Figure 9-1 is an instrumentation amplifier that combines the high input impedance of the differential-to-differential amplifier circuit and the common-mode rejection of the differential-to-single-ended amplifier circuit. This circuit is often used in applications where high input impedance is required (such as taps from a differential line) or in cases where the signal source is a high impedance.

If V_IN+ = V_CM + V_SIG+ and V_IN– = V_CM + V_SIG–, the output of the amplifier may be calculated according to Equation 3.

Equation 3.

The signal gain of the circuit is . V_CM is rejected, and V_REF provides a level shift around which the output signal swings. The single-ended output signal is in-phase with the differential input signal.

Figure 9-1 Instrumentation Amplifier

Integrated solutions are available, but the OPA835 device provides a much lower-power, high-frequency solution. For best CMRR performance, resistors must be matched. A good rule of thumb is CMRR ≈ the resistor tolerance; so 0.1% tolerance will provide approximately 60-dB CMRR.