TMP35/TMP36/TMP37

Rev. F | Page 15 of 20

USING TMP3x SENSORS IN REMOTE LOCATIONS

In many industrial environments, sensors are required to

operate in the presence of high ambient noise. These noise

sources take many forms, for example, SCR transients, relays,

radio transmitters, arc welders, and ac motors. They can also

be used at considerable distances from the signal conditioning

circuitry. These high noise environments are typically in the

form of electric fields, so the voltage output of the temperature

sensor can be susceptible to contamination from these noise

sources.

Figure 32 illustrates 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 are not capable of high output current

operation; thus, a standard PNP transistor is used to boost the

output current drive of the circuit. As shown in the table in

Figure 32, the values of R2 and R3 were chosen to produce an

arbitrary 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 ambient magnetic fields operating in the near vicinity of the

circuit/cable pair. Because the circuit uses an external transistor,

the minimum recommended operating voltage for this circuit is

5 V. To minimize the effects of EMI (or RFI), both the circuit

and the temperature sensor supply pins are bypassed with good

quality ceramic capacitors.

TWISTED PAIR

BELDEN TYPE 9502

OR EQUIVALENT

TMP3x

R2

R1

4.7kΩ

0.1µF

2N2907

0.01µF

GND

5V

R3

SENSOR R2

R3

TMP35

634

634

TMP36

887

887

TMP37

1k

1k

Figure 32. Remote, 2-Wire Boosted Output Current Temperature Sensor

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, which can be used to power the signal conditioning

circuitry of the transmitter. The full-scale output signal in these

transmitters is 20 mA.

Figure 33 illustrates a circuit that transmits temperature inform-

ation in this fashion. Using a TMP3x as the temperature sensor,

the output current is linearly proportional to the temperature of

the medium. The entire circuit operates from the 3 V output of

the REF193. The REF193 requires no external trimming because

of its tight initial output voltage tolerance and the low supply

current of the 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 Pin 3 of the OP193 is given by

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

×

+

×

×

⎟

⎠

⎞

⎜

⎝

⎛

=

R2

R3

V

R1

R3

TMP3x

R7

1

I

REF

OUT

For each temperature sensor, Table 5 provides the values for the

components P1, P2, and R1 to R4.

Table 5. Circuit Element Values for Loop Transmitter

Sensor

R1

P1

R2

P2

R3

R4

TMP35

97.6 kΩ

5 kΩ

1.58 MΩ

100 kΩ

140 kΩ

56.2 kΩ

TMP36

97.6 kΩ

5 kΩ

931 kΩ

50 kΩ

97.6 kΩ

47 kΩ

TMP37

97.6 kΩ

5 kΩ

10.5 kΩ

500 Ω

84.5 kΩ

8.45 kΩ

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

full-scale gain trim of the circuit 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 transients from pulling the noninverting input of the

OP193 more than 300 mV below its inverting input. Without

this diode, such transients can cause phase reversal of the

operational amplifier and possible latch-up of the transmitter.

The loop supply voltage compliance of the circuit is limited by

the maximum applied input voltage to the REF193; it is from

9 V to 18 V.