Post subject: Common mode voltage Posted: Sun Jul 31, 2005
I am working on an op amp project and was wondering if someone
could explain common mode voltage, in the context of op amps and in
general. From what I have read it is voltage that is common to both
inputs of the op amp and is not desirable.
Post subject: Posted: Sun Jul 31, 2005 9:27 pm
Equal voltage at both inputs should result in no voltage at the
How well an op amp acheives this is called common mode
Post subject: Common-mode
voltagePosted: Sat Aug 06, 2005 11:32 am
have two terminals (let's call them A and B), neither of which is ground,
you can talk about the voltages two ways:
1. You can measure
each to ground, getting VA and VB
2. You can measure the voltage
between the terminals, getting the differential voltage Vd = VA - VB,
and then measure the average voltage of the two terminals to ground:
VC = (VA + VB)/2. This last voltage is the common-mode voltage, and
whether it's good or bad depends on what you're trying to do.
For example, if you're using a 5-volt-only opamp, then the inputs
might both want to be at about 2.5 volts, so that they can both go up
and down equal amounts. That 2.5 volts would be common-mode voltage.
In some opamp circuits, you want to look ONLY at the voltage difference
between the two terminals. In that case, you don't want your opamp to
respond to ANY change in the common-mode voltage AT ALL. Unfortunately,
you can't build an opamp perfect in that way, so all opamps have a "common
mode rejection ratio".
An example of wanting only the difference
voltage would be a professional microphone with balanced output. Induced
hum due to nearby AC power will generally be equal on each of the two
signal wires, so it won't be in the difference at all. Any common mode
rejection less than perfect would mean that you'd hear some hum in the