Electronic Design

Composite Instrumentation Amp Extends CMRR Frequency Range 10 Times

Instrumentation amplifiers are the building blocks commonly used in industrial, medical, and military systems. The primary benefit of such a component is its ability to reject common-mode signals while amplifying a differential-input signal.

While all instrumentation amplifiers perform well at low frequencies, their ability to reject common-mode signals usually degrades rapidly as the frequency increases.

The circuit in Figure 1 is a composite instrumentation amplifier with a high common-mode rejection ratio (CMRR). It features an extended frequency range over which the instrumentation amplifier has good common-mode rejection (Fig. 2). The circuit consists of three instrumentation amplifiers. Two of these, U1 and U2, are correlated to one another and connected in antiphase. It is not necessary to match these devices because they are correlated by design. Their outputs, OUT1 and OUT2, drive a third instrumentation amplifier that rejects common-mode signals and amplifies differential signals. The overall gain of the system can be determined by adding external resistors. Without any external resistors, the system gain is 2 (Fig. 3). The performance of the circuit with a gain of 100 is shown in Figure 4.

Since U1 and U2 are correlated, their common-mode errors are the same. Therefore, these errors appear as a common-mode input signal to U3, which rejects them. In fact, if it is necessary, OUT1 and OUT2 can directly drive an analog-to-digital converter (ADC). The differential-input stage of the ADC will reject the common-mode signal, as seen in Figure 5.

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