Preliminary Technical Data

AD7357

Rev. PrD | Page 13 of 17

MODES OF OPERATION

The mode of operation of the AD7357 is selected by controlling

the (logic) state of the CS signal during a conversion. There are

three possible modes of operation: normal mode, partial power-

down mode and full power-down mode. After a conversion has

been initiated, the point at which CS is pulled high determines

which power-down mode, if any, the device enters. Similarly, if

already in a power-down mode, CS can control whether the

device returns to normal operation or remains in power-down.

These modes of operation are designed to provide flexible

power management options. These options can be chosen to

optimize the power dissipation/throughput rate ratio for the

differing application requirements.

NORMAL MODE

This mode is intended for applications needing fastest

throughput rates since the user does not have to worry about

any power-up times with the AD7357 remaining fully powered

at all times. Figure 14 shows the general diagram of the

operation of the AD7357 in this mode.

SCLK

LEADING ZEROS + CONVERSION RESULT

CS

114

10

Figure 14. Normal Mode Operation

The conversion is initiated on the falling edge of CS, as

described in the Serial Interface section. To ensure that the part

remains fully powered up at all times, CS must remain low until

at least 10 SCLK falling edges have elapsed after the falling edge

required to complete the conversion and access the conversion

result for the AD7357. The SDATA lines do not return to three-

state after 16 SCLK cycles have elapsed, but instead do so when

CS is brought high again. If CS is left low for another 2 SCLK

cycles, two trailing zeros are clocked out after the data. If CS is

left low for a further 16 SCLK cycles, the result for the other

ADC on board is also accessed on the same SDATA line as

shown in Figure 20 (see the Serial Interface section).

Once 32 SCLK cycles have elapsed, the SDATA line returns to

prior to this, the SDATA line returns to three-state at that point.

Thus, CS may idle low after 32 SCLK cycles until it is brought

high again sometime prior to the next conversion if so desired,

since the bus still returns to three-state upon completion of the

dual result read.

returned to three-state, another conversion can be initiated after

(assuming the required acquisition time has been allowed).

PARTIAL POWER-DOWN MODE

This mode is intended for use in applications where slower

throughput rates are required. Either the ADC is powered down

between each conversion, or a series of conversions may be

performed at a high throughput rate and the ADC is then

powered down for a relatively long duration between these

bursts of several conversions. When the AD7357 is in partial

power-down, all analog circuitry is powered down except for

the on-chip reference and reference buffers.

To enter partial power, the conversion process must be

shown in Figure 15. Once CS has been brought high in this

window of SCLKs, the part enters partial power-down, the

conversion that was initiated by the falling edge of CS is

remains in normal mode and does not power down. This avoids

accidental power-down due to glitches on the CS line.

SCLK

THREE-STATE

CS

114

10

2

Figure 15. Entering Partial Power-Down Mode

To exit this mode of operation and power up the AD7357 again,

a dummy conversion is performed. On the falling of CS, the

device begins to power up, and continues to power up as long as

device is fully powered up after approximately TBD μs has

elapsed, and valid data results from the next conversion, as

edge of SCLK, the AD7357 again goes into partial power-down.

This avoids accidental power-up due to glitches on the CS line.

Although the device may begin to power up on the falling edge

of CS, it powers down again on the rising edge of CS. If the

AD7357 is already in partial power-down mode and CS is

device enters full power-down mode.

FULL POWER-DOWN MODE

This mode is intended for use in applications where throughput

rates slower than those in the partial power-down mode are

required, as power-up from a full power-down takes

substantially longer than that from a partial power-down. This