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Amplifiers: Op Amps
Using fully differential op amps as
attenuators, Part 2: Single-ended bipolar
input signals
By Jim Karki
Member, Technical Staff, High-Performance Analog
Introduction
Fully differential operational amplifiers (FDAs) can easily
be used to attenuate and level-shift high-voltage input
signals to match the input requirements of lower-voltage
ADCs. This article is Part 2 of a three-part series. In Part 1
(see Reference 2) we considered a balanced, differential
bipolar input signal and proposed an architecture utilizing
an FDA to accomplish the task. In Part 2 we will show how
to adapt the circuits presented in Part 1 to a high-voltage,
single-ended (SE) bipolar input. Part 3, which will appear
in a future issue of the Analog Applications Journal , will
show the more generic case of an SE unipolar input with
arbitrary common-mode voltage. As mentioned in Part 1,
the fundamentals of FDA operation are presented in
Reference 1, which provides definitions and derivations.
Attenuator circuit for SE bipolar input
Using an input attenuator
Now consider a high-amplitude, SE bipolar input signal
that needs to be attenuated and level-shifted to the appro-
priate levels to drive a lower-voltage input ADC. The first
step is to modify the differential bipolar input circuit pre-
sented in Part 1 to accept an SE bipolar input and keep
the amplifier balanced. This is accomplished by grounding
one side of the signal source, splitting R T in half, and
groundingthecenterpoint.Otherwisethecircuitisthe
same. Splitting R T in half and grounding the center point
are key to keeping the resistances that set the gain on
each side of the amplifier balanced so that no offsets are
generated. Figure 5 shows the modified circuit.
We can build the circuit as shown (with appropriate
values), but we can get the equivalent circuit shown in
Figure 6 with a few simple changes: Combine R S , R G , and
R T /2 on the alternate input from the signal into an equiv-
alent resistor R G – ; use reference designator R G+ on the
positive side; and replace R T /2 with R T . The circuit analy-
sis of Figure 6 is very similar to that of Figure 1 in Part 1
of this series, but the changes in the input configuration
result in a new gain equation:
Figure 5. Differential bipolar input circuit
modified to accept SE bipolar input
R S
R G
R F
V S+
R /2
T
V OUT–
+
–
FDA
V Sig
–
V OUT+
+
R /2
T
V OCM
V S–
R S
R G
R F
Figure 6. Equivalent SE bipolar input circuit
Z IN_Amp
Z IN
R S
R G+
R F
V S+
R T
V OUT–
+
–
FDA
V Sig
–
V OUT+
+
V OCM
V S–
R G–
R F
With this constraint, the overall gain equation reduces to
V
V
R
RR
R
RRR
OUT
Sig
±
T
F
=
×
(4)
V
V
R
RR
+
+
OUT
Sig
± =
T
S
T
G
+
S
T
.
(6)
+
S
T
The noise gain of the FDA can be set to 2 by making the
second half of Equation 4 equal to 1:
The design equations provide two degrees of freedom for
choosing components. The input impedance is given by
(5)
RRRR
+ +
=
G
S
TF
21
Analog Applications Journal
3Q 2009
High-Performance Analog Products
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Amplifiers: Op Amps
Texas Instruments Incorporated
Z IN = R S + R T || Z IN_Amp , which is approximated by Z IN =
R S + R T || R G+ ; so we start by first choosing R S close to the
desired input impedance. We then select R F in the recom-
mended range for the device and calculate the required
value of R T to give the desired attenuation. These results
can be used to calculate R G+ and an equivalent value for
R G– . To see an example Excel ® worksheet, go to http://
www.ti.com/lit/zip/slyt341 andclickOpentoviewthe
WinZip ® directory online (or click Save to download the
WinZip file for offline use). Then open the spreadsheet file
FDA_Attenuator_Examples_SE_Bipolar_Input.xls and
select the Bipolar SE FDA Input Atten worksheet tab.
but care must be taken to double-check operation against
the data sheet, as not all SPICE models will show this
error. For instance, replacing the THS4520 model with the
THS4509 will simulate fine, but the actual device has a
similar input common-mode voltage range.
Onewaytocorrecttheproblemistousepull-upresis-
tors from the FDA input pins to the +5-V supply, as
described in the THS4520 data sheet. In this case, 2-kΩ
pull-up resistors will bring the input common-mode voltage
back into linear operation and will have no effect on the
gain of the signal. To see a TINA-TI simulation of this
corrected circuit (Example 3b), follow the same procedure
as for Example 3a, but view the middle circuit labeled
“Example 3b.” Note that this circuit provides the same
results as those shown in Figure 3 of Part 1.
Another way to eliminate the problem with input
common-mode voltage is to use the R F and R G gain-setting
resistors of the FDA as the attenuator, a method that is
described next.
Using an FDA’s R F and R G as an attenuator
The proposed circuit using gain-setting resistors to obtain
an SE bipolar input signal is shown in Figure 7. In this cir-
cuit, the FDA is used as an attenuator in a manner similar
to using an inverting op amp, as described in Part 1 for the
differential bipolar signal. The design equations are the
same as in Part 1, except that the input impedance is
reduced by approximately half. Thus, the gain (or attenua-
tion) is set by R F and R G :
V
V
Design Examples 3a and 3b
For Example 3a, let’s say that again we have a 20-V PP
bipolar (±10-V) input, but this time it is an SE signal.
We need a 1-kΩ input impedance and 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 6 and using substitution,
we can calculate
R
1
41
k
Ω
S
Sig
OUT
R
=
= − =
333 3
.
Ω
.
T
V
V
−
1
±
The nearest standard 1% value, 332 Ω, should be used.
Then, rearranging Equation 5 and using substitution, we
can calculate
RRRR
ΩΩ Ω ,
which is a standard 1% value. We can then calculate
RRRR
+ =− =
1
k
−
1
k
332
=
750
G
F
ST
R
R
OUT
Sig
± =
F
G
1 332 1,
which is a standard 1% value. These values will provide
the needed attenuation and keep the FDA stable. Again
the V OCM input on the FDA is used to set the output
common-mode voltage to 2.5 V.
The input impedance is Z IN = 1254 Ω, which is higher
than desired. If the input impedance really needs to be
closer to 1 kΩ, we can iterate with a lower value as before.
In this case, using R S = 787 Ω and R F = 1 kΩ will yield
Z IN = 999 Ω, which comes as close as is possible when
standard 1% values are used.
To see a TINA-TI™ simulation of the circuit in Example
3a,goto http://www.ti.com/lit/zip/slyt341 andclickOpen
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_SE_Bipolar_Input.TSC to view the
example (the top circuit labeled “Example 3a”). To down-
load and install the free TINA-TI software, visit www.ti.
com/tina-ti and click the Download button.
The simulation waveforms for Example 3a show that the
signal is distorted. Further investigation will show that the
input common-mode voltage range of the THS4520 used
in the simulation has been violated, causing nonlinear
operation. In this case the SPICE model shows a problem;
= +
=
750
ΩΩ ΩΩ
+
k
=
k
G
−
G
ST
R T is used to set the noise gain to 2 for stability; i.e.,
RR R
T
=
,
FG
2
and the input impedance is Z IN ≈ R G .
Figure 7. Using FDA’s R F and R G as attenuator
for SE bipolar input
R G
R F
V S+
+
V OUT–
–
FDA
V Sig
R T
–
V OUT+
+
V OCM
V S–
R G
R F
22
High-Performance Analog Products
3Q 2009
Analog Applications Journal
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Texas Instruments Incorporated
Amplifiers: Op Amps
Design Example 4
UsingthesameapproachasforExample3a,withR F = 1 kΩ,
we calculate R G = 4 kΩ (the nearest standard 1% value is
4.02 kΩ) and R T = 2.67 kΩ (the nearest standard 1% value
is 2.67 kΩ). This makes Z IN ≈ 4.02 kΩ, and SPICE shows it
to be more on the order of 4.46 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 .
To see an example Excel worksheet, go to http://www.
ti.com/lit/zip/slyt341 andclickOpentoviewtheWinZip
directory online (or click Save to download the WinZip file
for offline use). Then open the spreadsheet file FDA_
Attenuator_Examples_SE_Bipolar_Input.xls and select the
Bipolar SE FDA RF_RG Atten worksheet tab. To see a
TINA-TI simulation of the circuit in Example 4, follow the
same procedure as for Example 3a, but view the bottom
circuit labeled “Example 4.” Note that the circuit provides
the same results as those shown in Figure 3 of Part 1.
Conclusion
We have analyzed two approaches that attenuate and level-
shift high-amplitude, SE bipolar signals to the input range
of lower-voltage input ADCs. The first approach (Example
3a) 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. We saw in the simulation of
this example that there is a potential problem with input
common-mode voltage that we can solve by using pull-up
resistors from the inputs (Example 3b). The second
approach (Example 4) 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. Except for the potential problem
with the input common-mode voltage in Example 3a, the
approaches in Examples 3a and 4 yield the same voltage
translation that is needed to accomplish the interface task.
Otherperformancemetricswerenotanalyzedhere,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 in Example 3a is more
complex but allows the input impedance to be adjusted
independently from 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!”errorsforthisinputasittriestocalcu-
late 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.
The approach in Example 4 is easier, but the input
impedance is set as a multiplication of the feedback resis-
tor and attenuation: Z IN ≈ 2 × R F × 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.
Onefinalnote:Thesourceimpedancewillaffectthe
input gain or attenuation of either circuit and should be
included in the value of R S , especially if it is significant.
References
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
2.JimKarki,“UsingFullyDifferentialOpAmps
as Attenuators, Part 1: Differential Bipolar
Input Signals,” Analog Applications Journal
(2Q 2009) ............................... slyt336
Related Web sites
www.ti.com/sc/device/ partnumber
Replace partnumber with ADS8321, THS4509 , or
TINA-TI and spreadsheet support files for examples:
To download TINA-TI software:
23
Analog Applications Journal
3Q 2009
High-Performance Analog Products
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SLYT341
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