# Common Mode Voltage - RF Cafe Forums

The original RF Cafe Forums were shut down in late 2012 due to maintenance issues. Original posts:

Blix
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.

Thanks

Top

Guest
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.

Top

Guest
Post subject: Common-mode voltagePosted: Sat Aug 06, 2005 11:32 am
Hi!

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.

Good Luck!

Posted  11/12/2012

RF Cafe Software

Wireless System Designer

RF & EE Symbols Word
RF Stencils for Visio
Calculator Workbook
RF Workbench
Smith Chartâ„¢ for Visio
Smith Chartâ„¢ for Excel

About RF Cafe

Copyright
1996 - 2022
Webmaster:
Kirt Blattenberger,
BSEE - KB3UON

RF Cafe began life in 1996 as "RF Tools" in an AOL screen name web space totaling 2 MB. Its primary purpose was to provide me with ready access to commonly needed formulas and reference material while performing my work as an RF system and circuit design engineer. The Internet was still largely an unknown entity at the time and not much was available in the form of WYSIWYG ...

All trademarks, copyrights, patents, and other rights of ownership to images and text used on the RF Cafe website are hereby acknowledged.

My Hobby Website:
AirplanesAndRockets.com

Try Using SEARCH
to Find What You Need.
There are 1,000s of Pages Indexed on RF Cafe !