TMP35/TMP36/TMP37

Rev. F | Page 16 of 20

4

6

7

1µF

R5

100kΩ

250Ω

9V TO 18V

3

2

D1: HP5082-2810

REF193

TMP3x

R7

100Ω

4mA

ADJUST

D1

R6

100kΩ

20mA

ADJUST

GND

Q1

2N1711

0.1µF

2

4

6

3V

NOTE:

–

+

+

OP193

Figure 33. Temperature to 4–20 mA Loop Transmitter

TEMPERATURE-TO-FREQUENCY CONVERTER

Another common method of transmitting analog information

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

value 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 sig-

nal. When the conversions between temperature and frequency

are done accurately, the temperature data from the sensors can

be reliably transmitted.

The circuit in Figure 34 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

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

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

×

×

−

=

)

(

10

T

T

OFFSET

TPM

OUT

C

R

V

V

f

TMP3x

GND

6

4

2

3

7

8

5

1

AD654

10µF/0.1µF

5V

P2

100kΩ

470Ω

OFFSET

10Ω

R1

P1

0.1µF

5V

5kΩ

NOTE:

TMP35

TMP36

TMP37

11.8kΩ + 500Ω

16.2kΩ + 500Ω

18.2kΩ + 1kΩ

1.7nF

1.8nF

2.1nF

Figure 34. Temperature-to-Frequency Converter