AD7856 datasheet(20/33 Pages) AD

20 / 33 page

AD7856

–19–

REV. A

A combination of hardware and software selection can also be

used to achieve the desired effect.

Table VI. Power Management Options

PMGT1

PMGT0

SLEEP

Bit

Comment

Full Power-Down Between

Conversions (HW/SW)

Full Power-Up (HW/SW)

Normal Operation

(Independent of the

SLEEP

Pin)

Full Power-Down (SW)

Partial Power-Down Between

Conversions

NOTE

HW = Hardware Selection; SW = Software Selection.

POWER-UP TIMES

Using an External Reference

When the AD7856 is powered up, the part is powered up from

one of two conditions. First, when the power supplies are ini-

tially powered up and, secondly, when the part is powered up

from either a hardware or software power-down (see last section).

When AVDD and DVDD are powered up, the AD7856 should be

left idle for approximately 42 ms (6 MHz CLK) to allow for the

autocalibration if a 10 nF cap is placed on the

CAL pin, (see

Calibration section). During power-up the functionality of the

SLEEP pin is disabled, i.e., the part will not power down until

the end of the calibration if

SLEEP is tied logic low. The auto-

calibration on power-up can be disabled if the

CAL pin is tied to

a logic high. If the autocalibration is disabled, then the user must

take into account the time required by the AD7856 to power-up

before a self-calibration is carried out. This power-up time is the

time taken for the AD7856 to power up when power is first

applied (300

µs) typ) or the time it takes the external reference

to settle to the 14-bit level–whichever is the longer.

AUTO POWER

DOWN AFTER

CONVERSION

AUTO CAL ON

POWER-UP

AVDD DVDD

AIN(+)

AIN(–)

CREF1

CREF2

SLEEP

DIN

DOUT

SYNC

CONVST

AGND

DGND

CLKIN

SCLK

REFIN/REFOUT

AD7856

ANALOG

SUPPLY

+5V

0.1 F

10 F

0.1 F

0.01 F

MASTER CLOCK

INPUT

CONVERSION

START INPUT

SERIAL DATA OUTPUT

0.1 F

CAL

0.01 F

INTERNAL

REFERENCE

0V TO 2.5V

INPUT

4/6MHz OSCILLATOR

SERIAL CLOCK

INPUT

100kHz PULSE GENERATOR

OPTIONAL

EXTERNAL

REFERENCE

REF-192

SERIAL DATA INPUT

LOW POWER

C/ P

CURRENT, I = 12mA TYP

Figure 21. Typical Low Power Circuit

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