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Most instrumentation amplifiers do not like their reference inputs driven from a source with high source impedance. Typical instrumentation amplifiers use internally matched resistors that must be tied to a low impedance for good CMRR (common mode rejection ratio). When the reference driver has a large source impedance, it adds to the in amp’s internal resistors and ruins the careful matching. It is sometimes tempting to drive the reference with a resistor divider – for example when a mid-supply reference is required. However, the output resistance of the resistor divider will most likely ruin the instrumentation amplifier’s performance. To avoid this, buffer the resistor divider with an op amp. It is also advisable to add a large value capacitor between the resistor divider junction and ground – this capacitor will filter out high frequency noise on the power supply line.
