REV. C

TMP35/TMP36/TMP37

–12–

The 4 mA offset trim is provided by P2, and P1 provides the

circuit’s full-scale gain trim at 20 mA. These two trims do not

interact because the noninverting input of the OP193 is held at

a virtual ground. The zero-scale and full-scale output currents of

the circuit are adjusted according to the operating temperature

range of each temperature sensor. The Schottky diode, D1, is

required in this circuit to prevent loop supply power-on tran-

sients from pulling the noninverting input of the OP193 more

than 300 mV below its inverting input. Without this diode, such

transients could cause phase reversal of the operational amplifier

and possible latchup of the transmitter. The loop supply voltage

compliance of the circuit is limited by the maximum applied

input voltage to the REF193 and is from 9 V to 18 V.

A Temperature to Frequency Converter

Another common method of transmitting analog information

from a remote location is to convert a voltage to an equivalent in

the frequency domain. This is readily done with any of the

low cost, monolithic voltage-to-frequency converters (VFCs)

available. These VFCs feature a robust, open-collector output

transistor for easy interfacing to digital circuitry. The digital

signal produced by the VFC is less susceptible to contamination

from external noise sources and line voltage drops because the

only important information is the frequency of the digital signal.

As long as the conversions between temperature and frequency

are done accurately, the temperature data from the sensors can

be reliably transmitted.

The circuit in Figure 11 illustrates a method by which the

outputs of these temperature sensors can be converted to a

frequency using the AD654. The output signal of the AD654 is

a square wave that is proportional to the dc input voltage across

Pins 4 and 3. The transfer equation of the circuit is given by:

=

10

()













TMP3x

GND

6

4

2

3

7

8

5

1

AD654

10 F/0.1 F

5V

P2

100k

470

OFFSET

10

R1

P1

0.1 F

5V

5k

SENSOR

TMP35

TMP36

TMP37

11.8k

+ 500

16.2k

+ 500

18.2k

+ 1k

1.7nF

1.8nF

2.1nF

Figure 11. A Temperature-to-Frequency Converter

mum output voltage is reached. Potentiometer P1 is required to

calibrate the absolute accuracy of the AD654. The table in

Figure 11 illustrates the circuit element values for each of the

three sensors. The nominal offset voltage required for 0 Hz

output from the TMP35 is 50 mV; for the TMP36 and

TMP37, the offset voltage required is 100 mV. In all cases

for the circuit values shown, the output frequency transfer

characteristic of the circuit was set at 50 Hz/

°C. At the receiving

end, a frequency-to-voltage converter (FVC) can be used to

convert the frequency back to a dc voltage for further process-

ing. One such FVC is the AD650.

For complete information on the AD650 and AD654, please

consult the individual data sheets for those devices.

4

7

1 F

R5

100k

250

9V TO 18V

2

3

D1: HP5082–2810

REF193

TMP3x

R7

100

A1: OP193

*SEE TEXT

FOR VALUES

R3*

R1*

R2*

P2*

4mA

ADJUST

D1

R4*

R6

100k

P1*

20mA

ADJUST

GND

Q1

2N1711

0.1 F

2

4

6

3V

Figure 10. A Temperature to 4-to-20 mA Loop Transmitter