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Differential I/O low-power in-amplifier

26 Dec 2013  | Sandro Herrera

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These days, all commercially available three-op-amp instrumentation amplifiers (in-amps) provide 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.

The figure 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 1.2mA of supply current, the composite amplifier has 11nV/√Hz input noise, 2nA maximum input bias current, 75mV maximum input-referred offset voltage, and 0.9mV/K maximum input referred offset voltage drift.

Figure: 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 90dB 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 in-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 pin should be connected to the common-mode voltage required by the ADC.

About the author
Sandro Herrera is a senior IC designer.

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