EDN Access -- 8.4.94 RC network eliminates precision referenc

-August 04, 1994

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Design Ideas:August 4, 1994

RC network eliminates precision reference

Christophe Basso,
European Synchrotron Radiation Facility, Grenoble, France

The circuit in Fig 1 uses a magneto-resistive sensor to detect small magnet displacements without resorting to a precision voltage reference. A simple RC divider takes the place of the reference, eliminating an expensive component and obviating trimming each circuit.

In the quiescent state, the resistive divider R1 / R2 forces comparator IC1 to be in a high state. Now, if the output of op-amp IC2 starts to diminish, the voltage on the inverting input of IC1 stays constant because of C1. If IC2's voltage variation is large enough to reverse the voltage across R1 and reach the offset voltage of IC1, IC1's output switches to ground.

You can easily adjust IC1's switching threshold by changing the value of R1. The voltage difference across R1 sets the circuit's sensitivity. The bigger R1 is, the smaller the sensitivity.

Of course, the R1 - C1 time constant sets the time the comparator takes to toggle back to its initial state. But, because of the high value of R1 and R2, you can achieve long time constants. The system simply needs more time to reach its final state after a power-on cycle.

If you add another comparator wired as an OR to the first one, plus an RC circuit connected to the 5V supply, the circuit can also detect positive variations. (DI #1564)

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