AD680
LOW POWER, LOW VOLTAGE REFERENCE FOR
DATA CONVERTERS
TEMPERATURE OUTPUT PIN
The 8-lead package 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. VTEMP
at 25°C is 596 mV, and the temperature coefficient is 2 mV/°C.
Figure 16 shows the output of this pin over temperature.
The AD680 has a number of features that make it ideally suited
for use with ADCs and DACs. The low supply voltage required
makes it possible to use the AD680 with today’s converters that
run on 5 V supplies without having to add a higher supply
voltage for the reference. The low quiescent current (195 μA),
combined with the completeness and accuracy of the AD680,
make it ideal for low power applications, such as hand-held,
battery-operated meters.
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. This enables the TEMP pin to
be buffered by many inexpensive operational amplifiers that
have bias currents below this value.
The AD7701 is an ADC that is well-suited for the AD680.
Figure 18 shows the AD680 used as the reference for this
converter. The AD7701 is a 16-bit ADC with on-chip digital
filtering intended for the measurement of wide dynamic range
and low frequency signals, such as those representing chemical,
physical, or biological processes. It contains a charge balancing
(Σ–Δ) ADC, a calibration microcontroller with on-chip static
RAM, a clock oscillator, and a serial communications port.
760
720
680
640
600
560
520
480
440
This entire circuit runs on 5 V supplies. The power dissipation
of the AD7701 is typically 25 mW and, when combined with
the power dissipation of the AD680 (1 mW), the entire circuit
consumes just 26 mW of power.
+5V
ANALOG
SUPPLY
–50 –40 –30 –20 –10
0
10 20 30 40 50 60 70 80 90
0.1μF
10μF
TEMPERATURE (°C)
AD7701
AV
V
DD
Figure 16. TEMP Pin Transfer Characteristics
DV
DD
V
IN
0.1μF
SLEEP
MODE
DIFFERENTIAL TEMPERATURE TRANSDUCER
V
OUT
REF
0.1μF
Figure 17 shows a differential temperature transducer that can
be used to measure temperature changes in the environment of
the AD680. This circuit operates from a 5 V supply. The
temperature-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 temperature, use resistors with low temperature
coefficients, such as metal film resistors.
AD680
DATA READY
DRDY
CS
GND
READ (TRANSMIT)
SERIAL CLOCK
SERIAL DATA
SCLK
RANGE
SELECT
SDATA
BP/UP
CAL
CLKIN
CALIBRATE
ANALOG
INPUT
CLKOUT
SC1
A
IN
ANALOG
GND
AGND
SC2
0.1μF
DGND
0.1μF
DV
SS
AV
SS
5V
–5V
ANALOG
SUPPLY
2
5V
0.1μF
10μF
V
IN
7
3
TEMP
3
2
+
0.1μF
Δ
V
Δ
OUT
T
= 10mV/°C
6
Figure 18. Low Power, Low Voltage Supply Reference
for the AD7701 16-Bit ADC
OP90
AD680
–
4
GND
4
R
B
R
F
1.69kΩ
1%
6.98kΩ
1%
R
BP
100Ω
Figure 17. Differential Temperature Transducer
Rev. H | Page 9 of 12
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