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 output.
How well an op amp acheives
this is called common mode rejection.
Post subject: Common-mode voltagePosted: Sat Aug 06, 2005 11:32
Whenever you have two terminals (let's call them
A and B), neither of which is ground, you can talk about the voltages
1. You can measure each to ground, getting VA
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.
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".
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.