Re: Common Mode Rejection Question
- From: "BobW" <nimby_GIMME_SOME_SPAM@xxxxxxxxxxxxxx>
- Date: Sat, 13 Sep 2008 15:12:57 -0700
"RogerN" <regor@xxxxxxxxxxx> wrote in message
news:IuGdne2cltJBrFHVnZ2dnUVZ_hGdnZ2d@xxxxxxxxxxxxxxxx
I'm wanting to get an analog signal output equal to the differential
signal IN X gain. In other words, if I have input A = 11V and input B=12V
and Gain = 1, then I want an output signal of 1V. Is this what common
mode rejection is? ie. it takes the difference in the signals and not the
reference voltage of the signals. I want the same output if IN A=1V and
IN B=2V or IN A=5V and IN B = 6V. The application involves current sense
resistors and mosfets to switch in/out resistances, or perhaps high side
resistors. I want to isolate the voltage drop across the resistors and
ignore the actual voltage level the resistors are at.
Thanks!
Roger N
Roger,
An amplifier that outputs only the difference between two input signals is
called a differential amplifier.
Differential amplifiers, while by design only output the difference between
the inputs, always output a certain amount of the common-mode voltage of the
two inputs. Common-mode rejection refers to the ability of a given
differential amplifier to reject the common-mode component of the inputs.
The common-mode rejection of a given differential amplifier depends on the
magnitude of the common-mode signal and also the frequency of the
common-mode signal. In your first example (A=11V and B=12V) the common-mode
component of the inputs is equal to 11.5V.
If you want some help in designing and/or selecting a differential amplifier
then you'll have to provide some more details. For example:
- If your input A=11V and input B=12V, but then A changes to 15V and B
changes to 16V, how much of a change in the output signal can you tolerate?
Bob
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