June 5, 1997

Schmitt trigger has dynamic threshold

Isaac Eng, GSI Inc, Ottawa, ON, Canada

Schmitt triggers usually switch only at predefined static setpoints. For example, to allow user-defined setpoints, you can use a comparator in a standard hysteresis configuration (Figure 1). However, the design in Figure 2 allows dynamic setpoints, which are beneficial for variable-noise applications. This circuit is useful for rejecting noise that fluctuates in amplitude.

The circuit in Figure 2 has a low component count and uses common parts. V_LO and V_HI are the low and high setpoints, respectively, which you can change dynamically. V_IN drives the negative input of comparator LM311, which compares this signal to the noninverting input voltage that comes from the output of the CD4053 analog multiplexer. The multiplexer's output voltage (active threshold) switches depending on the state of V_OUT, just as in a regular Schmitt trigger. V_CC and V_EE can be 5V and ­5V, respectively. (DI #2030)

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