AD5689R datasheet(22/28 Pages) AD

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AD5689R/AD5687R

Data Sheet

Rev. B | Page 22 of 28

READBACK OPERATION

Readback mode is invoked through a software executable read-

back command. If the SDO output is disabled via the daisy-chain

mode disable bit in the control register, it is automatically enabled

for the duration of the read operation, after which it is disabled

again. Command 1001 is reserved for the readback function in

when DCEN is enable. This command, in association with

selecting one of the address bits, DAC B or DAC A, selects the

selected during readback. The remaining three address bits

(which include the two don’t care bits) must be set to Logic 0. The

remaining data bits in the write sequence are ignored. If more

than one address bit is selected or no address bit is selected, DAC

Channel A is read back by default. During the next SPI write, the

data that appears on the SDO output contains the data from the

previously addressed register.

For example, to read back the DAC register for Channel A,

implement the following sequence:

Write 0x900000 to the AD5689R/AD5687R input register.

This setting configures the part for read mode with the

Channel A DAC register selected. Note that all data bits,

DB15 to DB0, are don’t care bits.

Follow this write operation with a second write, a nop

operation, 0x000000 (0xF00000 in daisy-chain mode).

During this write, the data from the register is clocked out

on the SDO line. DB23 to DB20 contain undefined data,

and the last 16 bits contain the DB19 to DB4 DAC register

contents.

POWER-DOWN OPERATION

The AD5689R/AD5687R contain three separate power-down

modes. Command 0100 controls the power-down function (see

Table 9). These power-down modes are software-programmable

by setting eight bits, Bit DB7 to Bit DB0, in the input shift register.

There are two bits associated with each DAC channel. Table 11

explains how the state of the two bits corresponds to the mode of

operation of the device.

Either or both DACs (DAC B, DAC A) can be powered down to

the selected mode by setting the corresponding bits. See Table 12

for the contents of the input shift register during the power-down/

power-up operation.

Table 11. Modes of Operation

Operating Mode

PDx1

PDx0

Normal Operation Mode

Power-Down Modes

1 kΩ to GND

100 kΩ to GND

Three-State

When both Bit PDx1 and Bit PDx0 (where x is the channel that is

selected) in the input shift register are set to 0, the parts work

normally, with a normal power consumption of 4 mA at 5 V.

However, for the three power-down modes of the AD5689R/

AD5687R, the supply current falls to 4 μA at 5 V. Not only does

the supply current fall, but the output stage is also internally

switched from the output of the amplifier to a resistor network

of known values. This switchover has the advantage that the output

impedance of the part is known while the part is in power-down

mode. The three power-down options are as follows:



The output is connected internally to GND through a 1 kΩ

resistor.



The output is connected internally to GND through a 100 kΩ

resistor.



The output is left open-circuited (three-state).

The output stage is illustrated in Figure 46.

RESISTOR

NETWORK

VOUTX

DAC

POWER-DOWN

CIRCUITRY

AMPLIFIER

Figure 46. Output Stage During Power-Down

The bias generator, output amplifier, resistor string, and other

associated linear circuitry are shut down when the power-down

mode is activated. However, the contents of the DAC register

are unaffected when in power-down, and the DAC register can

be updated while the device is in power-down mode. The time

that is required to exit power-down is typically 4.5 μs for VDD = 5 V.

To further reduce the current consumption, the on-chip reference

can be powered off (see the Internal Reference Setup section).

Table 12. 24-Bit Input Shift Register Contents of Power-Down/Power-Up Operation1

DB23

(MSB)

DB22

DB21

DB20

DB19 to DB16

DB15 to DB8

DB7

DB6

DB5

DB4

DB3

DB2

DB1

DB0

(LSB)

PDB1

PDB0

PDA1

PDA0

Command bits (C3 to C0)

Address bits; don’t care

Power-down,

select DAC B

Set to 1

Power-down,

select DAC A

1 X = don’t care.

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