OP193FSZ Datasheet, PDF (16/20 Pages) Analog Devices – Precision, Micropower Operational Amplifiers

OP193/OP293
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 33, 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 results. With
a maximum output voltage of 4 V, the current through R3 is
only 2 μA, but this still produces an appreciable error.
R1
20kΩ
R2
1.98MΩ
5V
–
V+
A1
1/2 OP293
R3
20kΩ
R4
1.98MΩ
–IN
+
V–
5V
I_SINK
–
V+
A2
V_OUT
1/2 OP293
+IN
+
V–
Figure 33. 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 is 10 mA,
which is unacceptable in a low power application.
Figure 34 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, the 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 vs. 10 mA with a 400 Ω
resistor.
R1
20kΩ
–IN
R2
1.98MΩ
5V
–
V+
A1
1/2 OP293
+
V–
5V
10kΩ
R3
20kΩ
R4
1.98MΩ
Q1
VN2222
5V
Q2
–
V+
A2
1/2 OP293
+IN
+
V–
V_OUT
Figure 34. An Improved Single-Supply, 0 VIN, 0 VOUT Instrumentation Amplifier
A LOW POWER, TEMPERATURE TO 4 mA TO 20 mA
TRANSMITTER
A simple temperature to 4 mA to 20 mA transmitter is shown
in Figure 35. 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 TEMP pin, and the other half regulates the output current
to satisfy the current summation at its noninverting input.
( ) _IOUT
+ ^{VTEMP × R6 + R7}
R2 × R10
− V_SET


R2 + R6 + R7 _
R2 × R10 ^
The change in output current with temperature is the derivative
of the following transfer function:
( ) _∆IOUT
∆V_TEMP
= ^∆T
R6 + R7
∆T
R2 × R10
1N4002
REF43GPZ
V_IN 2
V_OUT 6
TEMP 3
R1, 10kΩ
GND 4
2_–
8
1/2
OP293
3₊
4
R4
20kΩ
R2
1 VTEMP 1kΩ
R3
100kΩ
R5
V_SET
5kΩ
ZERO
TRIM
SPAN TRIM
R6
3kΩ
R7
5kΩ
6
–
1/2
7
OP293
5₊
R8
1kΩ
R9
100kΩ
V+
8V TO 40V
2N1711
R10
100Ω
1%, 1/2 W
I_OUT
NOTES
1. ALL RESISTORS 1/4 W, 5% UNLESS OTHERWISE NOTED.
Figure 35. Temperature to 4 mA to 20 mA Transmitter
R_LOAD
Rev. C | Page 16 of 20