Analog Devices AN-614 Application Notes

9106

www.analog.com

word rate. It contains two differential analog input chan-

to power down the device between conversions, which

As shown in Figure 1, the AD7782 has a 3-wire serial

CS

AIN1(+)

AIN1(–)

AIN2(+)

AIN2(–)

CH1/CH2 RANGE

AD7782

V

DD

GND

BUF PGA

REFIN(–)REFIN(+)

MUX

24-BIT -

ADC

OSCILLATOR

AND

PLL

DOUT/RDY

SCLK

MODE

XTAL1 XTAL2

SERIAL

INTERFACE

AND

CONTROL

LOGIC

CS

will continuously convert with an update rate of 19.79 Hz.

CS

pin operates as a power-down pin as well as

CS

is high, continu-

RDY

becomes

three-stated, the AD7782 enters standby mode, and any

CS

goes low, the phase locked loop

then begins converting the selected channel.

MSB

DOUT/RDY

SCLK

AD7782 IS POWERED DOWN

CS

LSB

MSB

AD7782 IS POWERED UP

SCLK IS AN INPUT IN SLAVE MODE AND AN OUTPUT IN MASTER MODE.

–2

–

tered trademarks are the property of their respective companies.

When the AD7782 is powered up, it consumes 1.3 mA

typical at 3 V and 1.5 mA typical at 5 V. In power-down

typical at 5 V. By placing the AD7782 in power-down

CS

until the next conversion is required.

As seen in Figure 2, taking

CS

low powers up the AD7782.

After power-up, the ADC requires some time to settle

to convert. The sigma-delta ADC uses chopping, which

RDY

will remain high until a

valid conversion result is available. With an update rate

the AD7782 must remain powered-up for (2

50.5) +

Assuming a 3 V power supply, the average current equals

6) + (0.102

1300) = 138 µA.

With a 5 V power supply and, again, with one conversion

20) +

1500) = 171 µA.

functions in which the part performs one conversion each

6) + (0.00002833

1300) =

With a 5 V power supply, the average current equals

20) + (0.00002833

1500) = 20.04 µA.

which the ADC performs a single conversion, the AD7782

version. From the plot, the average current approaches

the power-down current speci cation when the time inter-

val between conversions is 15 seconds or greater.

3V

CONVERSION RATE – Minutes

180

0

0.01 1000.1

CURRENT – A

1 10

20

160

140

120

100

80

60

40

5V

CS

operates as both a chip-

ADC between conversions does not add additional