LM2903B-Q1: coupled noise on the input signal

Hello Sherry,

Yes. It is possible for the inputs to couple. The differential capacitance between the inputs can couple high frequency and fast edges between the inputs, particularly if the source impedance are high (>10Kohm) and the AC input amplitude is large.

The capacitance between the inputs is about 1pF, and any parallel traces can add a few pF more. So just imagine a capacitor of 1-2pF between the input pins. This is easily simulated.

This is why some bypassing of the reference voltage is recommend, to swamp out these "blow through" transients.

Also, some of the reference voltages are derived from voltage dividers off the supply voltage. If there is noise on the power supply, it will be injected into the input (attenuated by the division ratio) if there is no bypass capacitor on the divider output.

So the "fix" is to place a bypass capacitor (1nF to 1uF), depending on the source resistance) directly on the DC reference input. The one caveat is that if the reference input is also the hysteresis point (positive input), a capacitor is not recommended directly on the input as it causes issues with the hysteresis.

Do we have a schematic?