ST  (Schmitt Trigger)

ST Schmitt trigger calculator helps you calculate the resistor values to turn an analog comparator into a Schmitt trigger. The resistor values calculated can be any of the standard resistor values; you select which standard you want in the popup menu on the right. The comparator should be a push-pull output type; there are many good choices from several manufacturers. Examples include:

Analog Devices: ADCMP371.

National Semiconductor (now part of Texas Instruments) LMV7271.

ON Semiconductor NCS2200 (0.85 volt operation)

Texas Instruments: TLV3701, TLV3501, TLV3502, TLV3702, TLV3492, or TLC3702.

If you must use an open collector/open drain output device, then your pull up resistor on the output must be much smaller than R1 so you do not introduce errors to your threshold when your output is high.

Be careful with your resistor selection; if your resistor values are too high, your comparators input bias current can cause threshold errors, especially at higher temperatures.

You can select either inverting, non-inverting, or non-inverting with resistor divider Schmitt triggers from the popup at the top.

Each version has its advantages and disadvantages.

The non-inverting version’s input current changes when the output state changes, but if you are flexible with your Vref value, you may be able to use a part like the Analog Devices ADCMP361, or Maxim MAX9040 with a build-in reference.

The inverting versions offer high input impedance.

ST allows equation entry for all four input and output threshold voltages and the R2 edit fields. So if you know you need 0.65 volts of noise immunity, and your rising threshold has to be 2.461 volts, you simply enter  2.461-0.65  for your falling input. You don’t have to use a calculator to use this calculator! Your output voltage levels will typically be the voltage supply rails of the comparator chip used.

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