Design Idea; Differential I/O low-power instrumentation amp

January 03, 2014 // By Sandro Herrera
Currently, all commercially available three-op-amp instrumentation amplifiers (in-amps) offer a single-ended output, but many applications could benefit from an in-amp with a differential output.

Offering many advantages over its single-ended counterparts, a fully differential in-amp features increased immunity to common-mode noise sources, achieves lower second-harmonic distortion and higher signal-to-noise ratio, and provides an easy way to interface with modern differential-input ADCs.

Figure 1 shows the circuit implementation of a low-power, fully differential instrumentation amplifier formed by the cascade of an OP2177 precision low-power, dual op-amp (IC1) and an AD8476 fully differential amplifier/ADC driver (IC2). Drawing less than 1.2 mA of supply current, the composite amplifier has 11 nV/√Hz input noise, 2 nA maximum input bias current, 75 mV maximum input-referred offset voltage, and 0.9 mV/K maximum input referred offset voltage drift.

 

Figure 1 Low power, fully differential instrumentation amplifier

The OP2177 and gain-setting resistors RF1, RF2, and RG form the in-amp’s preamplifier and set the amplifier’s voltage gain to:

If RF1 = RF2, then

The AD8476 serves as the in-amp’s subtractor; thus, it receives the amplified signal from the preamplifier, rejects its common-mode component, and passes its differential component. The AD8476 has a 90dB common-mode rejection ratio (CMRR), enabling the in-amp to have 90 dB CMRR even at unity gain. At higher gains, errors induced by a common-mode input signal are further reduced by the preamplifier’s voltage gain when referred to the input.

Since the in-amp uses the three-op-amp topology, the match between discrete resistors RF1, RF2, and RG sets the amplifier’s gain accuracy—a parameter that can be easily calibrated—but does not limit the amplifier’s CMRR. The AD8476 also implements the in-amp’s differential output drive, allowing it to directly drive up to 500kSa/s differential input ADCs. Optional network RZ – CZ forms a single-pole low-pass filter that can be used as an antialiasing filter.

The instrumentation amplifier’s output common-mode voltage is set by driving the AD8476’s VOCM pin. If this pin is left unconnected, the amplifier’s output common-mode voltage sits at mid-supply. When using the in-amp to drive an ADC, the AD8476’s VOCM