Datasheet

AD680
REV. C
–6–
TEMPERATURE PERFORMANCE
The AD680 is designed for reference applications where tem-
perature performancc is important. Extensivc temperature test-
ing and characterization ensures that the device’s performance is
maintained over the specified temperature range.
Some confusion exists in thc area of defining and specifying ref-
erence voltage error over temperature. Historically, references
have been characterized using a maximum deviation per degree
centigrade, i.e., ppm/°C. However, because of nonlinearities in
temperature characteristics which originated in standard Zener
references (such as “S” type characteristics), most manufactur-
ers now use a maximum limit error band approach to specify
devices. This technique involves the measurement of the output
at three or more different temperatures to specify an output
voltage error band.
SLOPE = TC
=
V – V
MAX MIN
–6
(T – T ) x 2.5V x 10
MAX MIN
2.501 – 2.498
85 C – (–40 C) x 2.5V x 10
=
–6
= 9.6ppm/ C
2.501
2.500
2.499
2.498
–50 –30
–10 0
20 40
60 80
100
TEMPERATURE – C
Figure 9. Typical AD680AN/AP Temperature Drift
Figure 9 shows a typical output voltage drift for the AD680AN/
AR and illustrates the test methodology. The box in Figure 9 is
bounded on the sides by the operating temperature extremes,
and on the top and bottom by the maximum and minimum out-
put voltages measured over the operating temperature range.
The maximum height of thc box for the appropriate tempera-
ture range and device grade is shown in Table I. Duplication of
these results requires a combination of high accuracy and stable
temperature control in a test system. Evaluation of the AD680
will produce a curve similar to that in Figure 9, but output read-
ings may vary depending upon the test equipment utilized.
TEMPERATURE OUTPUT PIN
The 8-pin packaged versions of the AD680 provide a tempera-
ture output pin on Pin 3 of each device. The output of Pin 3
(TEMP) is a voltage that varies linearly with temperature.
V
TEMP
at 25°C is 596 mV, and the temperature coefficient is
2 mV/°C. Figure 10 shows the output of this pin over
temperature.
The temperature pin has an output resistance of 12 kΩ and is
capable of sinking or sourcing currents of up to 5 µA without
disturbing the reference output, enabling the temp pin to be
buffered by any of a number of inexpensive operational amplifi-
ers that have bias currents below this value.
760
440
90
520
480
–40–50
600
560
640
680
720
8060504020100–10–20–30 30 70
TEMPERATURE – C
TEMP PIN VOLTAGE – mV
Figure 10. Temp Pin Transfer Characteristic
DIFFERENTIAL TEMPERATURE TRANSDUCER
Figure 11 shows a differential temperature transducer that can
be used to measure temperature changes in the AD680’s envi-
ronment. This circuit operates from a +5 V supply. The tem-
perature dependent voltage from the TEMP pin of the AD680
is amplified by a factor of 5 to provide wider full-scale range and
more current sourcing capability. An exact gain of 5 can be
achieved by adjusting the trim potentiometer until the output
varies by 10 mV/°C. To minimize resistance changes with tem-
perature, resistors with low temperature coefficients, such as
metal film resistors, should be used.
3
6
7
2
3
4
4
V
IN
TEMP
GND
AD680
+5V
+5V
R
B
R
F
R
BP
100Ω
1.69kΩ
1%
6.98kΩ
1%
OP-90
2
0.1µF
= 10mV/ C
OUT
∆V
∆T
Figure 11. Differential Temperature Transducer
LOW POWER, LOW VOLTAGE REFERENCE FOR DATA
CONVERTERS
The AD680 has a number of features that make it ideally suited
for use with A/D and D/A converters. The low supply voltage
required makes it possible to use the AD680 with today’s
convertcrs that run on 5 V supplies without having to add a
higher supply voltage for the reference. The low quiescent cur-
rent (195 µA), combined with the completeness and accuracy of
the AD680 make it ideal for low power applications such as
handheld, battery operated meters.