnets to prove a new theory. This model took less that a day to design and construct (stain and


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varnish applied only after it proved successful). We intend to sell these Homopolar Generators as kits for students and teachers.

There are many instances when it is necessary to know the resistance of a PCB trace or a very small inductor. Unfortunately, the resistance of such objects is in the milliohm range which is far below the range of a typical DVM. This section will show you how to measure very small resistances with the DVMx1000. Small resistances are measured using the following circuit

VB

RR

R?

VR

VO

Figure 1-7 Circuit to measure unknown resistances

To measure an unknown resistance, the unknown resistance is placed in series with a known resistance (RR) and a battery (we use 1.5 volt D cell). The voltage across RR is measured using just a DVM. Once the voltage is measured and recorded, the DVMx1000 is connected to the DVM to measure the voltage across the unknown resistance. The value of resistance is determined from the following expression:


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RR

O RVV

R =?

The following spread sheet shows what is possible with a 1.5 volt battery and 3 different reference resistors.

Milliohm meter range calculations Voltmeter range Measurement range Reference resistor Vb Max Min Max Min Min useful DVMx1000 1000 1.5 0.003 0.000001 2 0.000666667 0.0333333 100 1.5 0.003 0.000001 0.2 6.66667E-05 0.0033333 10 1.5 0.003 0.000001 0.02 6.66667E-06 0.0003333 DVM (alone) on 200 mv range 1000 1.5 0.2 0.001 133.3333333 0.666666667 33.333333 100 1.5 0.2 0.001 13.33333333 0.066666667 3.3333333 10 1.5 0.2 0.001 1.333333333 0.006666667 0.3333333

Min useful = min*50 for 2% accuracy

The second set of data in the above table shows the range possible with just a DVM. By extending the concepts shown here it is possible to make measurements into the microohm range. The accuracy of the above measurement technique is largely a function of tolerance of the reference resistor used.


Rev 1.2 30 Dec 2003 SKU EE001

2 Placing the DVMx1000 into operation

To place the DVMx1000 into operation is very simple. The following parts are required

1) The DVMx1000. 2) A DVM or other voltage measuring instrument with 100K ohm or better input impedance. 3) Two 12-inch male-to-male jumpers (Section 8: Making Short Male-Male Jumpers) 4) (optional) One short 6 inch male-to-male jumper to act as a shorting strap.

Preparation:

1) Inspect unit for external signs of shipping damage. 2) Perform functional check (section 3.1). 3) Measure Test lead error to see if your test leads are usable (see section 4.1.2) 4) Connect the DVMx1000 to you DVM with the two 12” jumpers 5) Attach the DVM Probes to the DVMx1000 inputs (left terminals)

222...111...111 DDDCCC mmmeeeaaasssuuurrreeemmmeeennntttsss The following method of measuring DC accounts for the input error (input offset + test lead error) enabling measurements to 1 uV of accuracy. An example of the following steps is found in section 1.1 where emfs of about 10 uV are measured. In that example, the 1.9uV input error is significant and must be accounted for. For other experiments that produce outputs into the hundreds of uV the input error is of little consequence may be ignored. It is up to the researcher to determine when to ignore the input error. For each measurement where input offset is considered:

1) Short the test leads together, then allow the reading to stabilize for about 10 seconds, then write down the input error (or press the REL key on your meter if it has one). Note: every time the unit is disconnected it will go into saturation, when the unit is reconnected, it takes time for the temperature change caused by the saturating unit to renormalize. This is why we must wait when changing the connection (see section 3.2 for more details).

2) Connect the test leads to the device being measured, wait 10 seconds for the unit to stabilize (same reason as above) then write down the result. If you used the REL function above, the answer obtained is the actual voltage; otherwise, subtract the input error obtained from the previous step to arrive at the answer.

For Each measurement where input offset is ignored:

1) Connect the test leads to the device being measured, wait 10 seconds for the unit to stabilize the meter displays the answer. Note: every time the unit is disconnected it will go into saturation, when the unit is reconnected, it takes time for the temperature change caused by the


Rev 1.2 30 Dec 2003 SKU EE001

saturating unit to renormalize. This is why we must wait when changing the connection (see section 3.2 for more details).

Note1: It is best to measure the input error (the thermal error of the test leads + DVMx1000 input offset) often since the thermal test lead error changes constantly with temperature, humidity and even the way the leads are laid out on the table (at least with the crumby set shown). The heat of holding a test lead in the hand, alters the thermoelectric emf (as demonstrated by the example in section 1.1). Note2: The thermal errors of the jumpers that connect the DVMx1000 to the DVM do not contribute significant error. The reason is that the DVMx1000 output is in the millivolts range; therefore, the few microvolts of error in the jumpers are of no significance.

222...111...222 AAACCC mmmeeeaaasssuuurrreeemmmeeennntttsss When making AC measurements, the input error can be ignored. This occurs because DVMs remove the DC offset of a signal with a capacitor. Although AC measurements free you from the drudgery of the input error, you must be aware that each DVM handles AC differently when the signal being measured is not a sine wave. Measurements are made the same as the DC case where input offset is ignored. Dealing with interference and noise is important. More details about noise is found in section 4.2 and how to make actual RMS measurements is found in section 5.

The DVMx1000 is compatible with scopes; the following instructions explain how to employ the DVMx1000 with a scope. Note: a more sophisticated version of this product (SCOPEx1000); more appropriate for oscilloscopes, will be available shortly. To place the DVMx1000 into operation for scope readings is very simple. The following parts are required

1) The DVMx1000. 2) A Scope with at least a 1 M Ohm input impedance. 3) A Bannana jack-to-scope adaptor ( see section 9: Making a scope adaptor) 4) (optional) One short 6 inch male-to-male jumper to act as a shorting strap. 5) A set of leads from your DVM

Preparation:

1) Inspect unit for external signs of shipping damage.


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2) Perform functional check (section 3.1). 3) Measure Test lead error to see if your test leads are usable (see section 4.1.2) 4) Connect the DVMx1000 to scope with Scope adaptor 5) Attach the DVM Probes to the DVMx1000 inputs (left terminals) 6) Scope Bandwidth limit should be engaged (if available) 7) Scope averaging should be used.

Note: for complete example of using a scope with the DVMx1000 see Section 5.

222...222...111 DDDCCC mmmeeeaaasssuuurrreeemmmeeennntttsss The following method accounts for the input error (input offset + test lead error) enabling measurements to 1 uV of accuracy. An example of the following steps is found in section 1.1 where emfs of about 10 uV are measured. In that example, the 1.9uV input error is significant and must be accounted for. In the other examples, effects into the hundreds of uV are measured; therefore, the input error is of no consequence and subsequently ignored. For each measurement:

1) Short the test leads together, then allow the reading to stabilize for about 10 seconds, then adjust the scope trace to zero using the vertical position knob (for older scopes) or by adjusting the offset (if your scope has one). Note: each time the unit is disconnected it will go into saturation, when the unit is reconnected, it takes time for the temperature change caused by the saturating unit to renormalize. This is why we must wait when changing the connection (see section 3.2 for more details).

2) Connect the test leads to the device being measured, wait 10 seconds for the unit to stabilize (same reason as above) the reading obtained is the actual voltage.

Note1: It is best to measure the input error (the thermal error of the test leads + DVMx1000 input offset) often since the thermal test lead error changes constantly with temperature, humidity and even the way the leads are laid out on the table (at least with the crumby set shown). The heat of holding a test lead in the hand alters the thermoelectric emf (as demonstrated by the example in section 1.1). Note2: The thermal errors of the adaptor that connect the DVMx1000 to the Scope does not contribute significant error. The reason is that the DVMx1000 output is in the millivolts range; therefore, the few microvolts of error in the adaptor are of no significance. Note3: Noise may be a problem, see appropriate sections.

222...222...222 AAACCC mmmeeeaaasssuuurrreeemmmeeennntttsss AC measurements are same as the DC measurement except that the AC coupling can be enabled. This will eliminate the input error from your readings. For a complete example of AC measurements see section 5.


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3 Support Tests This chapter contains a set of test and definitions that enable you to diagnose any problems with your DVMx1000. This chapter will enable you to obtain maximum accuracy and prevent damage to the DVMx1000.

This is a quick check to determine if the unit is functioning. This check should be performed whenever the unit is exposed to mechanical or electrical shock, or when it is expected that the unit has failed to function. If the accuracy of the unit is in suspect, perform the accuracy test.

1) Slide the power switch to the on position (toward the battery compartment) 2) If the green power light fails to glow, then replace the battery. If the power light continues not

to glow, then the unit has been damaged. 3) Perform the saturation voltage test (section 3.3) to ensure that the unit has sufficient output

range. 4) Perform input offset check to ensure that the input offset is within design parameters.

Most problems are due to low battery conditions. Tests show that a fresh alkaline battery can power the unit for up to 12 hours.

The saturation voltages represent the maximum output voltage range (negative to positive) of the unit relative to the common (Black terminals). When the unit is functioning properly the output range is from -4.3 volts to + (battery voltage-4.3v); this is about -4.3V to +4.7V. This enables the unit to accurately amplify (multiply by 1000) input voltages in the range from -4.3mV to +4.7mV (input range = output range/1000). The unit is only specified from -3mV to +3mV because that is the guaranteed output range over the 12 hour life of the battery ( see Figure 7-3: Operational Characteristics). For example, if one were to apply 0.1 volts to the inputs of the DVMx1000, then the theoretical output voltage of the unit is 0.1 X 1000 = 100Volts. Since the output of the unit can only attain 4.7 volts, this unit is said to be in saturation. It will not damage the unit to be driven into saturation as long as the inputs voltage is not outside the supply voltages (Saturation voltages are within 20 mV of supply voltages). When driving the unit into saturation, there are some important operational differences that need to be understood:


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1) The input impedance drops from 1012 to about 100k ohms. 2) The unit consumes more battery energy. 3) The output recovery time is about 3ms. This means that when the inputs are brought back into

normal range, the output may take as much as 3ms to come out of saturation. 4) The offset null calibration recovery time is about 30 seconds. Since the device is driven into

saturation, the unit dissipates more heat. This internal heating aggravates the internal thermal emfs which are normally balanced. When the output of the unit comes out of saturation, it may take up to 30 seconds for the internal temperatures to settle, allowing the offset-null calibration circuit to settle.

The definition and meaning of the output range is described in the previous section (Section 3.2). To measure the positive saturation voltage, connect the unit as shown in the following picture:

To measure the negative saturation voltage, reverse the connection as shown:


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From the above picture we note that the output range of the device is from negative 4.53 volts to positive 4.55 volts. This represents a 9.08 volt output range which should correspond to battery voltage. The next picture shows the measured battery voltage.

The battery voltage is larger than the output voltage range because the battery in the photo is not under load (it has been disconnected from the unit). If the battery were measured while it was in the unit and


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the unit is saturated, the battery voltage would be within a few millivolts of the measured 9.08 volt output range shown above. The typical negative output saturation voltage (-Vos) is about -4.3 volts; however, it may range from -4.0 to -4.7 volts depending upon battery voltage (see Figure 7-3: Operational Characteristics). The positive output voltage is the difference of the battery voltage and |-Vos|. Thus for a strong battery, the output range is from -4.5 to +4.5 volts. For a weak battery (say 6 volts), the output range may be from -4 to +2 volts. For best results, replace the battery when the positive saturation voltage drops below +3 volts (battery voltage = 7Volts). This will keep the unit operating within specified parameters. If the battery is strong and the unit does not yield the proper saturation characteristics, then the unit has been damaged and should be sent in for repair. This type of damage can occur if the battery is accidentally touched to the contacts in reverse, or a DC adaptor is used and improperly connected. This type of damage can also occur if the unit is dropped from a sufficient height. Damage of this nature is not covered under warranty.


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4 Improving measurement accuracy To improve the accuracy of measurements made with the DVMx1000, the user needs to be able to identify sources of error and eliminate such error. There are two types of error, AC error and DC error. When making DC measurements, a good quality DVM will integrate out the AC error leaving just the DC component. Therefore, for DC measurements, the AC component is of little concern unless the AC error is sufficient to causes the unit to saturate (Saturation is discussed in section 3.2). When making DC measurements, the user needs to be aware of DC errors and corresponding remedies. When making AC measurements, a standard DVM will eliminate the DC error with a capacitor. Although the DC error is eliminated, the user needs to be familiar with such things as thermal input noise (not thermal emf), interference, and the manner in which AC instruments report RMS readings. When making measurements with an oscilloscope, both AC and DC errors may present problems. This is especially true when making measurements of signals below 100uV as in the example shown in section 5. Best results are obtained by minimizing all sources of error. This manual contains a number of examples using oscilloscopes.

To obtain the best use of the DVMx1000, the user must become aware of typical sources of error. Some errors are unavoidable (such as input offset) while other errors are introduced with faulty or poor quality connections (such as Thermal emf errors). When using the DMVx1000 to measure voltages over 100 microvolts (uV), these small errors may be of little concern; however, for accurate measurements below 100 uV, these errors must be accounted for.

444...111...111 IIInnnpppuuuttt OOOffffffssseeettt The input offset is essentially an error at the input to the amplifier. The DVM x1000 amplifier contains special nullification circuitry that minimizes this error. The typical input offset is 0.5 uV but can be as much as 2 uV. To measure the input offset of the DVMx1000, short the input to common with a very short jumper as shown in Figure 4-1. Allow the reading to stabilize (about 30 seconds) and mark down the reading. This reading includes the input offset DVMx1000 and the thermal emf errors (see next section) of the


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jumper. Next, take another reading with the jumper reversed (again, wait for the reading to stabilize for about 30 seconds).

Figure 4-1: First Measurement: -001.0 uV

Figure 4-2: Second Measurement: -000.5uV

The input offset of the DVMx1000 is found by averaging the two readings Vos = ((-1) + (-0.5))/2= -0.75 uV The thermal emf error in the jumper lead is ½ the difference of the readings Vth=((-1) – (-0.5))/2 = -0.25 uV The jumper lead error may be due to the fact the jumper is poorly made, or one end is a slight bit warmer than another. An example where a slight bit is temperature difference can lead to errors is shown in section 1.1.

444...111...222 TTTeeesssttt LLLeeeaaaddd EEErrrrrrooorrr A standard set of test leads (like the ones used in previous experiments) may contain many junctions involving dissimilar metals to include metal coatings and solder joints. Since each junction may introduce a thermal emf error(see section 1.1 for more details), it is important to measure the amount of error in the test leads to determine if the error is significant. Note1: The test leads in the following example typically generate about 0.25uV of error; however, we have intentional damaged the leads in such a way to generate more error for the purpose of demonstration. Note2: We believe that the normal 0.25 uV error is due to the color difference which may result in a temperature difference; however, we have not spent time to validated this theory.


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Figure 4-3: Forward Measurement: -4.1uV

Figure 4-4: Reverse Measurement +0.8

The Lead error is found by subtracting the two measurements, then divide by two. Lead Error= ((-4.1)-(0.8))/2 = -2.5uV (approximate)


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444...222...111 IIInnnttteeerrrfffeeerrreeennnccceee rrreeeccceeeiiivvveeeddd aaattt iiinnnpppuuuttt The inputs to the DVMx1000 are very sensitive. In fact, care must be taken to ensure that the coupling of AC energy to the system does not cause the unit to alternately saturate positive then negative. This condition is called AC saturation and is described in the next section. The amount of coupling from the environment to the unit depends upon the nature of AC sources in the laboratory and the nature of the input leads attached to the DVMx1000. The most prevalent AC interference source is going to be the 60 Hz AC wall power.

Kc X1000

Vout Vin 1012

Figure 4-5 Effective coupling of AC energy to DVMx1000

Figure 4-5 shows a useful model for understanding the manner in which electrical energy is coupled to the input of the DVMx1000. Kc is predominantly capacitive coupling (capacitive reactance) and therefore decreases with frequency. As the frequency of the source (Vin) increases, more energy is coupled to the DVMx1000. It is for this reason that the DVMx1000 has been design with a first order low pass frequency at 600 Hz and second order roll off starting at 1200 Hz. Without these low pass functions, the DVMx1000 would pick up and amplify all sorts of AM radio and switching power supply interference. Such interference would make the device unusable. We are currently developing a shielded ( more expensive) version predominantly for scope measurements. Since the most predominant source of interference within the bandwidth limitation of the DVMx1000 is 60Hz AC wall power, the remainder of this section is devoted to 60 Hz AC wall power interference. The same techniques apply to all AC interference sources that the user cares to model. At 60hz, the value of Kc is going to be much smaller than the 1012 ohm input impedance of the DVMx1000; therefore, the unit will go into AC saturation, as shown in Figure 4-6, if the leads are not shorted together (It is accidental that the picture shows the red lead plugged into the black terminal and visa-versa).


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Figure 4-6: Open input saturation caused by stray 60 Hz electromagnetic fields.

To measure the Kc constant, attach a resistor to the leads of the unit. Find the largest value from the following list ( 10M, 1M, 100K, 10K, 1K) where the scope shows a relatively clean sine wave that does not saturate (saturation causes the sine wave to clipped as shown above). In the following example (Figure 4-7, Figure 4-8), 10k ohms attached to the test leads gives us a 227 uV (RMS) sine wave (only a slight bit of other harmonics present).

Figure 4-7


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Figure 4-8

Substituting the values back into the effective model yields:

Kc X1000

227mVac (RMS) 120Vac

(RMS) 10K

227uVac (RMS)

Figure 4-9

Solving the model (using the resistor divider equation) for Kc yields, 227x10-6=120(10K)/(10K+Kc) Kc=5.3x109 ohms Kc will change if the unit is moved, the test leads re-arranged/changed, or the atmospheric conditions change; however, the value will remain in or near the giga-ohm range. To demonstrate the volatility of Kc, the leads are opened into a large loop with the 10k resistor at the other end of the loop. The following photos show that the amount of energy coupled to the system has nearly doubled.


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Figure 4-10: The effect of diverging the test leads

Figure 4-11

Because the test leads are separated, the ability of the DXMx1000 to eliminate common mode interference is reduced thus allowing more noise energy to pass to the scope.

444...222...222 MMMiiinnniiimmmiiizzziiinnnggg iiinnnttteeerrrfffeeerrreeennnccceee To minimize the reception of stray electromagnetic fields, you want to either increase the value of Kc or reduce the impedance between the read and black leads (input and common), or both.


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To increase the value of Kc:

1) Keep input leads as short as possible. 2) Input leads should both be same length and type of wire. 3) If possible, the leads should be twisted together; or kept as close together as possible. 4) If necessary, make shielded input leads when long leads are required. 5) The connection point between the tip of the black probe and the red probe should be as close

together as possible.

To reduce the impedance between black and red 1) Measure low impedances sources such as pick up coils. 2) Place capacitors in parallel to shunt the AC signal energy. Do this only if it will not also shunt

desired signal energy. This is done in the Homopolar generator example in section 1.3. Another good point is to keep the output leads as far away from the input leads as possible, otherwise the x1000 output energy may couple back to the input causing oscillation or other unwanted effects.

444...222...333 IIInnnpppuuuttt NNNoooiiissseee (((ttthhheeerrrmmmaaalll nnnoooiiissseee))) No device is free of input noise; this section will show how to derive precise uV measurements even in the presence of the 11uVp-p noise that the DVMx1000 generates. The following scope trace shows the output of the DVMx1000 with the input shorted:

Figure 4-12


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The yellow trace (top) is the actual output signal and the red trace (bottom) shows the spectral distribution of the yellow trace. The spectral display shows that the highest component of noise is at -72 dB (relative to 1V RMS). To understand what this means, we remember the dB relationship:

=

1log20 VsigdB

Plugging in -72dB and solving for Vsig yields 0.25 mV (This correlates 0.25uV at the amplifier input). Although each noise component is relatively small, it is the summation of all the noise components is what result in the seemingly large 11.4uV of input noise. This input noise has an interesting characteristic that enables us to defeat it; it is random with a significantly zero average. We can capitalize on this characteristic by realizing that DVMs, when measuring DC, perform an averaging function on the input signal. This averaging function effectively cancels the effect of the noise from the DC result. This enables very precise measurements in the uV range using the DVMx1000 in spite of the noise. AC measurements can take advantage of this averaging function depending on the device used. This is covered in the next chapter.


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5 Example of AC measurement For measuring AC signals, there is much variation in the way AC readings are handled by different instruments. Classic Analog (d’Arsonval type) and less expensive DVMs use a “Rectify-then–measure-the-DC-then-apply-fudge-factor” technique which only gives precise RMS readings if the wave being measured is sinusoidal. The old meter (shown on the right of Figure 5-1) utilizes a similar technique; however, it provides a lookup table (in the users manual) that, based on the shape of the wave you are measuring, allows you to convert the reading to true RMS, peak-peak (p-p) or whatever. The meter in the center of the picture claims that it is a “TRUE RMS” DVM; however, we shall see. In the following picture, we are setting up a comparison of the AC uV readings from three different instruments: an old DVM (right), a Brand New “TRUE RMS” DVM (middle) and a DPO Oscilloscope (back). For the experiment, we are going to drive a triangle wave into an attenuator (box at left next to DVMx1000). The output port of the attenuator that we have chosen divides the input signal (shown by yellow/top trace on scope) by 143351 before being sent into the DVMx1000. The output of the DVMx1000 is then fed to all three measurement devices at the same time. The blue (bottom) trace on the scope is the output of the DVMx1000. The first problem seen in the photo is that the output signal (blue) is very fuzzy. To clean this up we can place the scope on bandwidth limit (if your scope has one). In our case we will set it to the lowest setting which is 20MHz.

Figure 5-1

The next bit of clean up is to twist the jumper leads between the attenuator and the DVMx1000 and then loop them as shown in Figure 5-2. These jumper leads are the critical point of reception for interference. For best results the user should keep the leads as short as possible or make shielded


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cables; however, this section will show that it is possible to make good measurements with what is shown.

Figure 5-2: Wire discipline

Your signal will look relatively clean if you are working above 100mV as is done in the magnetic sensor example at the beginning of this document; however, we are going to pump a 3Vp-p triangle wave into the attenuator which should produce a 3/143351=21uVp-p triangle wave at the input of our DVMx1000. REMEMBER that we have 11uV of noise (see Section 4.2.3); if your scope or DVM does not have an averaging capability (to be demonstrated) you should expect to see a signal similar to that in Figure 5-3. Before we continue, let us make some simple calculations to see what kind of RMS readings we should expect. The RMS value of a triangle wave is Vp-p/3.464. As such the TRUE RMS meter and the scope should read 21/3.464=6.1 uV and the older meter should return a value of Vp-p/3.6=5.83 uV (according to the users manual) Note: Throughout this manual, the pictures of the instruments show millivolts. The instrument readings are the output of the DVMx1000 which is 1000 times the input. This text talks in terms of the DVMx1000 input which is in microvolts.


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Figure 5-3: 200 Hz Triangle: before averaging the signal.

Figure 5-4: 200 Hz Triangle: before averaging the signal.


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If you were to compare all the readings of all devices, none of them agree; however, many digital meters are rated to be accurate to within ½ the last digit of display; therefore, the reading on the old meter is acceptable. A few pages back it was explained that the noise (11uV) is a random distribution of energy with an essentially zero average. Since DC meters average the input signal, the noise cancels itself from the result that is read. Most good quality digital scopes have an averaging capability that enables you to use this same technique to eliminate noise. The next photo shows the averaging function turned on.

Figure 5-5: 200 Hz Triangle wave with averaging enabled

The first thing that you will notice is that the RMS (not cRMS) value matches the value obtained with the old meter. The scope RMS value is calculated from all samples in the scope. The cRMS is a “Cycle RMS” or RMS of one cycle on the screen (I don’t know which one). We are not going to bother with cRMS for the rest of this discussion. The most important thing that you will notice is that the output wave is not as sharp as the input wave, in fact the amplitude of the output wave is only 16.6uV, not the 21uV that we expected, what’s going on?


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The answer requires us to explore what triangle waves are; a triangle wave can be modeled as a collection of sine waves; this is known as Fourier analysis. In Fourier analysis, a triangle wave consists of a main sine wave at the base frequency (in this case f=200Hz) called the “Fundamental”, and an infinite number of smaller sine waves (called “harmonics”) at frequencies 3*f, 9*f … to infinity. The harmonics are smaller in magnitude than the fundamental; in fact, the magnitude of each harmonic is inversely proportional to its frequency. This means that the 3*f is the largest and most important of the harmonics and the 9*f is the second largest harmonic. Since the higher frequency harmonics (higher than 9*f) diminish in importance, the triangle will probably still look like a sharp triangle even if the higher harmonics are removed. Thus, to maintain a decent triangular shape, a 200Hz Triangle wave requires that the 200 Hz (fundamental), 600 Hz (3*f harmonic) and 1800 Hz (9*f harmonic) components remain intact. We must remember that the DVMx1000 has an upper bandwidth limit of 300Hz; this means that most of the 2*f harmonic and all of the 3*f and above harmonics are removed from the signal as it passes through the DVMx1000. The loss of most of the harmonic energy is the reason why the wave appears to be more like a single sine wave. We can prove this by reducing the frequency of the triangle generator to 25 Hz. A 25 Hz triangle wave needs at least the 25 Hz, 75 Hz and 225 Hz components retained for good signal shape. These components all fit within the pass-band of the DVMx1000. This is shown in the following scope screen shot.

Figure 5-6: 25Hz Triangle wave: averaging enabled


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The 21uVp-p and 6.1uV RMS results agree with the calculations we performed earlier. For the above 25 Hz test, the “TRUE RMS” DVM reads 9.4uV and the old meter reads 6.1uV on its face which is multiplied by 1.038 to convert to RMS; this yields 6.3uV. Both meters readings are off. The old meter is acceptable since the reading is within ½ last digit of display (this is common standard of digital meter accuracy). The “TRUE RMS” meter is not too good. Perhaps the “TRUE RMS” meter does not reject the noise well. In the case of the scope, the averaging function is applied before the RMS calculation is done. This is true with the old meter as well; the old meter averages the input before applying its conversion. Perhaps the “TRUE RMS” includes the noise energy in its RMS calculation which might cause the 50% error that we see. We can verify this by increasing the ratio of the signal to noise. In the next test, we increase the signal to 250uV. This works out to an RMS value of 72uV. If the noise theory is correct, then the “TRUE RMS” meter should return the 72uV signal value + the 3uV noise value for a grand total of 75uV.

Figure 5-7

The comparison of the above results is in the following table.


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Table 1: Comparisons of uV RMS values

Reading (/1000) Correction Result (uV RMS) Calculated Input=250uVp-p 3.464 72.17 Scope 72.64 uV None 72.64 Old meter 69.4 uV 1.038 (from user man) 72.0 TRUE RMS meter 71.7 uV None 71.7 In this test, the “TRUE RMS” meter did excellent; it actually agrees with the other devices. In the previous test, the meter was reading about 3 uV too much. If the input noise were being added in to the RMS calculation as we had thought, then this 3 uV of error should have caused the above reading to be off by 3 uV as well. Since this did not occur (The reading is dead-on) we can only conclude that the “TRUE RMS” meter is not accurate at the lower end of its AC mV range. Knowing how your AC instrumentation behaves is very important!


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6 Miscellaneous This section represents techniques (how-to) and applications that do not fit into the previous discussion.

It is always good to know if the DVMx1000 unit is saturating; however, in some situations, an oscilloscope may not be available. If your meter is producing strange or unstable readings, it is most likely you are driving the DVMx1000 into saturation. The following procedure will enable you to determine if the unit is either AC saturating or DC saturating.

1) Switch the meter to DC, write down the actual absolute (drop the minus sign) voltage shown on the face of the DVM (don’t convert to uV like we do elsewhere in this manual). If the reading unstable, then you are most likely in saturation, or have an intermittent connection someplace.

2) Switch the meter to AC. Write down the measurement and multiply by 1.414 (converts to peak value).

3) Add the absolute value of result of step 1 to the value of step 2. If the total is greater than or equal to 3 volts then you are probably saturating the unit.

Saturation is normally something you want to avoid; however, it can be used as a means to determine battery power of the device without having to open the device and measure the battery voltage. To do this we first must develop relationship between the DVMx1000 while it is in AC saturation and the battery voltage. The actual peak to peak voltage at the output of the unit while in saturation is within 20 millivolts of the actual battery voltage. As such a scope will always give you accurate battery conditions. Since DVMs vary in the way that they measure RMS voltage for square waves, the following procedure will enable you to develop a way to measure battery voltage using the AC saturation technique in conjunction with DVMS:


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1) Open the leads and touch the red lead (with your hand) to ensure to the DVMx1000 will be driven into AC saturation. By touching with your hand you ensure that enough energy is received by the DVMx1000 that it drives into AC saturation and develops a very clean square wave.

2) Using your DVM, measure the RMS voltage and write down the reading. 3) Next open the battery pack and measure the DC battery voltage. 4) Divide the battery voltage obtained in step 3 by the value obtained in step 2 to obtain a

“Battery Constant”. 5) The above result is a constant you should write down (perhaps on the back of the unit with a

marker). Our example meters (the ones used throughout this manual) yield the following: Table 2

Meter Measured battery voltage AC saturation reading Battery constant Old 7.77 3.52 2.2 TRUE RMS 7.77 3.39 2.3 Whenever you want to know the battery voltage of the unit, open the input to the DVMx1000 and touch the red lead, then measure the AC voltage at the output, then multiply by the appropriate battery constant to obtain battery voltage. Note: On the older product labels we specify that that the AC voltage multiplied by 1.85 will yield battery voltage. This was a misprint.


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7 Electrical Specifications Table 3: General Specifications

Parameter Symbol Conditions Min Typ Max Units Notes Input Offset Vos +/-0.5 +/-2 uV Input Offset drift +/-0.01 +/-0.05 uV/oC Input Bias Current Ib +/-10 +/-75 pA -3dB Frequency Flp 300 450 600 Hz

Frequency Response

0

10

20

30

40

50

60

701 10 100 1000 10000

Frequency

Gai

n dB

Figure 7-1 Frequency response


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Phase Response

-135

-120

-105

-90

-75

-60

-45

-30

-15

01 10 100 1000

Frequency

Phas

e (d

egre

es)

Figure 7-2 Phase Response

Operational Characteristics over Battery life

0

1

2

3

4

5

6

7

8

9

10

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Time (hours)

Vol

ts

VbattOffst (uV)Sat-Sat+

Figure 7-3: Operational Characteristics


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Figure 7-4: Noise Characteristics

Figure 7-4 shows the output of the DVMx1000 when the inputs are shorted. The Ch1 trace (yellow) is the actual output voltage showing about 11mV p-p of noise. The Math trace (red) is a FFT of the Ch1 trace showing the spectral distribution of the noise energy. The FFT trace is from 0Hz (at left) to 2500 Hz at right.

X1000 Vi

250 Ohms

0.1uF Output Terminals

Figure 7-5 Equivalent output circuit


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8 Making Short Male-Male Jumpers These instructions have been made into their own manual located under the accessories section (product AC006). The AC006 manual shows you how to make them yourself, or, you can order them preassembled from www.Distinti.com

Figure 8-1


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9 To make an Audio/Oscilloscope adaptor These instructions have been made into their own manual located under the accessories section (product AC008). The AC008 manual shows you how to make an audio scope adaptor yourself or, you can order one preassembled from www.Distinti.com

Although this scope/audio adaptor is unmatched and uncompensated, it performs well beyond 10 MHz. Performance details are located in the AC008 manual.

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