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Data Sheet

AD5123/AD5143

THEORY OF OPERATION

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Rev. C | 18 of 25

The AD5123/AD5143 digital programmable potentiometers are de-

signed to operate as true variable resistors for analog signals within

the terminal voltage range of VSS < VTERM < VDD. The resistor wiper

position is determined by the RDAC register contents. The RDAC

of resistance settings. A secondary register (the input register) can

preload the RDAC register data.

The RDAC register can be programmed with any position setting

using the I2C interface (depending on the model). 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 acknowledge 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 (AD5143, 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 8).

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

using Command 9 (see Table 8). 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 8).

Alternatively, the EEPROM can be written to independently using

Command 11 (see Table 11).

Table 8. 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 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 means don't care.

Table 9. Reduced Address Bits Table

Channel

Stored Channel Memory

All channels

Not applicable

RDAC1

RDAC2

RDAC3

RDAC4

1 X means don't care.

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