Common mode voltage - RF Cafe Forums
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Post subject: Common mode voltage Posted: Sun Jul 31, 2005 4:08 pm
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 output.
How well an op amp acheives this is called common mode
Post subject: Common-mode voltagePosted: Sat Aug 06, 2005 11:32 am
Whenever you 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 output.