Figure 77 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.
The output of the amplifier can be calculated according to Equation 7 if VIN+ = VCM + VSIG+ and VIN– = VCM + VSIG–.
Equation 8 shows the signal gain of the circuit. The input VCM is rejected, and VREF provides a reference voltage or level shift around which the output signal swings. The single-ended output signal is in-phase to the lower input signal polarity.
Integrated INA solutions are available, but the OPAx837 device provides a high-frequency solution at relatively low power (< 1.8 mA for the three op-amp solution). For best CMRR performance, resistors must be matched. A good rule of thumb is CMRR ≈ the resistor tolerance; so a 0.1% tolerance provides approximately 60-dB CMRR. For higher gain INA implementations with higher bandwidths, apply the OPA838 to the circuit of Figure 77.