OP193/OP293

Rev. C | Page 16 of 20

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.

5V

V+

V–

5V

+

–

+

–

V+

V–

R1

20k

Ω

R2

1.98M

Ω

–IN

+IN

R4

1.98M

Ω

R3

20k

Ω

A1

1/2 OP293

A2

1/2 OP293

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.

5V

V+

V–

5V

Q1

Q2

5V

+

–

+

–

V+

V–

R1

20k

Ω

R2

1.98M

Ω

A1

1/2 OP293

A2

1/2 OP293

–IN

+IN

R3

20k

Ω

10k

Ω

R4

1.98M

Ω

VN2222

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.

(

)

















×

+

+

−

×

+

×

+

10

2

7

6

2

10

2

7

6

R

R

R

R

R

V

R

R

R

R

V

I

SET

TEMP

OUT

The change in output current with temperature is the derivative

of the following transfer function:

(

)

10

2

7

6

R

R

R

R

T

V

T

I

TEMP

OUT

×

+

∆

∆

=

∆

∆

NOTES

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

SPAN TRIM

8

4

2N1711

1

2

3

REF43GPZ

6

5

7

1N4002

V+

8V TO 40V

R10

100

Ω

1%, 1/2 W

R8

1k

Ω

R9

100k

Ω

R4

20k

Ω

R6

3k

Ω

R7

5k

Ω

ZERO

TRIM

R5

5k

Ω

R2

1k

Ω

1/2

OP293

–

+

1/2

OP293

–

+

R3

100k

Ω

R1, 10k

Ω

TEMP

GND

2

6

3

4

Figure 35. Temperature to 4 mA to 20 mA Transmitter