AD5142ABCPZ10-RL7 datasheet(18/29 Pages) AD

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Data Sheet

AD5122A/AD5142A

THEORY OF OPERATION

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Rev. D | 18 of 29

The AD5122A/AD5142A digital programmable potentiometers are

designed to operate as true variable resistors for analog signals

within the terminal voltage range of VSS < VTERM < VDD. The resis-

tor wiper position is determined by the RDAC register contents. The

RDAC register acts as a scratchpad register that allows unlimited

changes of resistance settings. A secondary register (the input shift

The RDAC register can be programmed with any position setting

using the I2C interface. When a desirable wiper position is found,

this value can be stored in the EEPROM memory. Thereafter, the

wiper position is always restored to that position for subsequent

power-ups. The storing of EEPROM data takes approximately 15

ms; during this time, the device is locked and does not acknowl-

edge any new command, preventing any changes from taking

place.

RDAC REGISTER AND EEPROM

The RDAC register directly controls the position of the digital poten-

tiometer wiper. For example, when the RDAC register is loaded with

0x80 (AD5142A, 256 taps), the wiper is connected to half scale of

the variable resistor. The RDAC register is a standard logic register;

there is no restriction on the number of changes allowed.

It is possible to both write to and read from the RDAC register using

the digital interface (see Table 10).

The contents of the RDAC register can be stored to the EEPROM

using Command 9 (see Table 10). Thereafter, the RDAC register

always sets at that position for any future on-off-on power supply

sequence. It is possible to read back data saved into the EEPROM

with Command 3 (see Table 10).

Alternatively, the EEPROM can be written to independently using

Command 11 (see Table 13).

INPUT SHIFT REGISTER

For the AD5122A/AD5142A, the input shift register is 16 bits wide,

as shown in Figure 2. The 16-bit word consists of four control bits,

followed by four address bits and by eight data bits.

If the AD5122A RDAC or EEPROM registers are read from or

written to, the lowest data bit (Bit 0) is ignored.

Data is loaded MSB first (Bit 15). The four control bits determine the

function of the software command, as listed in Table 10 and Table

13.

Table 10. Reduced Commands Operation Truth Table

Command

Number

Control

Bits[DB15:DB12]

Address

Bits[DB11:DB8]1

Data Bits[DB7:DB0]1

Operation

NOP: do nothing

Write contents of serial register data to

RDAC

Write contents of serial register data to

input shift register

Read back contents

Data

EEPROM

RDAC

Copy RDAC register to EEPROM

Copy EEPROM into RDAC

Software reset

Software shutdown

Condition

Normal mode

Shutdown mode

1 X = don’t care.

Table 11. Reduced Address Bits Table

Channel

Stored Channel Memory

All channels

Not applicable

RDAC1

RDAC2

Not applicable

RDAC2

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