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The Design of a Calculable AC voltage reference using digital waveform generation
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Abstract
The design of an AC voltage reference source using a digital to analogue converter controlled by a microcontroller to produce a calculable RMS AC voltage reference with accuracy suitable for calibrating high performance Digital multimeters.
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Technical Objectives
• To provide a method for secondary and below laboratories with the ability to generate high accuracy AC Voltages
• Investigate errors associated with digitally generated AC Voltages, as well as the practical application of digitally generated waveforms in commercial calibration
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Learning Objectives
• To investigate alternatives to traditional AC voltage verification methods
• To enhance calibration laboratories understanding of verification of high performance modern DMM’s
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The calibration of Today’s Modern high performance Digital Meters which offer ACV Accuracy in the order of 80-100 ppm
To achieve a stand off ratio of just 4 to 1requires an accuracy of around 20 ppm. This is not possible with a multi-product calibrator
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The Requirement
The Solutions
1) Multi–Junction Thermal Transfer Standard
2) Calculable AC voltage reference
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The Solutions : Multi-Junction Thermal Transfer Standard
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Advantages Disadvantages
Wide Frequency Range Requires both a stable DC and AC voltage source
Proven Accuracy Equipment is expensive
Calibration is costly, especially when a wide range of points is required
Even with protection, older Thermal Transfer devices are easy to damage by accidently over-ranging
The Solutions : Calculable AC Voltage Reference
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Advantages Disadvantages
Due to the nature of the device, does not require an AC or DC source
Limited frequency range
Instrument can be internally verified with a DC volt meter
Output voltages are limited
Due to solid state design, instrument is hard to damage with incorrect connections
Low cost due to ‘simple’ hardware
The Concept of theCalculable waveform
• Digitally create an AC waveform using a digital to analogue (DAC) converter. In our experiments we chose a waveform based on 256 steps.
• Single step the waveform to allow the level of each step to be measured as a DC level.
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The Concept of theCalculable waveform
The RMS value of the waveform is then calculated using the following formula
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Where :
Vrms = RMS Ac voltage outputV = measured DC voltage of ‘step’n = number of steps
History
This is not a new idea.
This technique was pioneered as
long ago as 1988 where results against
Thermal converters gave uncertainties at
the 7 volt level of just 5ppm.
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What has changed
There is now a much greater need for high accuracy ACV. In 1988 we were only seeing the first developments in High accuracy AC DMM’s. Now almost every laboratory has a high performance DMM
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What has changed
Digital Electronics along with the rapid development of digital to analogue converters for high quality audio has now made it much easier to implement this concept.
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The Circuit
The design uses a PIC micro controller, programmed with the waveform to directly drive a DAC. The PIC is directly
controlled from an RS232 interface.
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Control Interface
Micro ControllerPIC 16F84
Clock
DAC
10V Reference Input
AC Voltage Output
The DC Reference
The D to A converter needs a short term stable 10Volt DC reference.
The Linear Technology LTZ100 reference can easily provide better than 1ppm stability over the short
term 2/3 hours required.
As the LTZ1000 is temperature stabilised
temperature variations over the measurement period will not add any significant contributions to an
uncertainty budget16th July 2013 NCSLI 2013, Nashville 16
Improvements Vs Performance
The circuit used is extremely simple. Early design’s used two converters, with the output being switched between them
to remove glitches.
With modern converters this is not necessary.
The PIC micro controller directly drives the converter, loading the code into the converter directly from it’s
memory, also clocking the converter after each data load. This very simple approach requires some machine code for the PIC to make it run as fast as possible. All timing comes
from the crystal oscillator driving the PIC.
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The Software
The AC reference is very simple to control, with just a few commands needed.
1 - 15Hz
2 - 60Hz
3 - 200Hz
4 - 1000Hz
S - Stop waveform and set to zero
X - Advance 1 step
Y - Advance 10 steps
Z - Advance 64 steps
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Taking the DCV Measurements
The DCV measurements can be taken with a high performance 8 digit DMM.
This will typically give full scale & linearity uncertainties of less than 5ppm.
As there are over 256 measurements to be made it is recommend that the process be automated by using a PC &
software.
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Measuring Software
A procedure to measure each step was written using Procal. As each step is measured it is squared and
added to a running total.
The procedure being 256 steps long, takes about 40 minutes to run automatically.
Although the measurement procedure could be written in any language Procal has the advantage to
work with any DMM without changing code.
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Basic Measurement Diagram
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Practical Set up
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Voltage Range
With a 10V input the peak output
of the converter will be 10Volts;
giving an RMS voltage of 7.07V
To get a 1V output the 10Volt output
can be resistively divided down allowing the output to still be single stepped and measured on DC.
Note an IVD cannot be used to divide down the DC output. However it can be used on the AC output
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Errors Switching Glitches and transition
This error is most difficult to evaluate. It is dependent on the DAC used. The DAC we have chosen is typically used for audio will be very ‘clean’. The approach used by Transmille has been to look at an individual step on a scope and estimate an error based on the size and duration of the transitions.
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Errors Switching Glitches and transition
Typical figures for this, at a frequency of 50Hz, where there is a step every 80uS, worst case measurements give glitch times of around 8nS, with a glitch size of around 10% of the step. This gives transition errors around the 10ppm level. In practice as some glitches will add and some will subtract the real error will be much less.
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Errors from Rise / fall times & DC offsets
Rise and Fall time
Providing the rise and fall times are equal they do not contribute to errors. The error caused by a difference in rise/fall times can be mathematically calculated
DC offsets
Any DC offsets in the DAC will be measured and added to the calculated RMS figure. However if the device being calibrated is AC coupled DC offsets in the AC output will give an error and it may be best to trim any offsets out to avoid this problem.
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Errors from Loading effects
Output Loading
When measuring the output with a thermal converter the load effect on the 2 wire output will need to be considered.
When using measuring the output with a typical DMM loading effects will be negligible
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Errors from drift in DC Reference Source & DAC
As the AC source is a transfer device it is only the short term drift between the DC measurement of the system and the subsequent use on AC that contributes to the error.
Long term changes in the reference or the DAC linearity or gain do not contribute as the system is calibrated before use.
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Uncertainties
Typical laboratory figures for uncertainty
Imported DC Voltage Measurement 5ppm
Resolution of DC voltage Measurement 0.1ppm
Switching transition errors 10ppm
Short term stability of DC reference and DAC* 2ppm
Combining for 95% (K=2) gives 13ppm
* Includes effects of temperature for +/- 2’C from Tcal
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Verifying the accuracy
Measurements made by TransmilleTransmille best AC measurement capability uses
our Fluke 540b thermal transfer, the accuracy of this
Instrument is enhanced by measuring the output of the thermal converter directly. Loading effects on both AC and
DC measurements were compensated for. Tests performed against a Wavetek 4920, Transmille 8081,
Agilent 3458A and Fluke 8508A provided similar results.
Typical readings obtainedSingle step calculation = 7.055681
Measurement by thermal Transfer 540B = 7.055779
Error = 14 ppm16th July 2013 NCSLI 2013, Nashville 30
Frequency Range
To date Transmille’s interest has been
In the frequency range 40Hz to 1kHz.
The design could generate frequencies
below 1Hz with no additional errors.
Frequencies up to 10kHz could also be
generated with increased error from switching.
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Improvements……
Extending the Voltage range.
A simple resistive divider
to provide 1V and 100mV outputs.
A DC voltage amplifier to provide a 100V output.
Both could be again ‘calibrated’ at DC using a known DMM to provide the calculated AC RMS value
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Other Functions……
Non sine wave outputs
With this method it would be very easy to generate other wave shapes to evaluate performance of
converters including crest factor.
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Power and Phase…..
Future development of this technique could be to add additional converters to allow for several
outputs. Up to 6 outputs could easily be provided for 3 phase simulation of power, with an accurate phase,
or delay between the outputs.
If all the outputs were at the same 7V level this would provide a very affordable solution for a phase
and power reference.
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References
[1] R. Kinard, and L. A. Harris “ A digitally synthesized sine wave source with +/- 1 ppm amplitude stability. “ in Euromeas ’77 Conf
Digest. Institute Elect. Eng. Conf. Pub no 152, Sept. 1977
[2] Oldham N., Hetrick P., Zeng X., “A Calculable, Transportable Audio Frequency AC Reference Standard”. IEEE Transactions on I & M, April
1989, Vol. 38.
[3] Oldham N., Bruce W., FU C., Cohee A., Smith A., “An Intercomparison of AC Voltage Using A Digitally Synthesized Source”.
IEEE Transactions on I & M Vol. 39 No. 1, February 1990
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Conclusions
Digital synthesized waveforms can
provide a low cost and easy to use solution for DC to AC voltage transfer.
Copies of our paper and presentation are available by signing up to our newsletter
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