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Texas Instruments Incorporated
Amplifiers: Op Amps
Using fully differential op amps as
attenuators, Part 1: Differential bipolar
input signals
By Jim Karki
Member, Technical Staff, High-Performance Analog
Introduction
Conditioning high-voltage input signals to drive ADCs from
high-voltage sources can be challenging. How can a higher-
voltage signal like ±10 V be attenuated and level-shifted to
match the significantly lower differential and common-
mode-voltage input required by the ADC? In this article,
Part 1 of a three-part series, we consider a balanced, differ-
ential bipolar input signal and propose an architecture
utilizing a fully differential operational amplifier (FDA) to
accomplish the task.
We consider this type of circuit first because it most
clearly shows how to approach the design, keep a balanced
circuit, and not introduce unwanted offsets. Parts 2 and 3
will appear in future issues of the Analog Applications
Journal . Part 2 will show how to adapt the circuit to a
single-ended bipolar input. Part 3 will show the more
generic case of a single-ended unipolar input with arbitrary
common-mode voltage. The level of complexity will
increase with each step, but ordering the presentation in
this manner should help the reader better understand why
values are chosen for the final circuit the way they are.
Differential bipolar input
The fundamentals of FDA operation are presented in
Reference 1. Since the principles and terminology pre-
sented there will be used throughout this article, please
see Reference 1 for definitions and derivations.
FDAs can easily be used to attenuate large signals, con-
vert single-ended signals to differential signals, and level-
shift voltages to match the input requirements of lower-
voltage ADCs. The trick is to implement them in a way that
will perform these tasks while keeping the amplifier stable.
FDAs have been compared to two standard, inverting,
single-ended output op amps configured in a differential
architecture. While this has some validity, one important
difference is that a unity-gain, stable op amp is compen-
sated for a noise gain* of 1, while a unity-gain, stable FDA
is typically compensated for a noise gain of 2. The implica-
tion of this in the context of implementing an attenuator
Figure 1. Attenuator circuit for differential
bipolar input
R S
R G
R F
V S+
+
V OUT–
–
FDA
V Sig
R T
–
V OUT+
+
V OCM
V S–
R S
R G
R F
circuit is that the gain resistors can no longer be chosen
simply to provide the attenuation. Two approaches are
identified in this article; one implements an input attenu-
ator with resistor values chosen to provide a noise gain of
2, and the other implements the attenuator using the gain-
setting resistors with added components to get a noise
gain of 2.
Using an input attenuator
The proposed input-attenuator circuit for a balanced, dif-
ferential bipolar input signal is shown in Figure 1, whose
parameters are defined as follows:
•V S+ and V S– are the power supplies to the amplifier.
•V Sig is the input-signal source.
•R S and R T are the resistors that provide attenuation of
the signal from the source. Their parallel combination
also affects the noise gain of the amplifier.
•R G and R F are the main gain-setting resistors for the
amplifier.
* Noise gain is used to define the stability criteria of an op amp and is calculated
as the gain from the input terminal of the op amp to the output. Generally, one
speaks of op amp stability in terms of the minimum noise gain required, where
larger values are fine, but lower values may lead to instability or oscillation.
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2Q 2009
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Amplifiers: Op Amps
Texas Instruments Incorporated
Figure 2. Thevenin-equivalent input source and attenuator
R S
2
R R
S
T
2
R
RR
T
ST
V
×
V Sig
R T
Sig
2
+
2
R R
S
T
2
R S
For analysis, it is convenient to assume that the FDA is
an ideal amplifier with no offset and has infinite gain. The
first step in analyzing the circuit in Figure 1 is to simplify
it by using only its attenuator portion and the Thevenin
equivalent of the input source. This is shown in Figure 2.
With the circuit in this form, it is easier to see that its
overall gain can be calculated by the formula
up in a spreadsheet. To see an example Excel ® worksheet,
go to http://www.ti.com/lit/zip/slyt336 and click Open to
view the WinZip ® directory online (or click Save to down-
load the WinZip file for offline use). Then open the file
FDA_Attenuator_Examples_Diff_Bipolar_Input.xls and
select the Diff Bipolar FDA Input Atten worksheet tab.
Design Example 1
As a design example, let’s say we have a 20-V PP differential
bipolar (±10-V) signal, and we need a 2-kΩ differential
input impedance. We want to use the ADS8321 SAR ADC
with a 5-V PP differential input and a 2.5-V common-mode
voltage. We choose R S = 1 kΩ and R F = 1 kΩ. Rearranging
Equation 3 and using substitution, we can calculate
V
V
R
RR
R
OUT
Sig
± =
T
F
(1)
×
.
2
+
2
RR
S
T
ST
R
+
G
2
The noise gain of the FDA can be set to 2 by making the
second half of Equation 1 equal to 1:
2
RR
ST
2
R
2
41
k
W
R
+
=
R
(2)
S
G
F
R
=
=
− =
666 7
.
W
.
2
T
V
V
Sig
OUT
−
1
With this constraint, the overall gain equation reduces to
±
V
V
R
RR
OUT
Sig
± =
T
(3)
.
The nearest standard 1% value, 665 Ω, should be used.
Then, rearranging Equation 2 and using substitution, we
can calculate
2
+
S
T
There are two degrees of freedom for choosing components
in the gain equation—an infinite number of combinations
of R S and R T that will give the desired input attenuation,
and an infinite number of R F and R G values to set the gain.
The differential input impedance of this amplifier circuit
is given by Z IN = 2R S + R T || 2R G . Depending on the atten-
uation needed, the input impedance is approximately 2R S .
It is recommended that R F be kept to a range of values
for the best performance. Too large a resistance will add
excessive noise and will possibly interact with parasitic
board capacitance to reduce the bandwidth of the ampli-
fier; and too low a resistance will load the output, causing
increased distortion. Design is best accomplished by first
choosing R S close to the desired input impedance, then
choosing R F within the recommended range for the device.
For example, the THS4521 performs best with R F at about
1 kΩ. Next, the value of R T required to give the desired
attenuation is calculated. Then R G is calculated for the
desired gain. These equations are easily solved when set
2
RR
2 k
WW W,
665
2
ST
RR
=−
=
1
k
W
−
=
750
GF
2
which is a standard 1% value. These values will provide
the needed attenuation function and will keep the FDA
stable. The V OCM input on the FDA is then used to set the
output common-mode voltage to 2.5 V.
The input impedance is
Z
= +
2
R
RR k
2
=
2
WWW
+
665
15
.
k
=
2461
W
,
IN
S
T
G
which is higher than desired. If the input impedance really
needs to be closer to 2 kΩ, we can iterate with a lower
value. In this case, using R S = 806 Ω and R F = 1 kΩ will
yield Z IN = 2014 Ω, which comes as close as is possible
when standard 1% values are used.
SPICE simulation is a great way to validate the design.
To see a TINA-TI™ simulation of the circuit in Example 1,
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Texas Instruments Incorporated
Amplifiers: Op Amps
Figure 3. TINA-TI simulation waveforms of differential
bipolar input in Example 1
10
V Sig_Diff
–10
3
V OUT1_Diff
–3
4
V OUT1+
1
4
V OUT1–
1
0
0.5
1.0
1.5
2.0
Time (µs)
go to http://www.ti.com/lit/zip/slyt336 and click Open to
view the WinZip directory online (or click Save to down-
load the WinZip file for offline use). If you have the TINA-
TI software installed, you can open the file FDA_
Attenuator_Examples_Diff_Bipolar_Input.TSC to view the
example (the top circuit labeled “Example 1”). To down-
load and install the free TINA-TI software, visit www.ti.
com/tina-ti and click the Download button.
The simulation waveforms in Figure 3 show that the
circuit simulates as expected. V Sig1_Diff is the 20-V PP input;
V OUT1_Diff is the differential output of the amplifier circuit;
and V OUT1+ and V OUT1 – are the individual outputs of the
amplifier.
Using an FDA’s R F and R G as an attenuator
The proposed circuit using gain-setting resistors to obtain
a balanced, differential bipolar input signal is shown in
Figure 4. In this circuit, the FDA is used as an attenuator
in a manner similar to using an inverting op amp. The gain
(or attenuation) is set by R F and R G :
V
V
Design Example 2
Using the same approach as for Example 1, with R F = 1 kΩ,
we calculate R G = 4 kΩ (the nearest standard 1% value is
4.02 kΩ) and R T = 2.67 kΩ (a standard 1% value). This
makes Z IN = 8.04 kΩ. The simulation results are the same
as before, but with this approach the only freedom of
choice given the design requirements is the value of R F .
Figure 4. Using FDA’s R F and R G as attenuator
for differential bipolar input
R G
R F
V S+
+
V OUT–
–
FDA
V Sig
R T
–
V OUT+
+
R
R
OUT
Sig
± =
F
G
V OCM
V S–
R T is used to set the noise gain to 2 for stability:
RR R
FG
R G
R F
T
=
2
The equation for input impedance is Z IN = 2R G .
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Amplifiers: Op Amps
Texas Instruments Incorporated
To see an example Excel worksheet, go to http://www.
ti.com/lit/zip/slyt336 and click Open to view the WinZip ®
directory online (or click Save to download the WinZip file
for offline use). Then open the file FDA_Attenuator_
Examples_Diff_Bipolar_Input.xls and select the Diff
Bipolar FDA Rf_Rg Atten worksheet tab. To see a TINA-TI
simulation of the circuit in Example 2, go to http://www.
ti.com/lit/zip/slyt336 and click Open to view the WinZip
directory online (or click Save to download the WinZip file
for offline use). If you have the TINA-TI software installed,
you can open the file FDA_Attenuator_Examples_Diff_
Bipolar_Input.TSC to view the example (the bottom circuit
labeled “Example 2”). Note that this circuit provides the
same results as for the circuit in Example 1. To download
and install the free TINA-TI software, visit www.ti.com/
tina-ti and click the Download button.
Conclusion
We have analyzed two approaches that attenuate
and level-shift high-amplitude, differential bipolar
signals to the input range of lower-voltage input ADCs.
The first approach uses an input attenuator with values
chosen to provide the required attenuation and to keep
the noise gain of the FDA equal to 2 for stability. The sec-
ond approach uses the gain-setting resistors of the FDA in
much the same way as using an inverting op amp, then a
resistor is bootstrapped across the inputs to provide a
noise gain of 2. The two approaches yield the same voltage
translation that is needed to accomplish the interface task.
Other performance metrics were not analyzed here, but
the two approaches have substantially the same noise,
bandwidth, and other AC and DC performance character-
istics as long as the value of R F is the same.
The input-attenuator approach shown in Example 1 is
more complex but allows the input impedance to be
adjusted independently of the gain-setting resistors used
around the FDA. At least to a certain degree, lower values
can easily be achieved if desired, but there is a maximum
allowable R S where larger values require the R G resistor to
be a negative value. For example, setting R S = 4 kΩ results
in R G = 0 Ω. The spreadsheet tool provided will generate
“#NUM!” errors for this input as it tries to calculate the
nearest standard value, which then replicates throughout
the rest of the cells that require a value for R G ; but this
value will work.
It should be noted that a circuit similar to the one in
Example 1, with a maximum R S value and R G = 0 Ω,
results in the same circuit as the one in Example 2 that
uses the gain-setting resistors as the attenuator. It should
also be noted that the source impedance will affect the
input gain or attenuation of either circuit and should be
included in the value of R S , especially if it is significant.
The approach in Example 2 is easier, but the input
impedance is set as a multiplication of the feedback resistor
and attenuation: Z IN = 2 x R F x Attenuation. This does
allow some design flexibility by varying the value of R F ,
but the impact on noise, bandwidth, distortion, and other
performance characteristics should be considered.
Reference
For more information related to this article, you can down-
load an Acrobat ® Reader ® file at www-s.ti.com/sc/techlit/
litnumber and replace “ litnumber ” with the TI Lit. # for
the materials listed below.
Document Title
TI Lit. #
1. Jim Karki, “Fully-Differential Amplifiers,”
Application Report........................ sloa054
Related Web sites
TINA-TI and spreadsheet support files for examples:
To download TINA-TI software:
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