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Amplifiers: Op Amps
Using fully differential op amps as
attenuators, Part 3: Single-ended
unipolar 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 3 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 (see Reference 3)
we showed how to adapt the circuits presented in Part 1 to
a high-voltage, single-ended (SE) bipolar input. In Part 3,
we will show how to adapt the circuits presented in Parts
1 and 2 to the more complex case where the input signal is
a high-voltage, SE unipolar input with arbitrary common-
mode voltage. As mentioned in Part 1, the fundamentals of
FDA operation are presented in Reference 1, which pro-
vides definitions and derivations.
Single-ended unipolar input
Using an input attenuator
Let’s consider a high-amplitude, SE unipolar input signal
that needs to be attenuated and level-shifted to the appro-
priate levels to drive a lower-voltage input ADC. We will
use the same basic approach as for the SE bipolar input
presented in Part 2; but, to offset the imbalance that
would otherwise be caused by the signal’s common-mode
voltage, we will modify the signal to provide biasing on its
alternate input. The proposed input-attenuator circuit for
an SE unipolar input signal is shown in Figure 8. R S – and
R T – have been added to the circuit in a manner that uses
V REF to provide biasing on the alternate input.
The circuit analysis of Figure 8 is very similar to that of
Figure 6 in Part 2. For the moment, assume that R S – on
the alternate input is grounded instead of tied to V REF . In
that case, the only changes in the gain equation are due to
changes in reference designators:
Figure 8. SE unipolar input-attenuator circuit
Z IN_Amp
R S+
R G+
R F
V S+
R T+
V Sig
V OUT–
+
–
FDA
V REF
V OUT+
–
+
V OCM
R S–
V S–
R G–
R F
R T–
If we choose to keep R F the same on both sides of the
FDA, then we need to balance the gain-setting resistances
by setting
|| || . (10)
In order to balance the offset due to the common-mode
voltage of V Sig , we multiply the common-mode voltage of
V Sig by the signal input attenuator (or voltage divider),
which equals V REF times the voltage divider on the alter-
nate input:
RRRRRR
+
= +
G
−
S
−
T
−
G
+
S
+
T
+
R
RR
R
RR
T
−
T
+
V
=
V
(11)
REF
SigCom
_
+
+
S
−
T
−
S
+
T
+
The input impedance is given by Z IN = R S+ + R T+ || Z IN_Amp ,
which is approximated by Z IN = R S+ + R T+ || R G+ .
These basic design equations provide the freedom to
choose one value in each of the following sets of interactive
components:
1. Signal input-attenuator resistors, R S+ and R T+
2. Gain-setting resistors, R F and R G±
3. V REF voltage-divider resistors, R S – and R T –
We start the design as before by first choosing R S+ close
to the desired input impedance. We then select R F in the
V
V
R
RR
R
RRR
OUT
Sig
±
T
+
F
=
×
(7)
+
+ ||
S
+
T
+
G
+
S
+
T
+
The noise gain of the FDA can be set to 2 by making the
second half of Equation 7 equal to 1:
(8)
With this constraint, the overall gain equation reduces to
RRRR
+
||
=
G
+
S
+
T
+
F
V
V
R
RR
OUT
Sig
±
T
+
=
.
(9)
+
S
+
T
+
19
Analog Applications Journal
4Q 2009
High-Performance Analog Products
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Amplifiers: Op Amps
Texas Instruments Incorporated
recommended range for the device and calculate the value
of R T+ required to provide the desired attenuation. The
result can be used to calculate R G+ .
For the alternate side of the input signal, we start simi-
larly by first choosing the value of R S – , which will basically
set the current in the voltage divider. It is generally best to
keep the current small to conserve power; but, since we
chose to keep R F the same on both sides of the FDA, there
are limitations because Equation 10 has to be satisfied.
The required value of R T – must be calculated to satisfy
Equation 11, and then the results can be used along with
R F to calculate R G – .
To see an example Excel ® worksheet, go to http://
www.ti.com/lit/zip/slyt359 andclickOpentoviewthe
WinZip ® directory online (or click Save to download the
WinZip file for offline use). Then open the file FDA_
Attenuator_Examples_SE_Unipolar_Input.xlsandselect
theUnipolarSEFDAInputAttenworksheettab.
The nearest standard 1% value, 499 Ω, should be used.
These values will provide the needed function 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
5,goto http://www.ti.com/lit/zip/slyt359 andclickOpento
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_SE_Unipolar_Input.TSCtoview
the example (the top circuit labeled “Example 5”). The
simulation waveforms are the same as those shown in
Figure 3 of Part 1. To download and install the free TINA-
TI software, visit www.ti.com/tina-ti and click the
Download button.
Using an FDA’s R F and R G as an attenuator
The proposed circuit using gain-setting resistors to obtain
an SE unipolar input signal is shown in Figure 9. In this
circuit, the FDA is used as an attenuator in a manner simi-
lar to that described in Part 2 for the SE bipolar signal,
and the design equations are the same:
V
V
Design Example 5
For Example 5, let’s say we have a 20-V PP SE unipolar
input signal that goes from 0 V to +20 V, we need a 1-kΩ
input impedance, and we want to use the ADS8321 SAR
ADC with a 5-V PP differential input and a 2.5-V common-
mode voltage. We also are using a +5-V single supply to
power both the FDA and the ADC, so we want to use that
as our reference voltage, V REF , on the alternate input. We
choose R S+ = 1 kΩ and R F = 1 kΩ. Rearranging Equation 9
and using substitution, we can calculate
R
R
OUT
Sig
± = ,
F
G
R
1 k
Ω
S
Sig
+
R
=
= − =
333.3
Ω.
T
+
V
V
41
and for stability we set
RR R
F
−
1
OUT
±
T
= ||
,
G
The nearest standard 1% value, 332 Ω, should be used.
Rearranging Equation 8 and using substitution, we can
calculate
RRRR
2
and Z IN ≈ R G .
To avoid a DC offset in the output, V REF is set to equal
the common-mode voltage of V Sig . Note that if a reference
voltage higher than the input common-mode voltage is
available in the system, a resistor divider can be used. This
ΩΩ ΩΩ
which is a standard 1% value. We then choose R S – = 1 kΩ
and calculate R T – by rearranging Equation 11 and using
substitution:
=−
||
=
1 k1 k
−
||
332
=
750
,
G
+
F
S
+
T
+
Figure 9. Using FDA’s R F and R G as attenuator
for SE unipolar input
R
S
−
R
=
T
−
1
−
1
V
R
RR
SigCom
REF
_
T
+
R G
R F
×
V
+
S
+
T
+
V S+
1 k
Ω
=
=
1 k
Ω
,
V Sig
1
V OUT–
−
1
R T
FDA
10 V
5 V
332
1 k3
Ω
Ω 32
×
V OUT+
+
Ω
V OCM
which is a standard 1% value. By rearranging Equation 10
and using substitution, we can calculate
RRRR RR
V REF
V S–
= +
||
−
||
G
−
G
+
S
+
T
+
S
−
T
−
R G
R F
=
750
ΩΩ ΩΩΩ
+
1 k
||
332
−
1 k1 k
||
=
500
Ω.
20
High-Performance Analog Products
4Q 2009
Analog Applications Journal
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Texas Instruments Incorporated
Amplifiers: Op Amps
is accomplished by keeping the parallel combination equal
to R G while simultaneously setting the voltage divider to
provide the input common-mode voltage at no load.
Design Examples 6a and 6b
For Example 6a, we will use the same approach as for
Example 5, with R F = 1 kΩ, and calculate R G = 4 kΩ (the
nearest standard 1% value is 4.02 kΩ) and R T = 2.6666 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. V REF should be set to the common-mode volt-
age of the input signal and is calculated by
provides the same results as those shown in Figure 3 of
Part 1. To download and install the free TINA-TI software,
visit www.ti.com/tina-ti and click the Download button.
Another way to eliminate the problem with input
common-mode voltage is to use the input attenuator to
the FDA as the circuit’s attenuator as described earlier.
Conclusion
We have analyzed two approaches that attenuate and level-
shift high-amplitude, SE unipolar signals to the input range
of lower-voltage input ADCs. The primary difference
between the unipolar input design and the bipolar designs
described in Parts 1 and 2 is that a reference voltage to the
alternate input must be provided in the unipolar design to
make sure the output swing is symmetrical about the
common-mode voltage. For the first approach (Example 5),
we chose input resistor values to provide the required
attenuation and to keep the noise gain of the FDA equal
to 2 for stability. This approach allows the use of a lower
value for V REF . The second approach (Example 6a) uses
the gain-setting resistors of the FDA in much the same
way as using an inverting op amp, then a resistor is boot-
strapped across the inputs to provide a noise gain of 2. We
saw in the simulation that this last approach caused a prob-
lem with the input common-mode voltage going too high on
the positive peaks of the input signal, but this was easily
compensated for by splitting the R T resistor and tying the
center to ground (Example 6b). The two approaches yield
the same voltage translation that is needed to accomplish
theinterfacetask.Otherperformancemetricswerenot
analyzed here, but the two approaches have substantially
the same noise, bandwidth, and other AC and DC perform-
ance characteristics as long as the value of R F is the same.
The input-attenuator approach in Example 5 is more
complexbutallowstheinputimpedancetobeadjusted
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 allow-
able 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!”errorsforthisinputasittriestocalculatethe
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 Examples 6a and 6b is easier, but the
input impedance is set as a multiplication of the feedback
resistor 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.
Also, as mentioned before, voltages at the amplifier nodes
should be checked against data-sheet specifications,
because SPICE models will not always show problems.
Onefinalnote:Thesourceimpedancewillaffectthe
input gain or attenuation of either circuit and should be
included in the value of R S+ , especially if it is significant.
V
+
V
0 V
+
20 V
Sigmin
_
Sigmax
_
V
=
=
=
10 V.
REF
2
2
The function is 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/slyt359 andclickOpentoviewthe
WinZip directory online (or click Save to download the
WinZip file for offline use). Then open the file FDA_
Attenuator_Examples_SE_Unipolar_Input.xlsandselect
theUnipolarSEFDARF_RGAttenworksheettab.Tosee
a TINA-TI simulation of the circuit in Example 6a, go to
the WinZip directory online (or click Save to download the
WinZip file for offline use). If you have the TINA-TI soft-
ware installed, you can open the file FDA_Attenuator_
Examples_SE_Unipolar_Input.TSCtoviewtheexample
(the middle circuit labeled “Example 6a”). To download
and install the free TINA-TI software, visit www.ti.com/
tina-ti and click the Download button.
The simulation waveforms for Example 6a show that the
signal is distorted. Further investigation will show that the
input common-mode voltage range of the THS4509 used
in the simulation has been violated at the most positive
peaks of the input signal, causing nonlinear operation. In
this case the SPICE model shows a problem; 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 THS4509 model with the THS4520 will simu-
late fine, but the actual device has a similar input common-
mode voltage range.
Onewaytocorrecttheproblemistousepull-downresis-
tors from the FDA input pins to ground as described in the
THS4509 data sheet. In this case, instead of placing the full-
value R T across the inputs, we place half the value (1.33
kΩ) from each input to ground. These resistors will act to
pull down the inputs and bring the common-mode voltage
back into linear operation. To see a TINA-TI simulation of
this corrected circuit (Example 6b), go to http://www.ti.
com/lit/zip/slyt359 andclickOpentoviewtheWinZip
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_
Unipolar_Input.TSCtoviewtheexample(thebottom
circuit labeled “Example 6b”). Note that the circuit
21
Analog Applications Journal
4Q 2009
High-Performance Analog Products
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Amplifiers: Op Amps
Texas Instruments Incorporated
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.
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:
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
3.JimKarki,“UsingFullyDifferentialOpAmps
as Attenuators, Part 2: Single-Ended Bipolar
Input Signals,” Analog Applications Journal
(3Q 2009) ............................... slyt341
22
High-Performance Analog Products
4Q 2009
Analog Applications Journal
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