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Loren Siebert 1

Aug 13, 2014

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LMH6881

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Part 1 - Avoid common missteps with the common mode

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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.

When the
common mode is unterminated the signal path is more likely to receive unwanted
common mode noise from external sources.

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.

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