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

µA, but this will still produce an appreciable error.

5V

V+

V–

5V

V+

V–

R4

1.98M

R3

20k

R2

1.98M

R1

20k

–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

Ω, 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.

5V

V+

V–

5V

V+

V–

+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

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

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

2N1711

1

2

3

R1 10k

2

6

3

4

REF-43BZ

GND

R5

5k

R3

100k

6

5

ZERO

TRIM

7

R6

3k

R7

5k

R8

1k

1N4002

R10

100

1%, 1/2 W

ALL RESISTORS 1/4W, 5% UNLESS OTHERWISE NOTED

Figure 11. Temperature to 4–20 mA Transmitter