Part 1 - Avoid common missteps with the common mode


 

Have you noticed how differential signaling is becoming more and more dominant in high performance signal paths?  Differential signaling offers several advantages! I’ve been thinking about the fact that every differential signal path has a parasitic common mode signal path associated with it. 

In a differential signal path most ambient noise is coupled as common mode noise.  Many differential devices are very good at rejecting this noise.  The following common mode rejection ratio (CMRR) plot is from the LMH6881 programmable differential amplifier (PDA).

The CMRR determines the “contamination” of the differential signal due to interference from common mode noise.  This number is very important, however, it is not the whole story. 

While it is important that the differential signal be preserved, it is also worth your consideration what happens to the common mode noise. If the noise is passed along another device that device is now required to reject it.  The plot below is the common mode gain for the same LMH6881 amplifier. 

From both of these plots we can determine a few key points. 

The first is that at low frequencies, both the common mode rejection and the common mode gain (attenuation) are very beneficial to you. 

What happens at higher frequencies, though?  Both the CMRR and common mode attenuation begin to degrade.  If the system in question has significant noise at very high frequencies (such as local oscillators or mixer spurs) what happens to this noise?   

As we can see from the CMRR and common mode gain plots, both the ability of the amplifier to keep common mode noise out of the differential signal as well as the ability of the amplifier to attenuate common mode noise decrease markedly at higher frequencies.  This means that system design to isolate and contain noise is important even with the use of differential signaling. 

Differential signaling transmission lines require termination for best performance.  This is no different than single ended transmission lines.  In order for you to reduce reflections and undesirable signal radiation, broadband termination of the transmission line is very important.    Most system designers are careful to terminate differential transmission lines, at least from a differential perspective.  However, many times the common mode termination is neglected. 

What are the implications of an unterminated common mode on a differential signal path? 

There are two main issues. 

  1. When the common mode is unterminated the signal path is more likely to receive unwanted common mode noise from external sources. 
  2. With a floating or unterminated common mode, the active devices (amplifier, mixer, ADC) may experience load conditions that result in reduced performance.  For example, many differential amplifiers have active circuits that control the common mode. Some common mode load conditions can decrease the phase margin of these circuits. 

Filters are often used to attenuate noise and spurs beyond the capabilities of the active system devices.  Differential filters, however, may be completely incapable of attenuating common mode noise unless they are specifically designed to attenuate common mode noise. 

Now that we are aware of some potential issues with respect to the parasitic common mode present in differential signal paths, a good question becomes how do we mitigate these issues?  I’m out of space for this blog post, so keep an eye out for my next blog post in September where I will outline some methods to help mitigate common mode noise.