ADCs with differential inputs are becoming increasingly popular. This popularity isn't surprising, because differential inputs in the ADC offer several advantages: good common-mode noise rejection, a doubling of the available dynamic range without doubling the supply voltage, and cancellation of even-order harmonics that accrue with a single-ended input. But the differential input structure doesn't eliminate the frequent need for additional gain between the signal source and the ADC. A frequently used gain stage is the classic, three-op-amp instrumentation amplifier (Figure 1). This popular circuit offers excellent common-mode rejection and high input impedance. The circuit also has an output-reference (ground-sense) terminal, allowing you to reference the output voltage to a voltage other than ground. However, this circuit has a single-ended output (relative to the reference terminal), so it's a poor match for a differential-input ADC.
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| Figure 1. | The classic three-op-amp instrumentation amplifier does not provide differential outputs. |
Figure 2 shows two easy ways to create a differential-input instrumentation amplifier. In Figure 2a, IC4 and its associated feedback resistors are connected in parallel with the original output amplifier but with inverted polarity relative to the original circuit. The two outputs together provide the desired function, but the circuit requires many matched resistors. Furthermore, the common-mode reference input could require several milliamperes of drive, depending on the resistor values and voltages involved. However, the circuit does the job, and you can build it by using a high-quality quad op amp and a handful of resistors. Figure 2b shows a more efficient and elegant approach, using only the four resistors required in the original output stage. In this circuit, a modern, fully differential op amp, such as the AD8138, replaces IC3 and IC4 in Figure 2a. The amplifier's two outputs swing symmetrically about its high-impedance, common-mode reference input. The differential outputs provide a clean, simple interface to a differential-input ADC.
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| Figure 2. | The four-op-amp instrumentation amplifier (a) provides differential outputs but requires many matched resistors. A differential-output op amp (b) reduces the IC count in Figure 2a to three. |
