ADT14GP AD [Analog Devices], ADT14GP Datasheet - Page 13

• Manufacturer Part Number: ADT14GP
• Description: Quad Setpoint, Programmable Temperature Monitor and Controller
• Manufacturer: AD [Analog Devices]

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.
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
V
REV. 0
REF
NC
NC
voltage will exactly cancel the VPTAT voltage at 0.0 C.
1
2
4
6
8
3
5
7
NC = NO CONNECT
ADT14
Figure 25. Out-of-Range Warning
HYSTERESIS
GENERATOR
REFERENCE
& VOLTAGE
SENSOR
TEMP
ADT14
VPTAT
V
REF
16
15
14
13
12
11
10
9
14
6
NC
6.49k
V
REF
Figure 27. Translating 5 mV/K to 10 mV/ F
100k
100k
121
2N1711
47k
2
3
200
LED
90.9k
1/2 OP297
10pF
V+
+15V
–15V
7
4
1k
–13–
6
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 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.
ADT14
VPTAT
100k
V
REF
Figure 26. Translating 5 mV/K to 10 mV/ C
14
6
4.12k
6
5
1/2 OP297
100k
100k
100k
487
7
2
3
105k
OP177
10pF
+15V
–15V
7
4
4.22k
6
ADT14
V
(V
@ T = 0.0 C)
OUT
OUT
(10mV/ C)
= 0.0V