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PICList Thread
'[PICLIST] [EE] operational amplifier or comparator'
2001\08\02@025644 by Vasile Surducan

On Wed, 1 Aug 2001, Bernard Tyers wrote:

> I am trying to use a CA3140E op-amp as a comparator.
> 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.
> Can anyone a) clarify this for me, or b)suggest a better way to measure
> the difference voltage.
 You seem to have problems with the question: What I need to measure a
diference between 2 voltages. In any case not a comparator.
An operational amplifier [OA] will always have negative reaction.
An OA output will be saturated only when the output swing will be greater
or almost the same like power suplies ( when the negative reaction will
disipear, the gain is to large or you have problems with common mode
rejection )
A comparator will have no reaction or better a positive reaction.
You need a diferential amplifier with input impedance at least 10 times
greater than the output impedance of your sensors.
The structure of a diferential amplifier in the folowing story:

R1 from the input1 to -input of OA, R2 as negative reaction from output to
-input of OA
R3 from the input2 to +input of the OA, R4 from +input of OA to AGND

input1, input2 are your signals

I will not tell you the equation, you must compute yourself using
superposition theorem: consider input 2 connected to AGND and write
ecuation 1 for an inverting OA with signal on input1 [thus you'll learn
first the inverting operational amplifier] , then consider input 1
conected to AGND and write ecuation 2 for a non-inverting OA with signal
on input2 [ that's it, you need to know the noninverting operational amplifier
Add ecuation 1  to equation 2 and find how works the diferential
To be much easy consider R1 = R3 and R2 = R4 if you need some gain
( vout = (R2/R1)x(input2 - input1)  or
R1=R2=R3=R4 if you need gain = 1 and the result at your OA output will be


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