REV. C

TMP35/TMP36/TMP37

–8–

Note the 0.1

µF bypass capacitor on the input. This capacitor

should be a ceramic type, have very short leads (surface mount

would be preferable), and be located as close a physical proxim-

ity to the temperature sensor supply pin as practical. Since these

temperature sensors operate on very little supply current and

could be exposed to very hostile electrical environments, it is

important to minimize the effects of RFI (radio frequency

interference) on these devices. The effect of RFI on these

temperature sensors in specific and analog ICs in general is

manifested as abnormal dc shifts in the output voltage due to

the rectification of the high frequency ambient noise by the IC.

In those cases where the devices are operated in the presence of

high frequency radiated or conducted noise, a large value tanta-

lum capacitor ( 2.2

µF) placed across the 0.1 µF ceramic may

offer additional noise immunity.

Fahrenheit Thermometers

Although the TMP3x temperature sensors are centigrade tem-

perature sensors, a few components can be used to convert the

output voltage and transfer characteristics to directly read Fahr-

enheit temperatures. Shown in Figure 5a is an example of a

simple Fahrenheit thermometer using either the TMP35 or the

TMP37. This circuit can be used to sense temperatures from

41

°F to 257°F, with an output transfer characteristic of 1 mV/°F

using the TMP35 and from 41

°F to 212°F using the TMP37

with an output characteristic of 2 mV/

°F. This particular

approach does not lend itself well to the TMP36 because of its

inherent 0.5 V output offset. The circuit is constructed with an

AD589, a 1.23 V voltage reference, and four resistors whose values

for each sensor are shown in the figure table. The scaling of the

output resistance levels was to ensure minimum output loading

on the temperature sensors. A generalized expression for the

circuit’s transfer equation is given by:

=

R1

R1

+ R2









TMP 35

R3

+ R4









AD589

()

where: TMP35 = Output voltage of the TMP35, or the TMP37,

AD589 = Output voltage of the reference = 1.23 V.

Note that the output voltage of this circuit is not referenced to

the circuit’s common. If this output voltage were to be applied

directly to the input of an ADC, the ADC’s common should be

adjusted accordingly.

SENSOR

TMP35

1mV/ F

45.3

10

10

374

TMP37

2mV/ F

45.3

10

10

182

R2 (k )R3 (k )R4 (k )

PIN ASSIGNMENTS

TMP35/37

GND

R1

R2

R3

R4

AD589

1.23V

0.1 F

Figure 5a. TMP35/TMP37 Fahrenheit Thermometers

The same circuit principles can be applied to the TMP36, but

because of the TMP36’s inherent offset, the circuit uses two less

resistors as shown in Figure 5b. In this circuit, the output

voltage transfer characteristic is 1 mV/

°F but is referenced to

the circuit’s common; however, there is a 58 mV (58

°F) offset

in the output voltage. For example, the output voltage of the

circuit would read 18 mV were the TMP36 placed in –40

°F

ambient environment and 315 mV at 257

°F.

TMP36

GND

0.1 F

R1

45.3k

R2

10k

Figure 5b. TMP36 Fahrenheit Thermometer Version 1

At the expense of additional circuitry, the offset produced by the

circuit in Figure 5b can be avoided by using the circuit in Figure 5c. In

this circuit, the output of the TMP36 is conditioned by a single-

supply, micropower op amp, the OP193. Although the entire

circuit operates from a single 3 V supply, the output voltage of the

circuit reads the temperature directly, with a transfer character-

istic of 1 mV/

°F, without offset. This is accomplished through

the use of an ADM660, a supply voltage inverter. The 3 V

supply is inverted and applied to the P193’s V– terminal. Thus,

for a temperature range between –40

°F and +257°F, the

output of the circuit reads –40 mV to +257 mV. A general

expression for the circuit’s transfer equation is given by:

=

R6

R5

+ R6









1

+

R4

R3









TMP 36

R3









2









Average and Differential Temperature Measurement

In many commercial and industrial environments, temperature

sensors are often used to measure the average temperature in a

building, or the difference in temperature between two locations

on a factory floor or in an industrial process. The circuits in

Figures 6a and 6b demonstrate an inexpensive approach

to average and differential temperature measurement.

In Figure 6a, an OP193 is used to sum the outputs of three

temperature sensors to produce an output voltage scaled by

10 mV/

°C that represents the average temperature at three loca-

tions. The circuit can be extended to as many temperature

sensors as required as long as the circuit’s transfer equation

is maintained. In this application, it is recommended that one

temperature sensor type be used throughout the circuit; other-

wise, the output voltage of the circuit will not produce an

accurate reading of the various ambient conditions.