
REV. B
OP193/OP293/OP493
–12–
A Single-Supply Instrumentation Amplifier
Designing a true single-supply instrumentation amplifier with
zero-input and zero-output operation requires special care.
The traditional configuration, shown in Figure 9, depends upon
amplifier A1’s output being at 0 V when the applied common-
mode input voltage is at 0 V. Any error at the output is multiplied
by the gain of A2. In addition, current flows through resistor R3
as A2’s output voltage increases. A1’s output must remain at 0 V
while sinking the current through R3, or a gain error will result.
With a maximum output voltage of 4 V, the current through R3
is only 2
A, but this will still produce an appreciable error.
5V
V+
V–
5V
V+
V–
VOUT
R4
1.98M
R3
20k
R2
1.98M
R1
20k
ISINK
–IN
+IN
1/2 OP293
A2
1/2 OP293
A1
Figure 9. A Conventional Instrumentation Amplifier
One solution to this problem is to use a pull-down resistor. For
example, if R3 = 20 k
, then the pull-down resistor must be
less than 400
. However, the pull-down resistor appears as a
fixed load when a common-mode voltage is applied. With a 4 V
common-mode voltage, the additional load current will be 10 mA,
which is unacceptable in a low power application.
Figure 10 shows a better solution. A1’s sink current is provided
by a pair of N-channel FET transistors, configured as a current
mirror. With the values shown, sink current of Q2 is about
340
A. Thus, with a common-mode voltage of 4 V, the addi-
tional load current is limited to 340
A versus 10 mA with a
400
resistor.
5V
V+
V–
5V
V+
V–
VOUT
+IN
1/2 OP293
A2
R4
1.98M
R3
20k
R2
1.98M
R1
20k
–IN
1/2 OP293
A1
5V
10k
Q1
Q2
VN2222
Figure 10. An Improved Single-Supply, 0 VIN, 0 VOUT
Instrumentation Amplifier
A Low-Power, Temperature to 4–20 mA Transmitter
A simple temperature to 4–20 mA transmitter is shown in Fig-
ure 11. After calibration, this transmitter is accurate to
±0.5°C
over the –50
°C to +150°C temperature range. The transmitter
operates from 8 V to 40 V with supply rejection better than
3 ppm/V. One half of the OP293 is used to buffer the VTEMP
pin, while the other half regulates the output current to satisfy
the current summation at its noninverting input:
I
VR6
R7
R2
R10
V
R2
R6
R7
R2
R10
OUT
TEMP
SET
+
×+
×
++
×
()
The change in output current with temperature is the derivative
of the transfer function:
I
T
V
T
R6
R7
R2
R10
OUT
TEMP
=
+
×
()
SPAN TRIM
8
4
8V TO 40V
V+
1/2 OP293
R4
20k
R9
100k
R2
1k
1/2 OP293
VTEMP
2N1711
1
2
3
R1 10k
2
6
3
4
REF-43BZ
VIN
VOUT
VTEMP
GND
R5
5k
R3
100k
6
5
ZERO
TRIM
VSET
7
R6
3k
R7
5k
R8
1k
1N4002
R10
100
1%, 1/2 W
RLOAD
IOUT
ALL RESISTORS 1/4W, 5% UNLESS OTHERWISE NOTED
Figure 11. Temperature to 4–20 mA Transmitter