TMP17

REV. 0

–7–

A variable temperature controlling thermostat can easily be built

using the TMP17 in the circuit of Figure 18.

TMP17

10k

Ω

+15V

AD790

COMPARATOR

(OPTIONAL)

C

62.7k

Ω

10k

Ω

C

TEMP > SETPOINT

OUTPUT HIGH

TEMP < SETPOINT

OUTPUT LOW

27.3k

Ω

REF01E

10V

Figure 18. Variable Temperature Thermostat

R

by the potentiometer R

temperature range

25

°C to 105°C. The reference maintains

a constant set point voltage and insures that approximately 7 V

appears across the sensor. If it is necessary to guardband for

extraneous noise, hysteresis can be added by tying a resistor

from the output to the ungrounded end of R

LOW.

Multiple remote temperatures can be measured using several

TMP17s with a CMOS multiplexer or a series of 5 V logic gates

because of the device’s current-mode output and supply-voltage

compliance range. The on-resistance of a FET switch or output

impedance of a gate will not affect the accuracy, as long as 4 V

is maintained across the transducer. Muxes and logic driving

circuits should be chosen to minimize leakage current related

errors. Figure 19 illustrates a locally controlled mux switching

the signal current from several remote TMP17s. CMOS or TTL

gates can also be used to switch the TMP17 supply voltages,

with the multiplexed signal being transmitted over a single

twisted pair to the load.

AD7501

D

E

C

O

D

E

R

/

D

R

I

V

E

R

REMOTE

TMP17s

S1

S2

S8

TTL DTL TO

CMOS I/O

CHANNEL

SELECT

+15V

–15V

10k

Ω

Figure 19. Remote Temperature Multiplexing

To minimize the number of muxes required when a large

number of TMP17s are being used, the circuit can be config-

ured in a matrix. That is, a decoder can be used to switch the

supply voltage to a column of TMP17s while a mux is used to

control which row of sensors are being measured. The maxi-

mum number of TMP17s which can be used is the product of

the number of channels of the decoder and mux.

An example circuit controlling 80 TMP17s is shown in Figure

20. A 7-bit digital word is all that is required to select one of

the sensors. The enable input of the multiplexer turns all the

sensors off for minimum dissipation while idling.

+15V

COLUMN

SELECT

4028 BCD TO DECIMAL DECODER

ROW

SELECT

+15V

–15V

80 – TMP17s

10k

Ω

Figure 20. Matrix Multiplexer

To convert the TMP17 output to

°C or °F a single inexpensive

reference and op amp can be used as shown in Figure 21.

Although this circuit is similar to the two temperature trim

circuit shown in Figure 11, two important differences exist.

First, the gain resistor is fixed alleviating the need for an

elevated temperature trim. Acceptable accuracy can be achieved

by choosing an inexpensive resistor with the correct tolerance.

Second, the TMP17 calibration error can be trimmed out at a

known convenient temperature (i.e., room temperature) with a

single pot adjustment. This step is independent of the gain

selection.

R

OP196

+5V

REF43

V–

C

F

≈ 9.1kΩ

≈ 9.8kΩ

100k

Ω

180k

Ω

TMP17

2.5V

Figure 21. Celsius or Fahrenheit Thermometer