Changing a voltage-feedback amplifier from a low-resistance feedback network to a proportionally larger one preserves ideal gain. It does not preserve the frequency response of the real loop. Input and PCB capacitance interact with the feedback impedance, changing the feedback factor and potentially reducing phase margin.
Preserve more than the resistor ratio
Analog Devices explains this interaction in The Truth About Voltage Feedback Resistors. Include the amplifier’s common-mode and differential capacitances together with pads, traces, protection devices, and switch capacitance. The exact equivalent capacitance depends on circuit configuration; simply adding all data-sheet capacitances can produce the wrong loop model.
For a useful sensitivity estimate, 10 kilohms interacting with 5 pF gives an RC frequency near 3.2 MHz. Increasing the resistance tenfold moves that scale to about 320 kHz. These are illustrative corner frequencies, not predicted oscillation frequencies. Whether the change matters depends on the amplifier’s open-loop response and the full feedback network.
Test the noise-gain shape
Analyze noise gain as a function of frequency and compare it with the amplifier’s available loop gain. A small capacitor across the feedback resistor may restore a more favorable response, but it also changes signal bandwidth and settling. Copying a compensation capacitor from another design without its resistor values and layout is unreliable.
| Change | Consequence to review |
|---|---|
| Larger feedback resistance | Lower parasitic RC corner and increased resistor noise |
| Longer summing-node trace | More capacitance at a sensitive node |
| Feedback capacitor | Altered noise gain and signal response |
| Probe on summing node | A measurement that changes the loop |
Validate the implemented geometry
Use a small-signal step to inspect ringing without entering slew-rate or output-current limits. Compare gain settings, component tolerances, and candidate compensation values. Then measure settling to the required accuracy, since an apparently quiet waveform on a coarse vertical scale can still contain an unacceptable tail. Test the actual ADC or cable load after stabilizing the core amplifier.
Keep the summing node compact and place feedback components where the intended loop is physically short. Use minimum noise-gain requirements when the amplifier is not unity-gain stable, and capacitive output-load testing for the separate output-pole mechanism. Record both resistor values and compensation details; gain alone is not an adequate circuit specification.
Move from reading to a build-ready project
Use the fabrication and assembly checklists to prepare drawings, production files and test requirements.
Open the DFM checklist