Don't fall in love with one type of instrumentation amp

The instrumentation amps I covered in my last column are difference amps made with internal resistors (Reference 1). Difference amps have high CMR errors when you use them with high-resistance sensors. Their input resistance is low—usually in the tens of kilohms—so any mismatch in sensor resistance causes a CMR error. Manufacturers set differential amps' gain because resistor-matching errors cause CMR errors. Also, the circuit configuration precludes input-resistance matching, so you can't use difference amps when the sensor is sensitive to load resistance. Inserting a buffer in front of the difference amp's inputs solves the input-resistance problem because only the IC design and mounting hardware limit a buffer's input resistance.

The circuit in Figure 1 shows an instrumentation amp with three op amps, and the associated transfer equation follows:

IC₁ and IC₂ are buffers that provide high-impedance inputs for the instrumentation amp. R_G provides a way to change the circuit gain with one resistor. When you leave R_G open, the gain is 1. A simple way to visualize the circuit is to look at R_G as two identical resistors each having half the value. You've now solved the input-resistance and gain-changing problems, but how do you get the CMR that you have sacrificed? The CMR comes from the difference amp IC₃, and the sacrifice is cost of the two extra op amps and the added resistors. The three-op-amp instrumentation amp costs more than a difference amp, but it provides just as good CMR, gains from 1 to several hundred, very high input resistance, matched input resistance, and good bandwidth.

The circuit in Figure 2 shows an instrumentation amp with two op amps, and the associated transfer equation follows:

The theoretical minimum gain of –2 occurs when R₁=R₂ and when you leave open R_G. However, in reality, the minimum gain is usually –4 or –5, so you can't use this configuration in buffer applications. The V₁ signal must propagate through two op amps, but the V₂ signal propagates through one op amp. When input signals contain frequencies greater than the flat portion of the op-amp gain curve (Reference 2), the V₁ signal attenuates more than the V₂ signal. The unequal attenuation causes the signal to unbalance, and CMR reduces at high frequencies. The gain of the two-op-amp instrumentation amp changes with one resistor, R_G.

The two-op-amp configuration can provide higher CMR, especially in low-voltage, single-supply applications. Two-op-amp configurations have simpler internal circuitry that enables lower cost, lower quiescent current, and smaller packages. The INA156 and INA122 are examples of two-op-amp instrumentation amps targeting single-supply operation. The newest three-op-amp instrumentation amps have overvoltage input protection for industrial applications (INA129), and some instrumentation amps (PGA204) include programmable-gain capability. The programmable feature enables users to always obtain the highest dynamic range by changing gain as the input signal approaches a limit. The three types of instrumentation amps all feature higher CMR than you can obtain with discrete components. The two- and three-op-amp instrumentation amps add high matched-input resistance and gain-changing capability.

Author Information

Ron Mancini is staff scientist at Texas Instruments. You can reach him at 1-352-569-9401, rmancini@ti.com.