TMP35FS View Datasheet(PDF) - Analog Devices

Part Name

Description

Manufacturer

TMP35FS

Low Voltage Temperature Sensors

Analog Devices 

TMP35/TMP36/TMP37

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:

( ) fOUT

=





VTMP

10 ×

− VOFFSET

RT × CT





5V

10␮F/0.1␮F

0.1␮F

VS

TMP3x VOUT 4

3

GND

R1

CT*

86

7

1

AD654

5

2

RPU

5k⍀

fOUT

RT*

5V

P1

P2

100k⍀

ROFF1

470⍀

NB: ATTA (min), fOUT = 0Hz

fOUT *RT AND CT – SEE TABLE

OFFSET

ROFF2

10⍀

SENSOR

TMP35

TMP36

TMP37

RT (R1 + P1)

11.8k⍀ + 500⍀

16.2k⍀ + 500⍀

18.2k⍀ + 1k⍀

CT

1.7nF

1.8nF

2.1nF

Figure 11. A Temperature-to-Frequency Converter

An offset trim network (fOUT OFFSET ) is included with this

circuit to set fOUT at 0 Hz when the temperature sensor’s mini-

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.

3V 6

2

REF193

R2*

1␮F

4

VS

R1*

TMP3x

VOUT

GND

P2*

4mA

ADJUST

P1*

20mA

ADJUST

R3*

3

7

2

4

D1

R4*

0.1␮F

R6

100k⍀

Q1

2N1711

R5

100k⍀

VLOOP

9V TO 18V

VOUT

RL

250⍀

*SEE TEXT

FOR VALUES

D1: HP5082–2810

R7

100⍀

A1: OP193

IL

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

–12–

REV. C

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