AD7858LBR3 datasheet(19/32 Pages) AD

19 / 32 page

AD7858/AD7858L

REV. B

–19–

Table VI. Power Management Options

PMGT1 PMGT0

SLEEP

Bit

Comment

Full Power-Down if Not

Calibrating or Converting

(Default Condition

After Power-On)

Normal Operation

(Independent of the

SLEEP Pin)

Full Power-Down

Partial Power-Down if Not

Converting

A typical connection diagram for a low power application is

shown in Figure 21 (AD7858L is the low power version of the

AD7858).

AUTO POWER

DOWN AFTER

CONVERSION

AUTO CAL

POWER-UP

OPTIONAL

EXTERNAL

REFERENCE

1.8MHz

OSCILLATOR

MASTER

CLOCK

INPUT

0.1 F

10 F

ANALOG

SUPPLY

+3V

0V TO 2.5V

INPUT

0.1 F

0.01 F

0.1 F

INTERNAL

REFERENCE

100kHz

PULSE

GENERATOR

CONVERSION

START INPUT

SERIAL CLOCK

INPUT

SERIAL DATA

OUTPUT

CLKIN

SCLK

CONVST

SYNC

DIN

DOUT

DGND

AGND

CAL

SLEEP

CREF2

CREF1

AIN(–)

AIN(+)

AVDD DVDD

AD7858/

AD7858L

LOW

POWER

C/ P

CURRENT,

I = 1.5mA

TYP

SERIAL DATA

INPUT

REFIN /REFOUT

REF192

Figure 21. Typical Low Power Circuit

POWER-UP TIMES

Using an External Reference

When the AD7858 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 AD7858 should be

left idle for approximately 32 ms (4 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 AD7858 to power-up

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

time taken for the AD7858 to power up when power is first

applied (300

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

to settle to the 12-bit level–whichever is the longer.

The AD7858 powers up from a full hardware or software

power-down in 5

µs typ. This limits the throughput which the

part is capable of to 104 kSPS for the AD7858 operating with a

4 MHz CLK and 66 kSPS for the AD7858L with a 1.8 MHz

CLK when powering down between conversions. Figure 22

shows how power-down between conversions is implemented

using the

CONVST pin. The user first selects the power-down

between conversions option by using the

SLEEP pin and the

power management bits, PMGT1 and PMGT0, in the control

cally enters a full power-down at the end of a conversion, i.e.,

when BUSY goes low. The falling edge of the next

CONVST

pulse causes the part to power up. Assuming the external refer-

ence is left powered up, the AD7858 should be ready for normal

operation 5

µs after this falling edge. The rising edge of CONVST

initiates a conversion so the

CONVST pulse should be at least

µs wide. The part automatically powers down on completion

of the conversion.

5 s

tCONVERT

POWER-UP

TIME

NORMAL

OPERATION

FULL

POWER-DOWN

POWER-UP

TIME

START CONVERSION ON RISING EDGE

POWER-UP ON FALLING EDGE

CONVST

BUSY

Figure 22. Power-Up Timing When Using

CONVST Pin

NOTE: Where the software CONVST is used or automatic full

power-down, the part must be powered up in software with an

extra write setting PMGT1 = 0 and PMGT0 = 1 before a con-

version is initiated in the next write. Automatic partial power-

down after a calibration is not possible; the part must be

powered down manually. If software calibrations are to be used

when operating in the partial power-down mode, then three

separate writes are required. The first initiates the type of cali-

bration required, the second write powers the part down into

partial power-down mode, while the third write powers the part

up again before the next calibration command is issued.

Using the Internal (On-Chip) Reference

As in the case of an external reference, the AD7858 can power-

up from one of two conditions, power-up after the supplies are

connected or power-up from hardware/software power-down.

When using the on-chip reference and powering up when AVDD

and DVDD are first connected, it is recommended that the power-

up calibration mode be disabled as explained above. When using

the on-chip reference, the power-up time is effectively the time

it takes to charge up the external capacitor on the REFIN/REFOUT

pin. This time is given by the equation:

tUP = 9

× R × C

where R

≅ 150 kΩ and C = external capacitor.

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