TMP01

REV. C

–13–

4 mA-20 mA Current Loop

Another, very common method of transmitting a signal over

long distances is to use a 4 mA-20 mA Loop, as shown in Fig-

ure 19. An advantage of using a 4 mA-20 mA loop is that the

accuracy of a current loop is not compromised by voltage drops

across the line. One requirement of 4 mA-20 mA circuits is that

the remote end must receive all of its power from the loop,

meaning that the circuit must consume less than 4 mA. Operat-

ing from +5 V, the quiescent current of the TMP01 is 500

µA

max, and the OP90s is 20

µA max, totaling less than 4 mA.

Although not shown, the open collector outputs and tempera-

ture setting pins can be connected to do any local control of

switching.

The current is proportional to the voltage on the VPTAT out-

put, and is calibrated to 4 mA at a temperature of –40

°C, to

20 mA for +85

°C. The main equation governing the operation

of this circuit gives the current as a function of VPTAT:

I

1

R6

VPTAT

× R5

R2

–

VREF

× R3

R3

+ R1

1

+

R5

R2

















The resulting temperature coefficient of the output current is

128

µA/°C.

5

8

1

4

R

L

2N1711

VREF

GND

V+

VPTAT

TMP01

R5

100k

Ω

R2

39.2k

Ω

7

6

4

3

2

OP90

R1

243k

Ω

R3

100k

Ω

R6

100

Ω

4–20mA

+5V TO +13.2V

Figure 19. 4-20 mA Current Loop

To determine the resistor values in this circuit, first note that

VREF remains constant over temperature. Thus the ratio of R5

VPTAT varies from 1.165 V at –40

°C to 1.79 V at +85°C. The

absolute value of the resistors is not important, only the ratio.

For convenience, 100 k

Ω is chosen for R5. Once R2 is calcu-

lated, the value of R3 and R1 is determined by substituting

values are shown in the circuit. The OP90 is chosen for this cir-

cuit because of its ability to operate on a single supply and its

high accuracy. For initial accuracy, a 10 k

Ω trim potentiometer

can be included in series with R3, and the value of R3 lowered

to 95 k

Ω. The potentiometer should be adjusted to produce an

output current of 12.3 mA at 25

°C.

Temperature-to-Frequency Converter

Another common method of transmitting analog information is

to convert a voltage to the frequency domain. This is easily

done with any of the low cost monolithic Voltage-to-Frequency

Converters (VFCs) available, which feature a robust, open-col-

lector digital output. A digital signal is very immune to noise

and voltage drops because the only important information is the

frequency. As long as the conversions between temperature and

frequency are done accurately, the temperature data can be suc-

cessfully transmitted.

A simple circuit to do this combines the TMP01 with an

AD654 VFC, as shown in Figure 20. The AD654 outputs a

square wave that is proportional to the dc input voltage accord-

ing to the following equation:

F

V

IN

10 (R1

+ R2) C

T

By simply connecting the VPTAT output to the input of the

AD654, the 5 mV/

°C temperature coefficient gives a sensitivity

of 25 Hz/

°C, centered around 7.5 kHz at 25°C. The trimming

resistor R2 is needed to calibrate the absolute accuracy of the

AD654. For more information on that part, please consult the

AD654 data sheet. Finally, the AD650 can be used to accu-

rately convert the frequency back to a dc voltage on the receiv-

ing end.

4

3

7

6

8

1

2

5

AD654

VPTAT

V+

R1

R2

R3

TEMPERATURE

SENSOR &

VOLTAGE

REFERENCE

1

2

3

4

HYSTERESIS

GENERATOR

WINDOW

COMPARATOR

TMP01

VPTAT

VREF

7

8

5

6

R1

1.8k

Ω

OSC

V+

F

OUT

C

T

0.1

µF

5k

Ω

V+

R2

500

Ω

Figure 20. Temperature-to-Frequency Converter