Introduction

An instrumentation amplifier is one of the most versatile signal processing amplifiers available.

It is used for precision amplification of differential dc or ac signals while rejecting large values

of common mode noise. By using integrated circuits, a high level of performance is obtained at

minimum cost. It is a type of differential amplifier that has been outfitted with input buffers,

which eliminate the need for input impedance matching and thus make the amplifier particularly

suitable for use in measurement and test equipment. Additional characteristics include very low

DC offset, low drift, low noise, very high open-loop gain, very high common-mode rejection

ratio, and very high input impedances. Instrumentation amplifiers are used where great accuracy

and stability of the circuit both short- and long-term are required. Although the instrumentation

amplifier is usually shown schematically identical to a standard op-amp, the electronic

instrumentation amp is almost always internally composed of 3 op-amps. These are arranged so

that there is one op-amp to buffer each input (+, −), and one to produce the desired output with

adequate impedance matching for the function.

Input offset voltage is differential voltage that must be applied to the differential amplifier input

to make the output voltage zero. Ideally, offset is expected to be zero volts.

The instrumentation amplifier gets its input from sources with a finite output resistance. High

input impedance is desired to ensure that the source is not loaded and the accuracy of the

measurement is not affected.

The 3 Op-Amp In-Amp

The 3 op-amp in-amp architecture is a common choice for both distinct and integrated in-amps.

The general gain transfer function is quite complicated, but if R3 = R4 = R5 = R6, the transfer

function simplifies to:

V out=(V ¿¿+¿−V−¿)(1+R2+R1

RG)¿

R2 and R1 are typically set to the same value, usually somewhere between 10 k½ and 50 k½.

The circuit’s overall gain can be adjusted from unity to an arbitrarily high value simply by

changing the value of RG.

Common Mode Gain

As we would expect, the common mode gain of the in-amp should ideally be equal to zero. To

work out the common mode gain, let’s imagine that there is only a common mode voltage of V cm

present at the inputs (i.e., Vin+ = Vin– = Vcm). As there is no voltage drop across RG, the voltage

on the outputs of each of the amplifiers, A1 and A2, is also equal to V cm. So to a first

approximation (assuming A1 and A2 are ideally matched) the common mode gain of the first

stage is equal to unity and is independent of the programmed gain. Assuming that op-amp A3 is

ideal, the common mode gain of the second stage is given by:

V outV ¿

=(

R4+R3

R3

∗R6

R5+R6

−R4

R3

)

Plugging this into Equation, the equation for the common mode rejection ratio becomes:

CMRR=20log¿)

The denominator of this equation is more complicated than it is for the 2 op-amp in-amp. The

denominator can be replaced by the percentage mismatch between the resistors: CMRR=20log¿)

Now, if all four resistors in Equation 9 are equal (or even if R3 = R5 and R4 = R6), the

denominator will reduce to zero. But any mismatch between the four resistors will cause a

portion of the common mode voltage to appear at the output. Similar to the case of the 2 op-amp

in-amp, any mismatch between the temperature drift of the resistors will further degrade the

CMRR as the temperature changes.

Common Mode Range

As we have previously mentioned, the common mode gain of the first stage of a 3 op-amp in-

amp is unity, with the result that the common mode voltage appears at the output of A1 and A2.

The differential input voltage, however, appears across the gain resistor. The resulting current

that must flow through R1 and R2 means that the voltage on A1 will rise above Vcm and the

voltage on A2 will drop below Vcm as the differential input voltage increases. Therefore, as the

gain and/or input signal increases, so does distribution of the voltages on A1 and A2, eventually

to be limited by the supply rails. We can conclude that the reachable ranges on the common

mode voltage, the differential input voltage, and the gain are consistent. For instance, increasing

the gain reduces both common mode range and input voltage range. By the same token,

increasing the common mode voltage tends to limit the differential input range and the maximum

achievable gain. If the output swings of the input stage op-amps are known, the connection

controlling input range, common mode range, and gain can be well defined for a particular 3 op-

amp in-amp. As the industry moves to lower supply voltages, this issue becomes more critical

with less and less headroom being available. As in the case of the 2 op-amp in-amp, the use of

rail-to-rail op-amps maximizes available headroom. A rail-to-rail output stage (A3) is of little

use, though, if the output voltages of the input stage, A1 and A2, are being cut because of

unnecessary input voltage, common mode voltage, or gain.

Product Description

The AD8231 is a low drift, rail-to-rail, instrumentation amplifier with software-

programmable gains of 1, 2, 4, 8, 16, 32, 64, or 128. The gains are programmed via

digital logic or pin strapping.

The AD8231 is ideal for applications that require precision performance over a wide

temperature range, such as industrial temperature sensing and data logging. Because the

gain setting resistors are internal, maximum gain drift is only 10 ppm/°C for gains of 1 to

32. Because of the auto-zero input stage, maximum input offset is 15μV and maximum

input offset drift is just 50nV/°C. CMRR is 80 dB for G = 1, increasing to 110 dB at

higher gains.

The AD8231 also includes an uncommitted op amp that can be used for additional gain,

differential signal driving, or filtering. Like the in-amp, the op amp has an auto-zero

architecture, rail-to-rail input, and rail-to-rail output.

The AD8231 includes a shutdown feature that reduces current to a maximum of 1μA. In

shutdown, both amplifiers also have high output impedance, which allows easy

multiplexing of multiple amplifiers without additional switches.

The AD8295 contains all the components necessary for a precision instrumentation

amplifier front end in one small 4 mm × 4 mm package. It contains a high performance

instrumentation amplifier, two general-purpose operational amplifiers, and two precisely

matched 10kΩ resistors.

The AD8295 has been designed to make PCB routing easy and efficient. The AD8295

components are arranged in a logical way so that typical application circuits have short

routes and few via. Unlike most chip scale packages, the AD8295 does not have an

exposed metal pad on the back of the part, which frees up additional space for routing

and via. The AD8295 comes in a 4 mm × 4 mm LFCSP that requires half the board space

of an 8-pin SOIC package.

The AD8295 includes a high performance, programmable gain instrumentation amplifier.

Gain is set from 1 to 1000 with a single resistor. The low noise and excellent common-

mode rejection of the AD8295 enable the part to easily detect small signals even in the

presence of large common-mode interference.

The AD8295 operates on both single and dual supplies and is well suited for applications

where ±10 V input voltages are encountered. Performance is specified over the entire

industrial temperature range of −40°C to +85°C for all grades. The AD8295 is

operational from −40°C to +125°C; see the Typical Performance Characteristics section

for expected operation up to 125°C.

The AD620 is a low cost, high accuracy instrumentation amplifier that requires only one

external resistor to set gains of 1 to 10,000. Furthermore, the AD620 features 8-lead

SOIC and DIP packaging that is smaller than discrete designs and offers lower power

(only 1.3 mA max supply current), making it a good fit for battery powered, portable (or

remote) applications.

The AD620, with its high accuracy of 40 ppm maximum nonlinearity, low offset voltage

of 50 µV max, and offset drift of 0.6 µV/°C max, is ideal for use in precision data

acquisition systems, such as weigh scales and transducer interfaces. Furthermore, the low

noise, low input bias current and low power of the AD620 make it well suited for medical

applications such as ECG and noninvasive blood pressure monitors.

The low input bias current of 1.0nA max is made possible with the use of Super ßeta

processing in the input stage. The AD620 works well as a preamplifier due to its low

input voltage noise of 9nV/Hz at 1 kHz, 0.28µV p-p in the 0.1 Hz to 10 Hz band,

0.1pA/Hz input current noise. Also, the AD620 is well suited for multiplexed

applications with its settling time of 15 µs to 0.01% and its cost is low enough to enable

designs with one in amp per channel.

Applications

Many industrial and medical applications use instrumentation amplifiers (INAs) to condition

small signals in the presence of large common-mode voltages and DC potentials. Examples of

applications where in-amps may be used include:

1) Audio applications involving weak audio signals or noisy environments;

2) High frequency signal amplification in cable RF systems;

3) High-speed signal conditioning for video data acquisition and imaging;

4) Data acquisition from low output transducers;

5) current/voltage monitoring;

6) Medical instrumentation;

7) Pressure and strain transducers;

8) Thermocouples and RTDs;

9) Programmable instrumentation;

10) Industrial process controls;

11) Weigh scales;

12) Wheatstone bridges;

13) Strain gauges;

14) Transducer interfaces;

15) Differential output;

Instrumentation amplifiers can be built with individual op-amps and precision resistors, but are

also available in integrated circuit form from several manufacturers. An IC instrumentation

amplifier typically contains closely matched laser-trimmed resistors, and therefore offers

excellent common-mode rejection.

Simulation of In-Amp with closest output to no. 4148 (student ID)

Circuit Scheme

Output graph “4159”

Conclusion:

The Op-Amp In-Amp circuit constructed and run successfully. By understanding the op-amp in-

amp concept and usage, the goal of the experiment has been achieved and the closest output to 4

last digit of student ID (4148) has been pointed in output graph (4159).