TL08x as comparator...

[Ingo Debus]

Duane R Balvage wrote:
> 
> Hello all...
> I have a Question... Someone mentioned that my design using a TL082
> as a comparator was "scary", I think mostly in rererence that it was
> capacitively coupled to the voltage it was comparing. Anyone have a
> better idea of how to do it?:

Ooops... sorry! I forgot to answer to your first mail.
I thought this was scary, because there could be some DC creeping to the
opamp's input, maybe through some dirt on the PCB or whatever. Since
both the capacitor and the TL's input have really high dc impedance a
parasitic impedance of many megohms can cause severe offsets.
Why not connect the opamp directly to the point you get your signal
from? If the signal is inside the TL's common mode range (as JH
mentioned) you can compensate the DC offset with the voltage applied to
the - input.

> The reason for the TL part was for HIGH input impedance - I didn't
> want to load down the integrating cap.

If it works this way, I'd stay with the TL. Never change a running
system :-)
But do you really need *that* high impedance? The input current of a
LM393 comparator is also pretty low (25nA typ, 250nA max).

Ingo