EVAL-AD7859CB4 datasheet(19/28 Pages) AD

Part #	EVAL-AD7859CB4

Description	3 V to 5 V Single Supply, 200 kSPS8-Channel, 12-Bit Sampling ADCs

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AD7859/AD7859L

REV. A

–19–

POWER VS. THROUGHPUT RATE

The main advantage of a full power-down after a conversion is

that it significantly reduces the power consumption of the part

at lower throughput rates. When using this mode of operation,

the AD7859/AD7859L is only powered up for the duration of

the conversion. If the power-up time of the AD7859/AD7859L

is taken to be 5

µs and it is assumed that the current during

power up is 4.5 mA/1.5 mA typ, then power consumption as a

function of throughput can easily be calculated. The AD7859

has a conversion time of 4.6

µs with a 4 MHz external clock and

the AD7859L has a conversion time of 9

µs with a 1.8 MHz

clock. This means the AD7859/AD7859L consumes 4.5 mA/

1.5 mA typ for 9.6

µs/14 µs in every conversion cycle if the parts

are powered down at the end of a conversion. The two graphs,

Figure 24 and Figure 25, show the power consumption of the

AD7859 and AD7859L for VDD = 3 V as a function of through-

put. Table VIII lists the power consumption for various

throughput rates.

Table VIII. Power Consumption vs. Throughput

Power

Throughput Rate

AD7859

AD7859L

1 kSPS

130

µW65 µW

10 kSPS

1.3 mW

650

µW

20 kSPS

2.6 mW

1.25 mW

50 kSPS

6.48 mW

3.2 mW

1.8MHz

OSCILLATOR

AVDD DVDD

AIN(+)

AIN(–)

CREF1

CREF2

SLEEP

DB15

DB0

CONVST

AGND

DGND

CLKIN

REFIN/REFOUT

AD7859L

ANALOG

SUPPLY

+3V

0.1µF

10µF

0.1µF

0.01µF

CONVERSION

START SIGNAL

0.1µF

CAL

0V TO 2.5V

INPUT

OPTIONAL

EXTERNAL

REFERENCE

CS

RD

WR

W/B

BUSY

DVDD

REF192

CURRENT,

I = 1.5mA TYP

LOW

POWER

µC/µP

Figure 23. Typical Low Power Circuit

CONVST

BUSY

5µs

4.6µs

tCONVERT

START CONVERSION ON RISING EDGE

POWER UP ON FALLING EDGE

POWER-UP

TIME

NORMAL

OPERATION

FULL

POWER-DOWN

POWER-UP

TIME

Figure 21. Using the CONVST Pin to Power Up the AD7859

for a Conversion

Using The Internal (On-Chip) Reference

As in the case of an external reference, the AD7859/AD7859L

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 effec-

tively 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

≈ 150K and C = external capacitor.

The recommended value of the external capacitor is 100 nF;

this gives a power-up time of approximately 135 ms before a

calibration is initiated and normal operation should commence.

When CREF is fully charged, the power-up time from a hardware

or software power-down reduces to 5

µs. This is because an in-

ternal switch opens to provide a high impedance discharge path

for the reference capacitor during power-down—see Figure 22.

An added advantage of the low charge leakage from the refer-

ence capacitor during power-down is that even though the refer-

ence is being powered down between conversions, the reference

capacitor holds the reference voltage to within 0.5 LSBs with

throughput rates of 100 samples/second and over with a full

power-down between conversions. A high input impedance op

amp like the AD707 should be used to buffer this reference

capacitor if it is being used externally. Note, if the AD7859/

AD7859L is left in its powered-down state for more than

100 ms, the charge on CREF will start to leak away and the

power-up time will increase. If this long power-up time is a

problem, the user can use a partial power-down for the last con-

version so the reference remains powered up.

BUF

ON-CHIP

REFERENCE

TO OTHER

CIRCUITRY

SWITCH OPENS

DURING POWER-DOWN

REFIN/OUT

EXTERNAL

CAPACITOR

Figure 22. On-Chip Reference During Power-Down

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EVAL-AD7859CB4 Datasheet(PDF) 19 Page - Analog Devices

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