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AD680JN

Low Power, Low Cost 2.5 V Reference

Analog Devices 

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 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.

760

720

680

640

600

560

520

480

440

–50 –40 –30 –20 –10 0 10 20 30 40 50 60 70 80 90

TEMPERATURE (°C)

Figure 16. TEMP Pin Transfer Characteristics

DIFFERENTIAL TEMPERATURE TRANSDUCER

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.

5V

0.1μF

2

VIN

TEMP 3

AD680

GND

4

5V

3+ 7

OP90 6

2– 4

RBP

100Ω

RB

1.69kΩ

1%

RF

6.98kΩ

1%

ΔVOUT

ΔT

=

10mV/°C

AD680

LOW POWER, LOW VOLTAGE REFERENCE FOR

DATA CONVERTERS

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 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.

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

0.1μF

0.1μF

RANGE

SELECT

CALIBRATE

10μF

VIN

VOUT

AD680

GND

AD7701

AVDD

DVDD

VREF

SLEEP

MODE

BP/UP

CAL

DRDY

CS

SCLK

SDATA

CLKIN

ANALOG

INPUT

ANALOG

GND

0.1μF

AIN

AGND

CLKOUT

SC1

SC2

DGND

–5V

ANALOG

SUPPLY

0.1μF

AVSS

10μF

DVSS

0.1μF

DATA READY

READ (TRANSMIT)

SERIAL CLOCK

SERIAL DATA

0.1μF

Figure 18. Low Power, Low Voltage Supply Reference

for the AD7701 16-Bit ADC

Figure 17. Differential Temperature Transducer

Rev. H | Page 9 of 12

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