AD7854ARS datasheet(18/28 Pages) AD

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AD7854/AD7854L

–18–

REV. B

Using The Internal (On-Chip) Reference

As in the case of an external reference the AD7854/AD7854L can

power up from one of two conditions, power-up after the sup-

plies are connected or power-up from a 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

≈ 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 software

power-down reduces to 5

µs. This is because an internal switch

opens to provide a high impedance discharge path for the refer-

ence capacitor during power-down—see Figure 22. An added

advantage of the low charge leakage from the reference capacitor

during power-down is that even though the reference 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 AD7854/AD7854L 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 longer power-up time is a problem, the user can use a

partial power-down for the last conversion so the reference

remains powered up.

SWITCH OPENS

DURING POWER-DOWN

TO OTHER CIRCUITRY

REFIN/OUT

EXTERNAL

CAPACITOR

ON-CHIP

REFERENCE

BUF

AD7854/

AD7854L

Figure 22. On-Chip Reference During Power-Down

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 AD7854/AD7854L is only powered up for the duration of

the conversion. If the power-up time of the AD7854/AD7854L

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 AD7854

has a conversion time of 4.6

µs with a 4 MHz external clock,

and the AD7854L has a conversion time of 9

µs with a 1.8 MHz

clock. This means the AD7854/AD7854L 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 four graphs,

Figures 24, 25, 26 and 27, show the power consumption of the

AD7854 and AD7854L for VDD = 3 V as a function of through-

put. Table VII lists the power consumption for various throughput

rates.

Table VII. Power Consumption vs. Throughput

Power

Throughput Rate

AD7854

AD7854L

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

4MHz/1.8MHz

OSCILLATOR

AVDD DVDD

AIN(+)

AIN(–)

CREF1

CREF2

DB11

DB0

CONVST

AGND

DGND

CLKIN

REFIN/REFOUT

AD7854/

AD7854L

ANALOG

SUPPLY

+3V TO +5V

0.1 F

10 F

0.1 F

0.01 F

CONVERSION

START SIGNAL

0.1nF EXTERNAL REFERENCE

0.1 F ON-CHIP REFERENCE

0V TO 2.5V

INPUT

OPTIONAL

EXTERNAL

REFERENCE

BUSY

AD780/

REF192

C/ P

HBEN

FULL POWER-DOWN

AFTER A CONVERSION

PMGT1 = 0

PMGT0 = 1

Figure 23. Typical Low Power Circuit

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