Check an Op Amp Buffer with Its Actual Capacitive Load

A buffer that behaves well on an unloaded evaluation board can ring after connection to a cable or an ADC filter. The load changes the feedback system’s behavior. Review the complete output network and test it in the configuration the product will use, including the cable and measurement setup.

Inventory the load

List deliberate filter capacitors, receiving-device inputs, cable capacitance, connectors and expected variation. Mark where the amplifier feedback is taken relative to any output resistor. That connection determines which voltage the amplifier directly regulates and changes how compensation should be evaluated.

Analog Devices’ capacitive loading guidance explains how a capacitive load can reduce phase margin and discusses compensation approaches. Use the selected amplifier’s recommendations as the starting point. There is no single series resistor that guarantees stability for every buffer and load.

Separate small and large signal behavior

Use a suitable small step to examine ringing without immediately driving the amplifier into slew-rate or output-current limits. Then test representative larger transitions to evaluate the application’s actual settling behavior. Record amplitude, offset, load, supply and probe setup for both measurements.

Check the whole operating range rather than only a centered waveform. Output headroom and available drive can vary with voltage and load. If the product drives an ADC, evaluate the conversion timing as well as the visible waveform. A response that looks smooth at one time scale may not have settled to the required accuracy when sampled.

Evaluate the cost of isolation

A series resistor can isolate the amplifier from capacitance, but it can also create a load-dependent voltage drop and an RC response. Determine whether those effects fit the error and timing budgets. Do not assume the amplifier corrects a drop that occurs outside its feedback sensing point.

For an illustrative DC case, a 100 microamp load through 100 ohms produces a 10 millivolt drop. Whether that matters depends on the application. This arithmetic does not recommend either value as a compensation network; stability requires an appropriate model and verification.

Avoid testing a different circuit accidentally

A probe adds capacitance and its ground connection can introduce artifacts. Confirm that the instrument connection is appropriate for the node and bandwidth. Compare measurements with the expected physical load, not an arbitrary bench cable that is absent from the product.

Retain the simulation assumptions and measured response for the final amplifier, feedback network and load range. Recheck substitutions and cable options that change those conditions. The release criterion should include acceptable settling and error under the intended load, rather than simply the absence of visible oscillation on an unloaded board.