At 03:20 PM 8/1/01 +0100, you wrote:
>first post.

Hello!

>I am trying to use a CA3140E op-amp as a comparator.

Meaning you don't want to measure the difference, only detect go-no-go.
If you want to measure it, I'd suggest an instrumentation amplifier, which
has a controlled gain and two high-Z inputs. You can follow it up with
a comparator. But the below sounds like you need to compare the difference
to a fixed setpoint, so we are looking InAmp, I'd think.

>The two inputs are chemical sensors. The difference voltage between them
>can range between 400mV, to 50mV (100% concentration to 0% concentration).
>
>I remember reading somewhere in a text book, that if V1-V2 is greater than
>(I think) 100uV the op-amp will go into saturation.

Welll.. theoretically, if Vos is zero. You can null the offset on that
amplifier, and then you just have to think about the gain, which is typically
100kV/V (as the data sheet says), so to get a 5V swing you'll need 50uV on
the input. But see below...

>Can anyone a) clarify this for me, or b)suggest a better way to measure
>the difference voltage.

The CA3140 is a terrible amplifier to use as a low-level comparator.
Don't use it!!!!

The Vos drifts over a period of hours and months depending on the
differential input voltage. I found this out before RCA put it into their
datasheets, at the cost of much hair torn out and $$$.

Suggest you consider the LMC6464 quad and make an in-amp with 3 amplifiers
and use the 4th one as a comparator, with a trimpot to give you the 50-400mV
setpoint (or maybe you'll have multiplied it by 10 to 0.5 ~4V)
See: http://www.national.com/ds/LM/LMC6462.pdf  (figure 15)
This amplifier is low drift and has low input bias current. You'll need
+/- supplies with the circuits shown.

Best regards,
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