Checking Differential Input Clamp Current During Amplifier Transients

An amplifier input can stay within its supply rails while still forward-biasing protection between the two input pins. During normal closed-loop operation the input differential voltage is small. A rapid step, delayed supply, or temporary loss of feedback can separate the input voltages enough to activate a differential clamp.

Distinguish the protection paths

A clamp between inputs is different from a clamp from an input to a supply. Analog Devices describes the consequences in Amplifier Input Protection: Friend or Foe?. Read the selected device’s input schematic, differential-voltage rating, and current limitations. Do not assume every precision amplifier uses identical protection or that a rail-valid signal guarantees a harmless differential transient.

Consider a unity-gain buffer receiving a fast positive step. Its noninverting input moves immediately, while its output and inverting input follow with finite speed. If an internal path conducts, the source must supply additional current. With a high source resistance, the input waveform may be altered before the amplifier output is observed. A scope trace of only the output can make this look like ordinary slew-rate limiting.

Estimate and measure the current

Draw the transient conduction loop through the source, external resistances, input clamp, and feedback connection. Use the documented clamp behavior for a bounding calculation; a universal diode drop is not a reliable specification. Include source current limits and any second amplifier that drives the node. Added series resistance can reduce current but introduces noise, bias error, and a settling time with input capacitance.

• Capture both input pins and output with probes that do not dominate their impedance.
• Test the largest permitted input step in both directions.
• Repeat during each allowed supply startup sequence.
• Check behavior when a feedback switch is disabled or a connector is absent.
• Verify recovery after the transient, including residual offset and settling.

Choose protection with the signal path in mind

A different input architecture may avoid a problematic conduction path, but evaluate its noise, offset, and common-mode performance before substitution. External clamps also have leakage and capacitance. If firmware suppresses samples during a transition, the circuit must still satisfy electrical limits while those samples are ignored.

Connect this review to large-signal slew behavior and protection leakage and settling budgets. The design record should distinguish the allowed signal excursion, the possible differential excursion, and the maximum current in the resulting loop.