OP193_02 Datasheet, PDF (12/16 Pages) Analog Devices – Precision, Micropower Operational Amplifiers

OP193/OP293/OP493
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 µA, but this will still produce an appreciable error.
R1
20k⍀
–IN
+IN
R2
1.98M⍀
5V
V+
A1
1/2 OP293
R3
20k⍀
V–
I_{S I N K}
R4
1.98M⍀
5V
V+
A2
1/2 OP293
V–
V_OUT
Figure 9. A Conventional Instrumentation Amplifier
One solution to this problem is to use a pull-down resistor. For
example, if R3 = 20 kΩ, then the pull-down resistor must be
less than 400 Ω. 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 µA. Thus, with a common-mode voltage of 4 V, the addi-
tional load current is limited to 340 µA versus 10 mA with a
400 Ω resistor.
R1
20k⍀
R2
1.98M⍀
5V
V+
A1
1/2 OP293
–IN
V–
5V
10k⍀
R3
20k⍀
R4
1.98M⍀
Q1
VN2222
Q2
5V
V+
A2
1/2 OP293
+IN
V–
V_OUT
Figure 10. An Improved Single-Supply, 0 V_IN, 0 V_OUT
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 V_TEMP
pin, while the other half regulates the output current to satisfy
the current summation at its noninverting input:
( ) ^{V × R6 + R7}
I
+ ^TEMP
OUT
R2 × R10
 R2 + R6 + R7 
−
V
SET

R2 × R10

The change in output current with temperature is the derivative
of the transfer function:
( ) ^∆V
∆I
= ^OUT
TEMP _{R6 + R7}
∆T
∆T
R2 × R10
REF-43BZ
V_IN 2
V_{OUT 6}
V_TEMP 3
R1 10k⍀
GND 4
2
8
1
1/2 OP293
3
4
R2
V_TEMP ^1k⍀
R3
R5
100k⍀ _5k⍀
R4
20k⍀
V_SET
ZERO
TRIM
R6
3k⍀
SPAN TRIM
R7
5k⍀
6
1/2 OP293 ⁷
5
R8
1k⍀
R9
100k⍀
1N4002
V+
8V TO 40V
2N1711
ALL RESISTORS 1/4W, 5% UNLESS OTHERWISE NOTED
R10
100⍀
1%, 1/2 W
I_OUT
R_LOAD
Figure 11. Temperature to 4–20 mA Transmitter
–12–
REV. B