CMRR - What is it and How Do You Get a Good Number

So what is CMRR? It stands for "Common Mode Rejection Ratio." It's a number that describes how well an input or output will reject noise or how well "balanced" a balanced line is. So to understand CMRR, you have to understand what a balanced line is and how it works. It is relatively easy to calculate CMRR, it is a logarithmic scale and is expressed as so many dBs of level. It calculation comes out as a negative number and describes how "deep" the noise is compared to the actual signal. (If you read my blog on balanced lines, you could also say that CMRR describes the level of the common mode noise compared to the level of the differential signal on a pair of wires.)

If you can get a manufacturer to tell you the CMRR of a specific device, you can interpret the number like this:

-40 dB CMRR very poor noise rejection
-50 dB CMRR poor noise rejection
-60 dB CMRR average noise rejection
-70 dB CMRR good noise rejection
-80 dB CMRR very good noise rejection
-90 dB CMRR excellent noise rejection
-100 dB CMRR world class noise rejection

The best I have ever seen was on a massive Hollywood mixing console, well over $1 million, with exotic hand-wound European transformers on each microphone input. The transformers alone were $1,000 each and the CMRR was an amazing -115 dB at 50 Hz. The cheap mixers, with active balanced inputs and no transformers, could do around -70 dB CMRR at low frequencies. Minus 70 CMRR might be fine for a rock band on the road or small night club mixer, but you probably would want better performance if you're going to record or broadcast something.

The interesting thing is that there are now ICs (integrated circuits) that "mimic" a transformer and are getting pretty good with CMRR. One of those is the InGenius 1200 made by T.H.A.T Corp. This chip boasts CMRR of -90 dB at 60 Hz. Not shabby! The LMV831 chip from National Semiconductor has a CMRR of -93 dB although I don't know what frequency that is measured at. So are chips as good as wire-wound transformers? Let's just say we're sure heading that way.

So asking about CMRR can be a very clever thing. First, it will show you how much the salesperson knows and that's an indication of the quality of the equipment they manufacture. Second, it's a very good indication of the quality inside the box. CMRR is a number that is hard to manipulate, hard to "fake". And, finally, it makes you a dangerous customer; here's someone who knows how things work and what to ask for. At the very least, it makes you look good.

If you have questions or comments, I'm easy to reach.