Datasheet AD680 (Analog Devices) - 9

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AD680 TEMPERATURE OUTPUT PIN LOW POWER, LOW VOLTAGE REFERENCE FOR DATA CONVERTERS
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 The AD680 has a number of features that make it ideally suited (TEMP) is a voltage that varies linearly with temperature. V for use with ADCs and DACs. The low supply voltage required TEMP at 25°C is 596 mV, and the temperature coefficient is 2 mV/°C. makes it possible to use the AD680 with today’s converters that Figure 16 shows the output of this pin over temperature. run on 5 V supplies without having to add a higher supply voltage for the reference. The low quiescent current (195 μA), The temperature pin has an output resistance of 12 kΩ and is combined with the completeness and accuracy of the AD680, capable of sinking or sourcing currents of up to 5 μA without make it ideal for low power applications, such as hand-held, disturbing the reference output. This enables the TEMP pin to battery-operated meters. 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
760
converter. The AD7701 is a 16-bit ADC with on-chip digital
720
filtering intended for the measurement of wide dynamic range and low frequency signals, such as those representing chemical,
680 V)
physical, or biological processes. It contains a charge balancing
m 640
(Σ–Δ) ADC, a calibration microcontroller with on-chip static
GE (
RAM, a clock oscillator, and a serial communications port.
600 VOLTA
This entire circuit runs on ±5 V supplies. The power dissipation
560 PIN P
of the AD7701 is typically 25 mW and, when combined with
TEM 520
the power dissipation of the AD680 (1 mW), the entire circuit consumes just 26 mW of power.
480 +5V 440
00813-016
ANALOG –50 –40 –30 –20 –10 0 10 20 30 40 50 60 70 80 90 SUPPLY 0.1
μ
F 10
μ
F TEMPERATURE (
°
C) AD7701 AV
Figure 16. TEMP Pin Transfer Characteristics
DD DVDD VIN DIFFERENTIAL TEMPERATURE TRANSDUCER SLEEP 0.1
μ
F VOUT VREF MODE 0.1
μ
F
Figure 17 shows a differential temperature transducer that can
AD680 DRDY DATA READY
be used to measure temperature changes in the environment of
GND READ (TRANSMIT)
the AD680. This circuit operates from a 5 V supply. The
CS SERIAL CLOCK
temperature-dependent voltage from the TEMP pin of the
SCLK RANGE SDATA
AD680 is amplified by a factor of 5 to provide wider full-scale
BP/UP SERIAL DATA SELECT
range and more current sourcing capability. An exact gain of 5
CLKIN CALIBRATE CAL
can be achieved by adjusting the trim potentiometer until the
ANALOG A CLKOUT INPUT IN
output varies by 10 mV/°C. To minimize resistance changes
SC1 ANALOG AGND
with temperature, use resistors with low temperature
GND SC2 0.1
μ
F
coefficients, such as metal film resistors.
DGND 0.1
μ
F 5V AV DV SS SS –5V ANALOG 2 5V SUPPLY 0.1
μ
F 10
μ
F VIN TEMP 3 3 7 +
00813-018
0.1
μ
F
Δ
VOUT 6 AD680 OP90
Δ
= 10mV/
°
C
Figure 18. Low Power, Low Voltage Supply Reference
T 2 –
for the AD7701 16-Bit ADC
4 GND 4 RB RF 1.69k
Ω
6.98k
Ω
1% 1% RBP 100
Ω 00813-017 Figure 17. Differential Temperature Transducer Rev. H | Page 9 of 12 Document Outline FEATURES GENERAL DESCRIPTION CONNECTION DIAGRAMS PRODUCT HIGHLIGHTS SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS OUTPUT PROTECTION ESD CAUTION PIN CONFIGURATIONS AND FUNCTION DESCRIPTIONS THEORY OF OPERATION APPLYING THE AD680 NOISE PERFORMANCE TURN-ON TIME DYNAMIC PERFORMANCE LOAD REGULATION TEMPERATURE PERFORMANCE TEMPERATURE OUTPUT PIN DIFFERENTIAL TEMPERATURE TRANSDUCER LOW POWER, LOW VOLTAGE REFERENCE FOR DATA CONVERTERS 4.5 V REFERENCE FROM A 5 V SUPPLY VOLTAGE REGULATOR FOR PORTABLE EQUIPMENT OUTLINE DIMENSIONS ORDERING GUIDE