AD590

Rev. C | Page 11 of 16

30pF

V+

4mA = 17°C

12mA = 25°C

20mA = 33°C

–

+

–

+

AD581

5k

Ω

10

Ω

10k

Ω

12.7k

Ω

35.7k

Ω

5k

Ω

500

Ω

AD590

AD707A

–

+

0.01

µF

V–

Figure 18. 4 mA to 20 mA Current Transmitter

Figure 19 is an example of a variable temperature control circuit

calibrated multiturn pot, or a switched resistive divider.

Powering the AD590 from the 10 V reference isolates the

AD590 from supply variations while maintaining a reasonable

β of the power transistor and the current requirements of the

load.

LM311

–

+

2

3

4

1

7

10k

Ω

V–

V+

V+

AD590

–

+

AD581

OUT

HEATING

ELEMENTS

GND

10V

Figure 19. Simple Temperature Control Circuit

Figure 20 shows that the AD590 can be configured with an 8-bit

DAC to produce a digitally controlled set point. This particular

circuit operates from 0°C (all inputs high) to 51.0°C (all inputs

low) in 0.2°C steps. The comparator is shown with 1.0°C

hysteresis, which is usually necessary to guard-band for

extraneous noise. Omitting the 5.1 MΩ resistor results in no

hysteresis.

MC

1408/1508

DAC OUT

–15V

+5V

REF

OUTPUT HIGH-

TEMPERATURE ABOVE SET POINT

OUTPUT LOW-

TEMPERATURE BELOW SET POINT

1.25k

Ω

+5V

+2.5V

AD580

20pF

BIT 1

BIT 8

BIT 2

BIT 7

BIT 3

BIT 6

BIT 4

BIT 5

1.15k

Ω

200

Ω

200

Ω, 15T

6.98k

Ω

1k

Ω, 15T

–15V

AD590

–

+

–15V

+5V

+5V

1

4

2

3

8

7

LM311

1k

Ω

5.1M

Ω

6.8k

Ω

Figure 20. DAC Set Point

The voltage compliance and the reverse blocking characteristic

of the AD590 allows it to be powered directly from 5 V CMOS

logic. This permits easy multiplexing, switching, or pulsing for

minimum internal heat dissipation. In Figure 21, any AD590

connected to a logic high passes a signal current through the

current measuring circuitry, while those connected to a logic

zero pass insignificant current. The outputs used to drive the

AD590s may be employed for other purposes, but the additional

capacitance due to the AD590 should be taken into account.

5V

CMOS

GATES

AD590

1k

Ω (0.1%)

–

+

–

+

–

+

–

+

Figure 21. AD590 Driven from CMOS Logic

CMOS analog multiplexers can also be used to switch AD590

current. Due to the AD590’s current mode, the resistance of

such switches is unimportant as long as 4 V is maintained

across the transducer. Figure 22 shows a circuit that combines

the principle demonstrated in Figure 21 with an 8-channel

CMOS multiplexer. The resulting circuit can select 1–80 sensors

over only 18 wires with a 7-bit binary word.