AD7274 datasheet(19/29 Pages) AD

19 / 29 page

AD7273/AD7274

Rev. 0 | Page 18 of 28

edge of CS, the device begins to power up and continues to

power up until after the falling edge of the 10th SCLK as long as

CS is held low. The power-up time required must elapse before

a conversion can be initiated, as shown in Figure 33. See the

Power-Up Times section for the power-up times associated with

the AD7273/AD7274.

POWER-UP TIMES

The AD7273/AD7274 has two power-down modes, partial

power-down and full power-down, which are described in

detail in the Modes of Operation section. This section deals

with the power-up time required when coming out of either of

these modes.

To power up from partial power-down mode, one cycle is

required. Therefore, with a SCLK frequency of up to 48 MHz,

one dummy cycle is sufficient to allow the device to power up

from partial power-down mode. Once the dummy cycle is

complete, the ADC is fully powered up and the input signal is

acquired properly. The quiet time, tQUIET, must be allowed from

the point where the bus goes back into three-state after the

dummy conversion to the next falling edge of CS.

To power up from full power-down, approximately 1 μs should

be allowed from the falling edge of CS, shown in Figure 33 as

tPOWER-UP. Note that during power-up from partial power-down

mode, the track-and-hold, which is in hold mode while the part

is powered down, returns to track mode after the first SCLK

edge is received after the falling edge of CS. This is shown as

Point A in Figure 31.

When power supplies are first applied to the AD7273/AD7274,

the ADC can power up in either of the power-down modes or

in normal mode. Because of this, it is best to allow a dummy

cycle to elapse to ensure that the part is fully powered up before

attempting a valid conversion. Likewise, if the part is to be kept

in partial power-down mode immediately after the supplies are

applied, two dummy cycles must be initiated. The first dummy

cycle must hold CS low until after the 10th SCLK falling edge

(see Figure 29). In the second cycle, CS must be brought high

between the second and 10th SCLK falling edges (see Figure 30).

Alternatively, if the part is to be placed into full power-down

mode after the supplies are applied, three dummy cycles must

be initiated. The first dummy cycle must hold CS low until after

the 10th SCLK falling edge (see Figure 29); the second and third

dummy cycles place the part into full power-down mode (see

Figure 32). See also the Modes of Operation section.

SCLK

110

AD7273/AD7674

SDATA

VALID DATA

Figure 29. Normal Mode Operation

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