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Data Sheet

AD5121/AD5141

THEORY OF OPERATION

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Rev. E | 19 of 30

The AD5121/AD5141 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

be used to preload the RDAC register data.

The RDAC register can be programmed with any position setting

using the I2C or SPI 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 18 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 (AD5141, 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 13).

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

using Command 9 (see Table 13). 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 13).

Alternatively, the EEPROM can be written to independently using

Command 1 (see Table 13).

INPUT SHIFT REGISTER

For the AD5121/AD5141, 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 AD5121 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 11 and Table

13.

Table 11. Simple Command 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 = don’t care.

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