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Open collector comparator pulls up to higher voltage than the pullup voltage!

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The rounding on *both* ends of the 3.9 V hump is a clue to something, but don't know what yet.

I agree that 0.6 V is suspicious, but IMO there's no way there is an ESD diode on and open collector output; having the output drive something at a voltage greater than Vcc is just too common an application.

ak

Hi,

As Flyback says, it's the next stage ESD diode that would most likely kick into effect if there was an over voltage from some other source. The next stage is a uC and they usually have these diodes from pin to ground and from pin to +Vcc. The one from pin to +Vcc would be the one here.

One experiment i quoted would be to disconnect the uC pin and see how high the output pin of the comparator goes.
Another idea would be to look on the manufacturers site to try to find some applications where they have a much higher Vcc and much lower pullup voltage (like 3.3v). The equivalent shunt resistance would have to be 187.2k which is quite low for a parasitic element like this. ESD diodes are typically maybe like 1 Megohm to 10 Megohms, in case they did have one on the comparator chip output too.

The data sheet itself doesnt seem to help here because it shows an open collector NPN being driven by another transistor such that when the other transistor pulls the base down to a low voltage near 0v, the output transistor turns off, and that's the usual setup, and that means any leakage from a 3.3 to 3.9v source on the collector would be INTO the collector not OUT of the collector. That is of course assuming there is no built in component between the collector and the comparator Vcc+.

If it is hard to experiment with the actual circuit you could also create a second smaller circuit just to run some tests.
 
Thanks...weve just realised that the anomaly spoken of here is due to the TA041 diff probe's 4 megohm resistance to ground from its output.
This type of anomlay must be common with diff probe measurements where the impedance of the measured node is high?
 
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