Budget Comparator Delay at the Available Input Overdrive

A comparator’s headline propagation delay is measured under specified conditions. If the application crosses its threshold with only a small excess voltage, the delay may differ from a test using a large step. A timing budget should describe the actual input event before selecting a single datasheet number.

Define the threshold crossing

TI’s comparator overview distinguishes propagation delay, output rise and fall time, and input overdrive. It explains that overdrive is the input excursion beyond the comparison level and influences propagation delay. These terms should remain separate in the design record.

Sketch the input waveform and reference at the comparator pins. Mark the threshold crossing, final overdrive, input slope and the output level that the receiving circuit recognizes. Do not use the source generator’s programmed waveform as a substitute for the waveform after the board’s filtering and source impedance.

Build the delay path

Include any input-filter delay, comparator response, output transition and receiver timing requirement. Identify which terms are specified limits and which are typical characterization. If only typical curves cover the actual operating condition, preserve that limitation and obtain additional evidence appropriate to the product requirement.

Check rising and falling events independently. The output stage and load can produce different transition times in the two directions. For an open-drain output, the pull-up and capacitive load belong in the path to the receiver’s switching level.

Use representative stimuli

Test both the smallest relevant overdrive and larger signals across the intended common-mode and supply conditions. Keep input excursions inside the component’s permitted range. A fast, generous laboratory pulse can demonstrate that the circuit works while missing the slow event that determines the application’s timing margin.

Capture the input difference and the receiver-side output with a consistent timing definition. Document bandwidth, probe loading and the threshold used by the oscilloscope measurement. An automatic measurement based on the middle of a waveform may not correspond to the digital receiver’s guaranteed input level.

Separate noise behavior from speed

If the input repeatedly crosses the threshold because of noise, first describe that behavior. A propagation-delay measurement from an arbitrary crossing can become misleading. Review filtering or hysteresis with the resulting latency included, rather than treating those changes as free improvements.

The final timing budget should identify the event, operating conditions, accepted limits and measured margin. Keep the relevant datasheet condition beside any delay value. When a sensor gain, filter, pull-up or comparator changes, revisit the event definition as well as the component number. This makes the budget reflect the signal that the product actually has available.