AD7477AARMZ datasheet(19/28 Pages) AD

19 / 28 page

AD7476A/AD7477A/AD7478A

Rev. F | Page 19 of 28

SDATA

SCLK

AD7476A/AD7477A/AD7478A

VALID DATA

Figure 20. Normal Mode Operation

THREE-STATE

SDATA

SCLK

Figure 21. Entering Power-Down Mode

INVALID DATA

SDATA

SCLK

VALID DATA

THE PART IS FULLY

POWERED UP WITH

VIN FULLY ACQUIRED

THE PART

BEGINS TO

POWER UP

Figure 22. Exiting Power-Down Mode

When power supplies are first applied to the AD7476A/AD7477A/

AD7478A, the ADC can power up in either the power-down or

normal modes. 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 it is intended to keep

the part in the 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 that the device is in power-down by

executing a cycle such as that shown in Figure 22. Once supplies

are applied to the AD7476A/AD7477A/AD7478A, the power-up

time is the same as that when powering up from the 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, a 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 an adequate acquisition time has been 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

initially powers up after supplies are applied, the track-and-hold

is already in track. This means, assuming one 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 the mode, a dummy cycle is not required to

place the track-and-hold into track.

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