# opamp offset

Discussion in 'General Electronics Chat' started by MikeA, Jan 11, 2014.

1. ### MikeA Thread Starter Member

Jan 20, 2013
125
17
I'm trying to amplify a signal in the mV range. Say I have a base signal of 1.000v. And I want to amplify that 3000 times, so 1.001v signal will output as 3v from opamp output.

So I'll have a reference voltage of 1.000v fed into the opamp, and have it listen to the signal. Am I correct in that if I want to measure 1mv accurately, I need an opamp with an input offset voltage spec much lower than 1mv for it to work correctly?

2. ### MikeA Thread Starter Member

Jan 20, 2013
125
17
p.s. why can't a post subject line be longer than like 12 characters? descriptive longer subject doesn't post

3. ### ScottWang Moderator

Aug 23, 2012
4,924
776
Your descriptions has something wrong.
The base voltage is 1.000V.
then 1.000v * 3000 = 3KV, the output is 3KV?

Or your base voltage is 1mV, so the output equal to 1mV * 3000 = 3V.

It depends on how low the input offset voltage you want, you can see the OP07,OP27,OP37 or LF356,LF357.

4. ### MikeML AAC Fanatic!

Oct 2, 2009
5,450
1,066
You can use the reference supply to get rid of the opamp's offset (make it adjustable to null out the offset). The problem is the temperature dependence of the offset...

5. ### MikeA Thread Starter Member

Jan 20, 2013
125
17
MikeML, do you mean I'll have trouble with the offset voltage drifting with temperature? Is there a better approach to what I'm trying to do then?

6. ### ramancini8 Member

Jul 18, 2012
447
119
1. Pick a very low offset op amp; they usually have excellent drift parameters. Don't forget the resistors because the cheap 5% type usually have poor tempco.
2. If the signal is ac use a blocking capacitor.

7. ### wayneh Expert

Sep 9, 2010
12,368
3,223
Are you primarily concerned with the ∆V between the two signals, with both voltages possibly drifting around, or do you want a ∆V against an absolute reference that is as solid as you can get it? A differential op-amp with a gain of 3000 will give you the former. Both voltages need to be within the common mode input range and ∆V*gain must remain within the output range of the op-amp.