Comparator doesn't work as expected?


I'm having a strange problem with the LT1016

When the -input is at 0.630V, and the +input is at 0.555V, and the comparator is fed with 5V,
the outputs are because of some reason Q(inverted) = 2.10V and Q = 2.67V
2.67V for Q is just out of specifications (minimum High voltage of the pin should be 2.7V).
2.10V for Q(inverted) is not good as it should be less than 0.3V in it's low state.

The "latch enable" pin is grounded.
Both output pins are not connected.

Why doesn't Q(inverted) go to 0.3V and Q to 3V, as the datasheet suggests?
I tested this with another (also new) comparator; also an LT1016, and it behaves exactly the same.

1 V+ 5V
2 +IN 0.555V
3 -IN 0.630V
4 V- 0V
5 Latch Enable 0V
6 GND 0V
7 Qout 2.67V
8 Qout(inverted) 2.10V

Any ideas?

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In Appendix A of the data sheet you linked to ,there is a sort-of "artist's interpretation" (figure A-1) of what the outputs of this comparator look like inside. It looks like two NPN transistors stacked on top of each other, with the emitter of the top one connected to the collector of the bottom one.

I don't know if that makes everything clear or not. I am guessing your IC is in fact working as designed. That is to say, those little transistors in its output stage really are turning on and off, but with no load attached, the voltage there is kinda floaty.

I like IrishJim's suggestion of connecting a 1K resistor from the output to ground, and then observing what this does to the output voltage.

I also suggest trying a 1K resistor connected from output to V+ = 5V, and see what that does.

Also try it with both the "pull-up" and "pull-down" resistors together: i.,e with a 1K resistor connected from +V to output and another 1K resistor connected from output to ground.

I am hopeful some arrangement of these pull-up, pull-down, resistors will make your comparator's outputs behave better.

Its a nice comparator for dual supplies, but as the late, great Jim Williams from Linear points out in another app note for another chip, it wasn't great at single ended uses.

Show your exact circuit.

irishjim682 years ago

Try putting a 1Kohm resistor on the outputs to ground, and then measure at the output pin. steveastrouk is correct that open collector outputs must have a load.

The outputs are open-collector, they NEED a load.