AD7822 datasheet(12/20 Pages) AD

12 / 20 page

AD7822/AD7825/AD7829

–12–

REV. B

OPERATING MODES

The AD7822, AD7825, and AD7829 have two possible modes

of operation, depending on the state of the CONVST pulse

approximately 100 ns after the end of a conversion, i.e., upon

the rising edge of the EOC pulse.

Mode 1 Operation (High-Speed Sampling)

When the AD7822, AD7825, and AD7829 are operated in

Mode 1 they are not powered-down between conversions. This

mode of operation allows high throughput rates to be achieved.

Figure 20 shows how this optimum throughput rate is achieved

by bringing CONVST high before the end of a conversion, i.e.,

before the EOC pulses low. When operating in this mode, a new

conversion should not be initiated until 30 ns after the end of a

read operation. This is to allow the track/hold to acquire the

analog signal to 0.5 LSB accuracy.

Mode 2 Operation (Automatic Power-Down)

When the AD7822, AD7825, and AD7829 are operated in

Mode 2 (see Figure 21), they automatically power down at the

end of a conversion. The CONVST signal is brought low to ini-

tiate a conversion and is left logic low until after the EOC goes

high, i.e., approximately 100 ns after the end of the conversion.

The state of the CONVST signal is sampled at this point (i.e.,

530 ns maximum after CONVST falling edge) and the AD7822,

AD7825, and AD7829 will power down as long as CONVST

is low. The ADC is powered up again on the rising edge of the

CONVST signal. Superior power performance can be achieved

in this mode of operation by only powering up the AD7822,

AD7825, and AD7829 to carry out a conversion. The parallel

interface of the AD7822, AD7825, and AD7829 is still fully

operational while the ADCs are powered down. A read may occur

while the part is powered down, and so it does not necessarily

need to be placed within the EOC pulse as shown in Figure 21.

VALID

DATA

CONVST

EOC

DB0–DB7

TRACK

HOLD

TRACK

HOLD

120ns

Figure 20. Mode 1 Operation

CONVST

EOC

DB0–DB7

tPOWER-UP

VALID

DATA

POWER

DOWN

HERE

Figure 21. Mode 2 Operation

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