I am using an LM2903 Comparator on my design to measure sensor signal. Constant current source is provided to the sensor by means of resistors (connected to 2.5V). I have an RC input filter (R=1K,C=0.47u) at the non inverting input of the comparator followed by R1=1k (input resistor) and RF=475k (feedback resistor) for external hysteresis. Reference voltage is 1.25V.
When I tested the circuit, I found my comparator output oscillates instead of saturating. So I put small cap (10nF) across the feedback resistor. Now the output bounces from high to low and vice versa even though my sensor input is constant.

  • 2
    \$\begingroup\$ I think you've got a circuit error, but maybe post a schematic. Is there a resistor from inverting input to ground.. or your reference voltage? \$\endgroup\$ – George Herold Oct 2 '14 at 20:15
  • \$\begingroup\$ Yes.1.25V is generated from 2.5VRef by means of voltage divider and given to inverting input.I don't know how to include my image file. \$\endgroup\$ – sanju Oct 2 '14 at 21:31
  • \$\begingroup\$ OK, What is the R value? (10k ohm? my fav. R) to slow down a comparator I think you want a C to ground on the output, not across the "hysterisis" resistor. (my spelling stinks.) I don't know the details of the LM2903. \$\endgroup\$ – George Herold Oct 2 '14 at 23:30

An LM2903 is an open collector device, you need a pull-up on the output. Also, the LM2903 Vcc must account for headroom, so it probably needs at least 3.3V if not 5V power.


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