APPLICATION BULLETIN
®
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EXTENDING THE COMMON-MODE RANGE
OF DIFFERENCE AMPLIFIERS
By R. Mark Stitt (602) 746-7445
OFFSETTING THE INPUT COMMON-MODE
RANGE WITH A CONSTANT VOLTAGE
Extending the common-mode range of difference amplifiers
allows their use in a wider variety of reduced power-supply
applications.
In many applications, the common-mode signal range is
known and the common-mode input range of the difference
amplifier can be adjusted to coincide with the required
range. For example, the ±60V common-mode range of the
INA117 operating on ±6V supplies could be shifted to range
from +0V to +120V, or +50V to +170V.
The INA117 has a specified common-mode input range of
±200V when operating on standard ±15V power supplies. At
power-supply voltages above ±13V, the INA117 input range
is limited to ±200V by the power capabilities of its internal
input resistors. On reduced power supplies, the input range
is limited by the common mode input range of the internal
op amp.
To offset the common-mode range, the reference connection
of the difference amplifier is connected to an offsetting
voltage, VX, instead of ground. With the reference connected
to an offsetting potential, a second difference amplifier must
be used to refer the output back to ground.
The linear common-mode input range of the internal op amp
extends to within 3V of its power supply voltage. For
example, with a ±15V power supply, the common-mode
input range of the internal op amp is ±12V. Because the
INA117 internal resistor network divides the input by 20, the
actual input range of the INA117 would be 20 • (±12V), or
240V, for ±15V power supplies. Similarly, reducing the
power supply voltage to ±6V will limit the input commonmode voltage to ±60V.
One way to offset the input voltage is to connect the
reference pins 1 and 5 to the negative supply voltage as
shown in Figure 1. Another possibility is to derive the offset
voltage from a zener diode connected to the negative power
supply as shown in Figure 2. In either case, the total
common-mode range of the INA117 is unchanged and
shifted by –19 • (VX).
There are two approaches to boosting the common-mode
input range for reduced power supply applications: Offsetting the common-mode range by a fixed amount, and dynamically adjusting the common-mode range to follow the
input common-mode signal.
V+
7
V2
V3
2
R1
380kΩ
R2
380kΩ
7
3
R3
380kΩ
6
R5
R4
20kΩ
21.1kΩ
2
1
5
R2
25kΩ
5
6
A1
INA117
8
R1
25kΩ
4
3
R3
25kΩ
A2
R4
25kΩ
INA105
1
VO = V3 – V2
–3V > VO > –10V
Swap A2 pins 2 and 3 for:
10V > VO > 3V
also;
((V+) – 5V) > VO > ((V–) + 3V)
4
V–
VX = V–
FIGURE 1. Offsetting the INA117 Common-Mode Input Range Using the Negative Power Supply as a Reference.
1990 Burr-Brown Corporation
AB-015
AB-015
Printed in U.S.A. August, 1990
V+
7
V2
V3
2
R1
380kΩ
R2
380kΩ
7
3
R3
380kΩ
6
R5
R4
20kΩ
21.1kΩ
1
5
R2
25kΩ
5
6
VO = V3 – V2
A1
INA117
8
2
R1
25kΩ
10kΩ
4
3
V X = (V–) + 3.3V
R3
25kΩ
R4
25kΩ
1N4684
3.3V
0V > VO > –10V
Swap A2 pins 2 and 3 for:
10V > VO > 0V
A2
INA105
1
also;
((V+) – 5V) > VO > ((V–) + 3V)
4
V–
FIGURE 2. Offsetting the INA117 Common-Mode Input Range with a Zener Reference.
Since the input voltage can swing to within 3V of the power
supply, the following relationships apply for the INA117:
ADJUSTING THE COMMON
MODE RANGE DYNAMICALLY
VR = 20 • [(V+) + |V–| – 6V]
Another way to boost the common-mode range of a difference amplifier is to drive the reference connection dynamically in response to changes in the input. A circuit to
boost the input range of the INA117 is shown in Figure 5.
A third amplifier, A3, along with resistors R7, R8, and R6 is
used to derive, invert, and scale the input level presented to
the reference connection.
VL = 20 • [(V–) + 3V] – 19 • V X
V H = V R – VL
Where
VR = total common-mode range [V]
VL = minimum common-mode signal [V]
The value for R6 depends on the power supply voltages and
op amp used for A 3. To maximize the common-mode range,
R6 should be selected so the output of A 3 is at its maximum
swing limit when the inputs to the difference amplifier op
amp are at 3V from the opposite power supply. The OPA1013
is a good choice for A 3 since its outputs are guaranteed to
swing within 2V of the power supply rails.
VH = maximum common-mode signal [V]
V+, V– = positive, negative power supply [V]
|V–| = absolute value of the negative power supply [V]
VX = offset voltage (from ground) [V]
POWER
SUPPLY
±15V
±12V
±9V
±6V
COMMON-MODE INPUT RANGE (V)
VX = V–(2)
VX = (V–) + 3.3 (3)
VX = 0(1)
–240 to +240(4)
–180 to +180
–120 to +120
–60 to +60
45 to 525(4)
48 to 408(4)
51 to 291(4)
54 to 174
Using the OPA1013 op amp for A 3, and considering the
allowed swing to within 3V of the power supply voltage, the
following relationships apply for the INA117.
–18 to 462(4)
–15 to 345(4)
–12 to 228(4)
–9 to 111
VCM = ±{20 • ((V+) – 3V) – 19 • VX}
NOTES: (1) Reference connected to GND (normal operation). (2) Reference connected to V– (see Figure 1). (3) Reference connected to V–
through 3.3V zener (see Figure 2). (4) Voltages greater than ±200V are
shown for reference only. INA117 maximum rated operating voltage is
±200V.
VX = (V–) + 2V
R6 = –0.5MΩ • VX/VCM
Where:
TABLE I. INA117 Common-Mode Input Range for Selected Power Supplies and Reference Offsets.
VCM = common-mode input range [V]
R6 = value of R6 [Ω]
The same principles can be applied to the INA105 difference
amplifier as shown in Figures 3 and 4. With an allowable
voltage swing to within 3V of the power supply, the following
relationships apply for the INA105:
VR = 2 • [(V+) + |V–| – 6V]
POWER
SUPPLY (V)
VCM
(V)
R6
(kΩ)
±15
±12
±9
±6
±487(1)
±370(1)
±253(1)
±136
13.3
13.7
13.7
14.7
NOTES: (1) Voltages above ±200V are shown for reference only. INA117
maximum rated operating voltage is ±200V.
VL = 2 • [(V–) + 3V] – VX
TABLE II. INA117 Common-Mode Input Range for Selected Power Supplies Using Figure 5 Circuit.
2
V+
7
7
V2
2
R1
25kΩ
R2
25kΩ
R1
25kΩ
2
5
R2
25kΩ
5
6
V3
3
6
A1
R3
25kΩ
R4
25kΩ
3
1
A2
R3
25kΩ
INA105
VO = V3 – V2
–3V > VO > –10V
Swap A2 pins 2 and 3 for:
10V > VO > 3V
R4
25kΩ
4
INA105
1
also;
((V+) – 5V) > VO > ((V–) + 3V)
4
V–
VX = V–
FIGURE 3. Offsetting the INA105 Common-Mode Input Range Using the Negative Power Supply as a Reference.
V+
7
7
V2
2
R2
25kΩ
R1
25kΩ
R1
25kΩ
2
5
R2
25kΩ
5
6
V3
3
6
A1
R3
25kΩ
R4
25kΩ
10kΩ
INA105
3
1
4
VO = V3 – V2
0V > VO > –10V
Swap A2 pins 2 and 3 for:
10V > VO > 0V
A2
R3
25kΩ
R4
25kΩ
INA105
1
also;
((V+) – 5V) > VO > ((V–) + 3V)
4
1N4684
3.3V
V–
VX = (V–) + 3.3V
FIGURE 4. Offsetting the INA105 Common-Mode Input Range with a Zener Reference.
V2
V3
2
3
R1
380kΩ
R2
380kΩ
R3
380kΩ
6
R5
R4
20kΩ
21.1kΩ
8
1
2
25kΩ
5
6
A1
INA117
5
3
25kΩ
R7
1MΩ
25kΩ
R6
R8
1MΩ
A3
FIGURE 5. Boosting the INA117 Common-Mode Input Range Dynamically.
3
25kΩ
A2
INA105
1
VO = V3 – V2
If bipolar output swing is required, offset from the rail must
be large enough to accommodate the common-mode offset
as well as the output swing. When using the Figure 5 or 6
circuit, the VX terms in the equations must be replaced by:
In the circuit of Figure 5, the true common-mode signal
drives the INA117 reference, V X. Often it is adequate to
drive the reference in response to the common-mode signal
at just one input pin (either pin 2 or 3). In this case one
common-mode sense resistor (either R 7 or R8) can be omitted. The value of the feedback resistor, R 6, must then be
doubled.
[(V–) + 3V + VSW] for negative swings
and by
[(V+) – 5V – VSW] for positive swings
The same principles can be applied to the INA105 difference
amplifier as shown in Figure 6. Using the same 3V, 2V rules,
the following relationships apply for the INA105.
Where:
VCM = ±{2 • ((V+) – 3V) – VX}
VSW = difference amp output swing relative to reference,
VX.
VX = (V–) + 2V
(VSW = difference amp differential input since the gain = 1)
R6 = –0.5MΩ • VX/VCM
The boosted common-mode range for positive inputs is
different than for negative inputs due to the differences in
the difference amplifier output swing limitations:
OUTPUT RANGE LIMITATIONS
For positive common-mode inputs:
Keep in mind that with any of these techniques, the
common-mode range refers to the input of the difference
amp only. To make use of the extended common-mode
range, the output swing limitations of the difference amp
must also be observed.
VCMH = 20 • ((V+) – 3V) – 19 • V XH
R6H = –0.5MΩ • VXH/VCMH
VXH = [(V–) + 3V + V SW]
For negative common-mode inputs:
The output of the INA117 or INA105 is guaranteed to swing
at least ±10V on ±15V power supplies. However, a negative
output can actually swing to within 3V of the negative power
supply (to –12V on ±15V supplies).
VCML = 20 • ((V–) + 3V) – 19 • VXL
R6L = –0.5MΩ • VXL/VCMH
VXL= [(V+) – 5V – VSW]
With zero differential input voltage to the difference amplifier,
the output will be at zero volts with respect to the reference
connection, VX. The circuits in Figure 1 or 3 will not work
with zero differential input. Since the difference amp reference
pin is connected to V–, the output of the difference amp
would saturate to its negative swing limit in an attempt to
swing to V–. For the circuit to work, the differential input
must be at least 3V so that the output of the difference
amplifier is at 3V from V–. The input to the difference
amplifier can be either +3V or –3V and the input connections (pins 2 and 3) can be interchanged to provide the
proper output swing.
Where:
VCMH = highest common-mode input voltage [V]
VCML = lowest common-mode input voltage [V]
Since only one value for R6 can be used, the smaller value
must be selected if the common-mode input is bipolar. The
total common-mode swing is limited by this value.
Reducing the difference amplifier output swing increases the
possible common-mode input range. If a higher output
swing is needed, add gain after the difference amplifier as
shown in Figure 7. In this circuit an optional f –3dB = 1kHz
filter is added ahead of the gain stage. The filter reduces
output noise at the expense of signal bandwidth reduction. If
filtering is not desired, connect the difference amp output
directly to the noninverting input of the output amplifier.
The circuits in Figures 2 and 4 will function with zero
differential input since the reference is connected 3.3V from
V–. Output swing in response to input signal must, however,
always be in the positive direction. As before, the inputs of
both difference amplifiers can be connected to accommodate
any polarity of input/output swing.
The information provided herein is believed to be reliable; however, BURR-BROWN assumes no responsibility for inaccuracies or omissions. BURR-BROWN assumes
no responsibility for the use of this information, and all use of such information shall be entirely at the user’s own risk. Prices and specifications are subject to change
without notice. No patent rights or licenses to any of the circuits described herein are implied or granted to any third party. BURR-BROWN does not authorize or warrant
any BURR-BROWN product for use in life support devices and/or systems.
4
2
V2
R1
25kΩ
R2
25kΩ
2
5
R2
25kΩ
R1
25kΩ
5
6
3
V3
6
A1
R3
25kΩ
R4
25kΩ
R7
1MΩ
INA105
3
1
VO = V3 – V2
A2
R3
25kΩ
R4
25kΩ
INA105
1
R6
R8
1MΩ
A3
FIGURE 6. Boosting the INA105 Common-Mode Input Range Dynamically.
V2
V3
2
3
R2
380kΩ
R1
380kΩ
R3
380kΩ
6
R5
R4
20kΩ
21.1kΩ
8
1
2
25kΩ
25kΩ
5
6
A1
INA117
5
3
25kΩ
25kΩ
R9
15.8kΩ
C1
0.01µF
A2
INA105
1
1/2
OPA1013
A4
VOUT – 10 (V3 – V2 )
R11
10.2kΩ
R7
1MΩ
R6
R12
1.13kΩ
R8
1MΩ
A3
FIGURE 7. Boosted Common-Mode Input Range INA117 with Noise Filtering and Added Gain after the 2nd Difference
Amplifier to Further Extend INA117 Common-Mode Input Range.
5