REV. C

TMP35/TMP36/TMP37

–11–

A Temperature to 4–20 mA Loop Transmitter

In many process control applications, 2-wire transmitters are

used to convey analog signals through noisy ambient environ-

ments. These current transmitters use a “zero-scale” signal

current of 4 mA that can be used to power the transmitter’s

signal conditioning circuitry. The “full-scale” output signal in

these transmitters is 20 mA.

A circuit that transmits temperature information in this fashion

is illustrated in Figure 10. Using a TMP3x as the temperature

sensor, the output current is linearly proportional to the tem-

perature of the medium. The entire circuit operates from the

3 V output of the REF193. The REF193 requires no external

trimming for two reasons: (1) the REF193’s tight initial output

voltage tolerance and (2) the low supply current of TMP3x, the

OP193 and the REF193. The entire circuit consumes less than

3 mA from a total budget of 4 mA. The OP193 regulates the

output current to satisfy the current summation at the noninverting

node of the OP193. A generalized expression for the KCL

equation at the OP193’s Pin 3 is given by:

=

1

R 7









×

TMP 3x

× R3

R1

+

× R3

R2









For each of the three temperature sensors, the table below illus-

trates the values for each of the components, P1, P2, and R1–R4:

Table II. Circuit Element Values for Loop Transmitter

Sensor

R1( )

P1( )R2( )

P2( )R3( )R4( )

TMP35

97.6 k

5 k

1.58 M

100 k

140 k

56.2 k

TMP36

97.6 k

5 k

931 k

50 k

97.6 k

47 k

TMP37

97.6 k

5 k

10.5 k

500

84.5 k

8.45 k

Illustrated in Figure 9 is a way to convert the output voltage of a

TMP3x sensor into a current to be transmitted down a long

twisted-pair shielded cable to a ground referenced receiver. The

temperature sensors do not possess the capability of high output

current operation; thus, a garden variety PNP transistor is used

to boost the output current drive of the circuit. As shown in the

table, the values of R2 and R3 were chosen to produce an arbi-

trary full-scale output current of 2 mA. Lower values for the

full-scale current are not recommended. The minimum-scale

output current produced by the circuit could be contaminated

by nearby ambient magnetic fields operating in the vicinity of

the circuit/cable pair. Because of the use of an external transis-

tor, the minimum recommended operating voltage for this

circuit is 5 V. Note, to minimize the effects of EMI (or RFI),

both the circuit’s and the temperature sensor’s supply pins are

bypassed with good quality, ceramic capacitors.

TWISTED PAIR

BELDEN TYPE 9502

OR EQUIVALENT

TMP3x

R2

R1

4.7k

0.1 F

2N2907

0.01 F

GND

5V

R3

SENSOR

R2

R3

TMP35

634

634

TMP36

887

887

TMP37

1k

1k

Figure 9. A Remote, 2-Wire Boosted Output Current Tem-

perature Sensor

TMP35

0.1 F

GND

OP193

0.1 F

R1*

24.9k

R4

4.99k

R5*

1.21M

TYPE K

THERMO-

COUPLE

CU

CU

R2*

102

0V – 2.5V

R6

100k

5%

R3

10M

5%

COLD

JUNCTION

CHROMEL

ALUMEL

ISOTHERMAL

BLOCK

NOTE:

ALL RESISTORS 1%

UNLESS OTHERWISE NOTED

0 C

T

250 C

7

6

4

3

2

P1

50k

Figure 8. A Single-Supply, Type K Thermocouple Signal Conditioning Circuit with Cold-Junction Compensation