|
OP193 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 31, depends upon
amplifier A1âs output being at 0 V when the applied common-
mode input voltage is at 0 V. Any error at the output is multi-
plied by the gain of A2. In addition, current flows through
resistor R3 as A2âs output voltage increases. A1âs 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
R2
20k
1.98M
R1
R2
20k
1.98M
+5V
V+
A1
1/2 OP293
âIN
Vâ
+5V
10k
Q1
VN2222
+IN
R3
R4
20k
1.98M
Q2
+5V
V+
A2
1/2 OP293
Vâ
VOUT
âIN
+IN
+5V
V+
A1
R3
20k
1/2 OP293
Vâ
IS I N K
R4
1.98M
+5V
V+
A2
1/2 OP293
Vâ
VOUT
Figure 31. A Conventional Instrumentation Amplifier
Figure 32. An Improved Single-Supply, 0 VIN, 0 VOUT
Instrumentation Amplifier
A Low-Power, Temperature to 4â20 mA Transmitter
A simple temperature to 4â20 mA transmitter is shown in Fig-
ure 33. After calibration, this transmitter is accurate to ± 0.5°C
over the â50°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:
One solution to this problem is to use a pull-down resistor. For
example, if R3 = 20 kâ¦, then the pull-down resistor must be
less than 400 â¦. 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.
IOUT
+ VTEMP Ã (R6 +
R2 Ã R10
R7)
â
VSET

ï£ï£¬
R2 + R6 + R7
R2 à R10 
The change in output current with temperature is the derivative
of the transfer function:
Figure 32 shows a better solution. A1âs 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 ⦠resistor.
âIOUT
=
âVTEMP (R6 + R7)
âT
âT
R2 Ã R10
REF-43BZ
VIN 2
VOUT 6
VTEMP 3 R1 10kâ¦
GND 4
2
8
1
1/2 OP293
3
4
R2
VTEMP 1kâ¦
R3
R5
100k⦠5kâ¦
R4
20kâ¦
VSET
ZERO
TRIM
R6
SPAN TRIM
3kâ¦
R7
5kâ¦
6
1/2 OP293 7
5
R8
1kâ¦
R9
100kâ¦
1N4002
V+
+8V TO +40V
2N1711
ALL RESISTORS 1/4W, 5% UNLESS OTHERWISE NOTED
R10
100â¦
1%, 1/2 W
IOUT
RLOAD
Figure 33. Temperature to 4â20 mA Transmitter
â12â
REV. A
|
▷ |