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AD7908/AD7918/AD7928

REV. A

Normal Mode (PM1 = PM0 = 1)

This mode is intended for the fastest throughput rate performance

as the user does not have to worry about any power-up times

with the AD7908/AD7918/AD7928 remaining fully powered at

all times. Figure 12 shows the general diagram of the operation

of the AD7908/AD7918/AD7928 in this mode.

The conversion is initiated on the falling edge of

CS and the

track-and-hold will enter hold mode as described in the Serial

Interface section. The data presented to the AD7908/AD7918/

AD7928 on the DIN line during the first 12 clock cycles of the

data transfer are loaded into the Control Register (provided

WRITE bit is set to 1). If data is to be written to the SHADOW

sented on the DIN line during the first 16 SCLK cycles is loaded

into the SHADOW Register. The part will remain fully powered

up in Normal mode at the end of the conversion as long as PM1

and PM0 are both loaded with 1 on every data transfer.

Sixteen serial clock cycles are required to complete the conversion

and access the conversion result. The track-and-hold will go back

into track on the 14th SCLK falling edge.

CS may then idle high

until the next conversion or may idle low until sometime prior to

the next conversion, (effectively idling

CS low).

Once a data transfer is complete (DOUT has returned to three-

state), another conversion can be initiated after the quiet time,

tQUIET, has elapsed by bringing CS low again.

SCLK

DOUT

DIN

1 LEADING ZERO + 3 CHANNEL IDENTIFIER BITS

+ CONVERSION RESULT

NOTES

1. CONTROL REGISTER DATA IS LOADED ON FIRST 12 SCLK CYCLES

2. SHADOW REGISTER DATA IS LOADED ON FIRST 16 SCLK CYCLES

DATA IN TO CONTROL/SHADOW REGISTER

Figure 12. Normal Mode Operation

Full Shutdown (PM1 = 1, PM0 = 0)

In this mode, all internal circuitry on the AD7908/AD7918/

AD7928 is powered down. The part retains information in the

Control Register during full shutdown. The AD7908/AD7918/

AD7928 remains in full shutdown until the power management

bits in the Control Register, PM1 and PM0, are changed.

If a write to the Control Register occurs while the part is in Full

Shutdown, with the power management bits changed to PM0 =

PM1 = 1, Normal mode, the part will begin to power up on the

CS rising edge. The track-and-hold that was in hold while the

part was in Full Shutdown will return to track on the 14th SCLK

falling edge.

To ensure that the part is fully powered up, tPOWER UP, should have

elapsed before the next

CS falling edge. Figure 13 shows the

general diagram for this sequence.

Auto Shutdown (PM1 = 0, PM0 = 1)

In this mode, the AD7908/AD7918/AD7928 automatically

enters shutdown at the end of each conversion when the control

hold is in hold mode. Figure 14 shows the general diagram of

the operation of the AD7908/AD7918/AD7928 in this mode. In

shutdown mode, all internal circuitry on the AD7908/AD7918/

AD7928 is powered down. The part retains information in the

Control Register during shutdown. The AD7908/AD7918/

AD7928 remains in shutdown until the next

CS falling edge it

receives. On this

CS falling edge, the track-and-hold that was in

hold while the part was in shutdown will return to track. Wake-

up time from auto shutdown is 1 µs, and the user should ensure

that 1 µs has elapsed before attempting a valid conversion.

When running the AD7908/AD7918/AD7928 with a 20 MHz

clock, one dummy cycle should be sufficient to ensure the part

is fully powered up. During this dummy cycle the contents of

the Control Register should remain unchanged; therefore the

WRITE bit should be 0 on the DIN line. This dummy cycle

effectively halves the throughput rate of the part, with every

other conversion result being valid. In this mode, the power

consumption of the part is greatly reduced with the part enter-

ing shutdown at the end of each conversion. When the Control

at the end of the conversion. The user can move the ADC in

and out of the low power state by controlling the

CS signal.

Powering Up the AD7908/AD7918/AD7928

When supplies are first applied to the AD7908/AD7918/AD7928,

the ADC may power up in any of the operating modes of the

part. To ensure the part is placed into the required operating

mode, the user should perform a dummy cycle operation as out-

lined in Figure 15.

The three dummy conversion operation outlined in Figure 15

must be performed to place the part into the Auto Shutdown

mode. The first two conversions of this dummy cycle operation

are performed with the DIN line tied high; for the third conver-

sion of the dummy cycle operation, the user should write the

desired Control Register configuration to the AD7908/AD7918/

AD7928 in order to place the part into the Auto Shutdown

mode. On the third

CS rising edge after the supplies are applied,

the Control Register will contain the correct information and

valid data will result from the next conversion.

Therefore, to ensure the part is placed into the correct operating

mode, when supplies are first applied to the AD7908/AD7918/

AD7928, the user must first issue two serial write operations

with the DIN line tied high, and on the third conversion cycle the

user can then write to the Control Register to place the part into

any of the operating modes. The user should not write to the

SHADOW Register until the fourth conversion cycle after the

supplies are applied to the ADC, in order to guarantee the

Control Register contains the correct data.

If the user wants to place the part into either the Normal mode

or Full Shutdown mode, the second dummy cycle with DIN tied

high can be omitted from the three dummy conversion operation

outlined in Figure 15.

–19–

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