HighCMRR instrumentation amp works with low supply voltages
Modern batterycell voltages of 3 to 3.6V require circuits that offer efficient lowvoltage operation. This Design Idea proposes an accoupled instrumentationamplifier design that features high CMRR (commonmoderejection ratio), wide dc inputvoltage tolerance, and a firstorder highpass characteristic. Most of these features stem from a highgain firststage design. The circuit uses popularvalue and tolerance components. Figure 1a shows the simplified amplifier circuit. The general principle is that the capacitor, C, and the R_{3} resistors buffer and accouple the input signal. The second stage comprises two differential amplifiers, A_{D}. Each of them amplifies half the differential input signal. A summing operation yields the following expression for V_{OUT}:
In Figure 1a, V_{A}, V_{B}, V_{C}, and V_{D} are the two differential amplifiers' inputs, and A_{D} is the gain. The time constant 2R_{3}C defines the highpass cutoff frequency. Figure 1b shows the detailed circuit. The input stage comprises op amps A_{1}, A_{2}, A_{3}, and A_{4}. A_{1} and A_{2} are the main gain stages. Because their inverting and noninverting inputs are at the same potential, the input voltages supply the R_{3} resistors. The buffers A_{3} and A_{4}, along with the R_{2} resistors, produce an amplification factor, 1+R_{3}/R_{2}, for the current in R_{3}, because R_{2} and R_{3} connect to equal potentials. This circuit structure is the heart of the design. The voltage on capacitor C has no ac component, and A_{1} and A_{2} each amplifies onehalf of the differentialinput ac signal. C filters the input dc component, which appears at the A_{3} and A_{4} outputs. The second stage is a unitygain, fourinput addersubtracter stage. It implements the above equation, where A_{D} is 1+R_{1}/(R_{2}R_{1}). Assuming R_{3}>>R_{2}, A_{D}=1+R_{1}/R_{2}.
Another possible implementation for the second stage could use two differentialchannel ADCs, producing a digitized V_{OUT}, ready for microcomputer processing. If a ±5V supply is available, it is possible to obtain V_{OUT} by using two difference amplifiers on one chip, such as the INA2134. You can calculate the minimum CMRR as:
where A_{D(14)} is the differential gain of amplifiers A_{1} through A_{4}, A_{CM(14)} is the commonmode gain of these amplifiers, A_{D5} is the differential gain of amplifier A_{5}, and A_{CM5} is the commonmode gain of A_{5}. Δ is the tolerance of the R_{4} resistors in the circuit. A very important parameter is the op amps' input offset voltage, especially for A_{3} and A_{4}. The A_{1} and A_{2} offsets do not contribute to error, because they add to the input signal's dc component, which capacitor C cancels. The maximum outputvoltage error attributable to opamp offset voltage is:
where V_{IOMAX} are the maximum offset voltages of the corresponding op amps. In selecting op amps, you should note the following: A_{3}, A_{4}, and A_{5} should be lowoffset and highCMRR types, and A_{1} and A_{2} should have high openloop gain, CMRR, and gainbandwidth products. Figure 2 shows a practical amplifier circuit. The power supply is one 3V lithium battery. You can use several opamp types, such as MCP607s or OPA2336s. Because of the input commonmode voltage range, you set the signal ground to onethird of the supply voltage. The D_{1} diodes prevent the circuit from latching up. The R_{7}C_{4} networks provide RFnoise filtering at the inputs. You derive the network's values from the following consideration: With R_{7}C_{4}=(R_{1}R_{2}R_{3})C_{2}~R_{2}C_{2}, the highfrequency zero in the amplifier's transfer function cancels:
The circuit has the following advantages:

The first stage ensures the overall gain, thus providing high CMRR without the use of highprecision resistors in the second stage.

By connecting the lowfrequencydetermining RC network to the inverting inputs of the opamp pair that amplifies the input signal, the circuit needs no additional input buffers.

The circuit delivers a standard, firstorder highpass characteristic, using passive components with popular values and tolerances.

The differentialinput range is as high as 2V, using a 3V supply.

The circuit consumes low supply current and power: approximately 120 μA, 0.4 mW.
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