AD7910ARMZ2 datasheet(17/24 Pages) AD

17 / 24 page

AD7910/AD7920

Rev. C | Page 17 of 24

VALID DATA

SCLK

SDATA

110

AD7910/AD7920

Figure 19. Normal Mode Operation

THREE-STATE

SDATA

SCLK

110

Figure 20. Entering Power-Down Mode

SCLK

SDATA

THE PART

BEGINS TO

POWER UP

THE PART IS FULLY

POWERED UP WITH

VIN FULLY ACQUIRED

VALID DATA

INVALID DATA

Figure 21. Exiting Power-Down Mode

When power supplies are first applied to the AD7910/AD7920, the

ADC can power up in either power-down mode or in normal

mode. Because of this, it is best to allow a dummy cycle to elapse to

ensure the part is fully powered up before attempting a valid

conversion. Likewise, if the intention is to keep the part in power-

down mode while not in use and the user wishes the part to power

up in power-down mode, the dummy cycle can be used to ensure

the device is in power-down mode by executing a cycle such as that

shown in Figure 20. Once supplies are applied to the

AD7910/AD7920, the power-up time is the same as that when

powering up from power-down mode. It takes approximately 1 μs

to power up fully if the part powers up in normal mode. It is not

necessary to wait 1 μs before executing a dummy cycle to ensure

the desired mode of operation.

Instead, the dummy cycle can occur directly after power is

supplied to the ADC. If the first valid conversion is performed

directly after the dummy conversion, care must be taken to

ensure that adequate acquisition time is allowed. As mentioned

earlier, when powering up from the power-down mode, the part

returns to track upon the first SCLK edge applied after the

falling edge of CS. However, when the ADC powers up initially

after supplies are applied, the track-and-hold is in track. This

means, assuming the user has the facility to monitor the ADC

supply current, if the ADC powers up in the desired mode of

operation and thus a dummy cycle is not required to change

mode then a dummy cycle is required to place the track-and-

hold into track.

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