ADT14

–13–

REV. 0

Out-of-Range Warning

In Figure 25, connecting an open-collector output of the

ADT14 and an inverted open-collector output together into a

wired-OR configuration, a temperature “out-of-range” warning

signal is generated. This can be useful in sensitive equipment

calibrated to work over a limited temperature range.

TEMP

SENSOR

& VOLTAGE

REFERENCE

HYSTERESIS

GENERATOR

13

14

15

9

10

11

12

16

6

7

8

1

2

3

4

5

NC

NC

47k

Ω

200

Ω

LED

V+

NC

NC = NO CONNECT

ADT14

2N1711

Figure 25. Out-of-Range Warning

Translating 5 mV/K to 10 mV/ C

A useful circuit is shown in Figure 26 that translates the

VPTAT output voltage, which is calibrated in Kelvins, into an

output that can be read directly in degrees Celsius on a voltme-

ter display. To accomplish this, an external amplifier is config-

ured as a differential amplifier. The resistors are scaled so the

°C.

6

7

2

3

4

OP177

+15V

105k

Ω

4.22k

Ω

10pF

–15V

100k

Ω

100k

Ω

4.12k

Ω 487Ω

@ T = 0.0

°C)

ADT14

VPTAT

14

6

Figure 26. Translating 5 mV/K to 10 mV/ C

However, the gain from VPTAT to the output is two, so that

5 mV/K becomes 10 mV/

°C. Thus, for a temperature of +80°C,

the output voltage is 800 mV. Circuit errors will be due prima-

rily to the inaccuracies of the resistor values. Using 1% resistors

the observed error was less than 10 mV, or 1

°C. The 10 pF

feedback capacitor helps to cancel the effects of stray capaci-

tance that could cause oscillations. For improved accuracy, an

adjustment potentiometer can be added in series with either

100 k

Ω resistor.

Translating VPTAT to the Fahrenheit Scale

A similar circuit to the one shown in Figure 26 can be used to

translate VPTAT into an output that can be read directly in

degrees Fahrenheit, with a scaling of 10 mV/

°F. Only unity gain

or less is available from the first stage differentiating circuit, so

the second amplifier provides a gain of two to complete the

conversion to the Fahrenheit scale. Using the circuit in Figure

27, a temperature of 0.0

°F gives an output of 0.00 V. At room

temperature (77

°F) the output voltage is 770 mV. A –40°C to

+85

°C operating range translates into –40°F to +85°F. The

errors are essentially the same as for the circuit in Figure 26.

6

7

2

3

4

1/2 OP297

+15V

90.9k

Ω

1k

Ω

10pF

–15V

100k

Ω

100k

Ω

6.49k

Ω 121Ω

ADT14

VPTAT

14

6

7

6

5

1/2 OP297

100k

Ω

100k

Ω

Figure 27. Translating 5 mV/K to 10 mV/ F