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Loren Siebert 1

Nov 3, 2014

hsamps_fda

fully
differential amplifier

hsamps

LMH6517

PGA/VGA

LMH6521

loren
siebert

Operational
Amplifier (Op Amp)

variable
gain amplifier

LMH3401

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Related

Part 2 - Mitigating common mode noise

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In my previous blog post, I outlined the fact that differential
signaling can still be negatively impacted by common mode noise. In this post, I
will outline some methods that you can use to reduce the impact of common mode
noise on differential signal paths. I’ll cover three main categories of devices:
passive circuits, high-impedance active circuits and low-impedance active
circuits.

Let’s start with the simplest example,
passive circuits. System designers often use an LC filter to eliminate noise. As
commonly implemented, a differential filter will be floating with respect to
ground. This is fine for the differential response, as shown in Figure 1.
However, as shown in Figure 2, there is no attenuation of common mode signals.
By implementing a simple change in the filter topology, as shown in Figure 3, we
get the response shown in Figure 4.

Figure 1: Filter
differential response

Figure 2: Filter common
mode response with 2.5k Ohm of common mode “termination”

Figure 3: Simple filter
change to shunt common mode energy with one extra capacitor

Figure 4: Filter common
mode response with change of Figure 3

Like filters, some active differential
devices do not have any ground reference (or termination) for common mode
signals. One example is the LMH6521 and other amplifiers with inductors connected to the
output pins. The LMH6521 amplifier is a very high-speed, digitally controlled
variable gain amplifier (DVGA). It has a fully differential signal path, with a
2.5k-Ohm common mode resistance, which is not sufficient for it to be a major
attenuator of common mode energy, as shown in Figure 2.

You can add more common mode energy
dissipation to the LMH6521 by adding some resistors to the circuit, as shown in
Figure 5. Through the beauty of differential signaling, the two 500-Ohm
resistors add only 1000 Ohms of load to the differential circuit, but they
provide 250 Ohms of common mode termination. The amplifier output load changes
from 180 Ohms before the resistors are added to 152 Ohms after. This change in
load condition will have very little impact on the amplifier’s performance, yet
the common mode termination is much improved. The same technique could also be
used at the amplifier input.

Figure 5: Common mode
termination of a differential amplifier (additional resistors shown in
green)

Not all differential devices are high
impedance with respect to the common mode. Some amplifiers, such as the LMH3401, have common mode control circuits that set the
amplifier output common mode with a low-noise, low-impedance circuit. The LMH3401 has a common mode circuit that is basically a unity
gain amplifier. It takes the voltage at CM and buffers it at the amplifier
outputs, such that the common mode of the amplifier output pins is
fixed.

In Figure 6 below, I show the impact of this
circuit as dotted capacitors. With respect to the common mode, the amplifier
outputs are a virtual short to ground for any AC noise in the
system.

Figure 7 shows the common mode output
impedance of the LMH3401. By selecting an amplifier with these features, you
achieve built-in common mode rejection as an extra benefit, and you don’t have
to do anything extra with your design.

Figure 6: Differential
amplifier with low impedance common mode control

Figure 7: LMH3401 common mode output impedance (including on
chip resistors)

As you can see, there are several ways you
can help mitigate common mode noise pickup on differential signal paths. Some of
them require a bit of planning and some extra components. An easier approach is
to select an amplifier that’ll do the work for you.

Bonus:

Do you see the error in Figure 3? Leave a
comment if you find it…

Related resources:

View the LMH3401 and LMH6521 datasheets

Between the amplifier and the ADC: Managing filter loss in
communications systems (application report)

Circuit board design for LMH6517/21/22 and other high-speed
IF/RF feedback amplifiers (application report)

Read more of my blog posts on high-speed amplifier design

Eric Edin89219

The error in figure 3 is C1 and C3 should be 17pF in the changed circuit to
be equivalent to the original circuit.

Nov 3, 2014

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The Watcher

The error is that the source is single-ended when it should be
differential. R2 should connect to the (-) terminal of VG1 instead of R1 and
the ground connection should be removed.

Nov 17, 2014

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Richard Williams

I agree with Eric but not the Watcher as we are testing common mode
response...

The newly added 10M resistors though are not symmetrical!  Surely one
should be added to each arm of the differential signal.

Nov 17, 2014

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Loren Siebert 1

Thanks for the comments!

Eric has the correct response.  Two capacitors in series have 1/2 the
capacitance of a single capacitor.  In order to be equivalent they each
must be double the original value.

The single ended drive is correct, this is how you must drive the circuit
to model the common mode response.

Nov 17, 2014

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Loren Siebert 1

Richard;  The 10M resistors are also an error of sorts.  With
"perfect" inductors and capacitors TINA-TI cannot find a DC operating point
for differential circuits.  The solution is to use imaginary ground ties
at different points of the circuit.  Another solution is to edit the
properties of the capacitors and use 1G Ohm instead of infinite for the
parallel resistance.  Likewise the inductors should have an equivalent
series resistance entered into their properties.  Most RF inductors are
around 50 to 400 milli Ohms of resistance.  Once these properties have
been changed the resistors to ground can be removed.

Nov 17, 2014

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