# common mode

#### lost in analog

Joined Sep 23, 2005
1
If we have a perfect opamp and we input the same signal on both the positive and the negative terminal, the output should be zero. But in reality this never happens. What would be the best way to get as close as posible to a common voltage gain of 0?.

#### hgmjr

Joined Jan 28, 2005
9,027
Post a copy of your schematic and I will be more that happy to assist you with your common mode problem.

hgmjr

#### Brandon

Joined Dec 14, 2004
306
Originally posted by lost in analog@Sep 23 2005, 10:39 PM
If we have a perfect opamp and we input the same signal on both the positive and the negative terminal, the output should be zero. But in reality this never happens. What would be the best way to get as close as posible to a common voltage gain of 0?.
[post=10550]Quoted post[/post]​
What are you trying to do?

The fact you don't have exactly zero doesn't mean a thing. If your under 0.7-0.5 volts most micros and logic gates see that as off.

#### Gorgon

Joined Aug 14, 2005
113
Originally posted by lost in analog@Sep 24 2005, 06:39 AM
If we have a perfect opamp and we input the same signal on both the positive and the negative terminal, the output should be zero. But in reality this never happens. What would be the best way to get as close as posible to a common voltage gain of 0?.
[post=10550]Quoted post[/post]​
Hi,
Common mode error adjustment is normally done with a small voltage adjustment of the common ( '0v' reference voltage) input.

To do this you just connect a potmeter between + and - supply(or better, proper reference voltages), and connect the viper with a relative high value resistor to a low value resistor(Rx). This point is your new reference. The other end of the low value resistor is connected to the(old) common ref. point. You can now adjust the voltage over Rx, 'off' to both sides of the old common point. The ratio between the two resistors and the voltage swing will give the amount of adjustment. A general rule is to keep it as low as possible.

Short the two inputs on the opamp and adjust for optimal 'zero' output.

I suppose there are several ways to do this, and I don't know if its optimal or the best.

TOK