29
Analog Applications Journal
Texas Instruments Incorporated
1Q 2009 www.ti.com/aaj High-Performance Analog Products
Output impedance matching with fully
differential operational amplifiers
Introduction
Impedance matching is widely used in the transmission of
signals in many end applications across the industrial,
communications, video, medical, test, measurement, and
military markets. Impedance matching is important to
reduce reflections and preserve signal integrity. Proper
termination results in greater signal integrity with higher
throughput of data and fewer errors. Different methods
have been employed; the most commonly used are source
termination, load termination, and double termination.
Double termination is generally recognized as the best
method to reduce reflections, while source and load termi-
nation have advantages in increased signal swing. With
source and load termination, either the source or the load
(not both) is terminated with the characteristic imped-
ance of the transmission line. With double termination,
both are terminated with this characteristic impedance.
No matter what impedance-matching method the designer
chooses, the termination impedance to implement must be
accurately calculated.
Fully differential operational amplifiers (FDAs) can pro-
vide a broadband, DC-coupled amplifier for balanced differ-
ential signals. They also have a unique ability to convert
broadband, DC-coupled single-ended signals into balanced
differential signals.
A common method to provide output impedance match-
ing is to place resistors equal to the desired impedance in
series with the amplifier’s output. With double termination,
this has the drawback that the signal level delivered to the
line is reduced by –6 dB (or half) from the signal at the
amplifier’s output.
Synthetic impedance
matching allows lower-
value resistors to be used
in conjunction with posi-
tive feedback around the
amplifier. The benefit of
doing this is that the out-
put attenuation is reduced.
This increases efficiency
by lowering the loss and
allows support of higher-
amplitude signals on the
line than can be achieved
with standard termination.
Usingstandardseries
matching resistors to ana-
lyze the output impedance
of FDAs is very easy, but
synthetic imped ance matching is more complex. So we
will first look at the output impedance using only series
matching resistors, and then use that as a starting point to
consider the more complex synthetic impedance matching.
The fundamentals of FDA operation are presented in
Reference 1. Since the principles and terminology present-
ed there will be used throughout this article, please see
Reference 1 for definitions and derivations.
Standard output impedance
An FDA works using negative feedback around the main
loop of the amplifier, which tends to drive the impedance
at the output terminals, V
O–
and V
O+
, to zero, depending
on the loop gain. An FDA with equal-value resistors in
each output to provide differential output termination is
shown in Figure 1. As long as the loop gain is very high, the
output impedance, Z
OUT
, in this circuit is approximately
equal to 2 × R
O
.
Parameter definitions for Figure 1 are as follows:
• R
F
and R
G
are the gain-setting resistors for the amplifier.
• R
L
is the impedance of the load, which should be
balanced and, for double termination, equal to Z
Line
.
• R
O
is the output resistor.
• V
O±
is the output terminal.
• V
OCM
is the output common mode of the FDA.
• V
OUT±
is the differential output signal.
• V
S±
is the power supply to the amplifier.
• Z
Line
is the characteristic impedance of the balanced
transmission line from the amplifier to the load.
Amplifiers: Op Amps
By Jim Karki
Member, Technical Staff, High-Performance Analog
Output
Resistor
Gain-Setting
Resistors
Output
Resistor
V
S+
V
S–
R
F
R
F
R
G
R
G
R
L
V
OUT–
R
O
R
O
V
OUT+
V
O+
V
O
–
Balanced
Transmission
Line
Z
Line
V
OCM
V
IN+
V
IN–
+
+
–
–
FDA
Z
OUT
Figure 1. FDA with differential resistors for output termination