EDN Access


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. VLO and VHI are the low and high setpoints, respectively, which you can change dynamically. VIN 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 VOUT, just as in a regular Schmitt trigger. VCC and VEE can be 5V and 5V, respectively. (DI #2030)

Figure 1

This Schmitt trigger uses a comparator in a standard hysteresis configuration and has static setpoints.
Figure 2

You can dynamically set the high and low thresholds of this Schmitt trigger.

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