Checking Source-Impedance Balance Around a Chopper Amplifier

A zero-drift amplifier can provide excellent DC accuracy while interacting with its input network at internal switching frequencies. Review the complete source impedance around both inputs, particularly when the circuit uses large resistors, filtering or a sensor with frequency-dependent impedance.

Identify the relevant mechanism

Analog Devices’ wider-bandwidth zero-drift discussion explains how transient input currents can interact with unequal source impedances and create additional artifacts. It also describes interference mixing with chopping behavior, which makes the input spectrum relevant to the result.

Record the selected amplifier and the manufacturer’s guidance for its switching artifacts. Do not infer identical behavior across all devices described as zero drift. Keep DC offset performance and higher-frequency interference behavior as separate items in the design review.

Draw the impedances seen by both inputs

Include the sensor, gain network, protection parts, bias return and filter capacitors. Consider impedance over frequency, not just the resistance measured with a meter. A resistor added to one input can match a DC value while leaving capacitive behavior very different.

Identify any asymmetry introduced by component tolerance, routing or switched sensor configurations. For a differential source, check both the common-mode and differential paths. Record which parts establish the source impedance during normal operation and which take over when the sensor is disconnected.

Test the actual network

Compare noise spectra and time records with the intended sensor or a representative impedance model attached. Include the signal bandwidth and likely out-of-band interferers. Keep analyzer bandwidth, averaging and amplifier gain consistent when comparing circuit options.

If an artifact changes after adjusting the input network, verify that the improvement remains under tolerance and temperature variation. Check the resulting settling, noise and input loading as well. An added component should solve the identified interaction without quietly violating another channel requirement.

Coordinate filtering with acquisition

Review filtering before the amplifier and before any subsequent sampling event. A filter placed only after a signal has already mixed or aliased may not remove the resulting in-band error. Select filter locations from the observed mechanism and the manufacturer’s application guidance.

Keep the source-network schematic and measured spectral conditions with the approved amplifier choice. State which sensor configurations were tested. When a protection resistor or filter capacitor changes, revisit this record even if the circuit’s nominal DC gain is unchanged. The result should demonstrate that the complete channel preserves the intended precision, rather than relying exclusively on a low offset value in a component table.