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AD590JR-REEL

Two-Terminal IC Temperature Transducer

Analog Devices 

V+

4mA = 17°C

12mA = 25°C

20mA = 33°C

+ AD581

– VOUT

35.7kΩ

+

RT

AD590

5kΩ

–

0.01µF

10kΩ

12.7kΩ

10Ω

30pF

–

AD707A

+

5kΩ 500Ω

V–

Figure 18. 4 mA to 20 mA Current Transmitter

Figure 19 is an example of a variable temperature control circuit

(thermostat) using the AD590. RH and RL are selected to set the

high and low limits for RSET. RSET could be a simple pot, a

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

voltage (~7 V) across it. Capacitor C1 is often needed to filter

extraneous noise from remote sensors. RB is determined by the

β of the power transistor and the current requirements of the

load.

V+

V+

AD581

OUT

10V

V–

+

RH AD590

RB

RSET

– 2– 7

LM311

RL

3+ 4

1

C1

10kΩ

HEATING

ELEMENTS

GND

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.

AD590

20pF

–15V

DAC OUT

BIT 1

BIT 2

BIT 3

BIT 4

MC

1408/1508

1.25kΩ

REF

+5V

BIT 8

BIT 7

BIT 6

BIT 5

1.15kΩ

200Ω, 15T

+5V

+2.5V

AD580

200Ω

6.98kΩ

+5V +5V

1kΩ, 15T 3

8

+

LM311 7

AD590 2

–

1

4

1kΩ

OUTPUT HIGH-

TEMPERATURE ABOVE SET POINT

OUTPUT LOW-

TEMPERATURE BELOW SET POINT

–15V

–15V

5.1MΩ

6.8kΩ

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

+–

+–

+–

–

1kΩ (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.

Rev. C | Page 11 of 16

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