TMP01

Rev. E | Page 14 of 20

The current is proportional to the voltage on the VPTAT

output, 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

⎟

⎠

⎞

⎜

⎝

⎛

⎟

⎠

⎞

⎜

⎝

⎛ +

+

×

−

×

=

2

5

1

1

3

3

2

5

6

1

R

R

R

R

R

VREF

R

R

VPTAT

R

I

OUT

The resulting temperature coefficient of the output current is

128 μA/°C.

5

8

1

4

2N1711

VREF

GND

V+

VPTAT

TMP01

OP90

4–20mA

5V TO 13.2V

7

4

3

6

2

R6

100

Ω

R1

243k

Ω

R2

39.2k

Ω

R3

100k

Ω

R5

100k

Ω

Figure 27. 4mA to 20 mA Current Loop

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

VREF remains constant over temperature. Thus, the ratio of

as 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 kΩ is chosen for R5. Once R2 is

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

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

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

high accuracy. For initial accuracy, a 10 kΩ trim potentiometer

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

to 95 kΩ. 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-

collector digital output. A digital signal is 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

successfully transmitted.

A simple circuit to do this combines the TMP01 with an AD654

VFC, as shown in Figure 28. The AD654 outputs a square wave

that is proportional to the dc input voltage according to the

following equation:

T

IN

OUT

C

R

R

V

F

)

2

1

(

10

+

=

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, consult the AD654

data sheet. Finally, the AD650 can be used to accurately convert

the frequency back to a dc voltage on the receiving end.

TEMPERATURE

SENSOR AND

VOLTAGE

REFERENCE

VREF

VPTAT

VPTAT

1

2

3

4

8

7

6

5

HYSTERESIS

GENERATOR

WINDOW

COMPARATOR

TMP01

R1

R2

R3

V+

V+

V+

4

3

1

2

5

AD654

OSC

7

8

6

R1

1.8k

Ω

R2

500

Ω

5k

Ω

0.1µF

Figure 28. Temperature-to-Frequency Converter