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AD5251/AD5252

Rev. 0 | Page 20 of 28

OPERATIONAL OVERVIEW

The AD5251/AD5252 are dual-channel digital potentiometers

in 1 kΩ, 10 kΩ, 50 kΩ, or 100 kΩ that allow 64 and 256 linear

resistance step adjustments. The AD5251/AD5252 employ

double-gate CMOS EEPROM technology that allows resistance

settings and user-defined data to be stored in the EEMEM

registers. The EEMEM is nonvolatile, such that settings remain

when power is removed. The RDAC wiper settings are restored

from the non-volatile memory settings during device power-up

and can also be restored at any time during operation.

The AD5251/AD5252 resistor wiper positions are determined

by the RDAC register contents. The RDAC register acts like a

scratch-pad register, allowing unlimited changes of resistance

settings. RDAC register contents can be changed using the

device’s serial I2C interface. The format of the data-words and

the commands to program the RDAC registers are discussed in

the I2C Interface Detail Description section.

The four RDAC registers have corresponding EEMEM memory

locations that provide nonvolatile storage of resistor wiper

position settings. The AD5251/AD5252 provide commands to

store the RDAC register contents to their respective EEMEM

memory locations. During subsequent power-on sequences, the

RDAC registers are automatically loaded with the stored value.

Whenever the EEMEM write operation is enabled, the device

activates the internal charge pump and raises the EEMEM cell

gate bias voltage to a high level, essentially erasing the current

content in the EEMEM register and allowing subsequent stor-

age of the new content. Saving data to an EEMEM register con-

sumes about 35 mA of current and lasts about 26 ms. Because

of charge pump operation, all RDAC channels may experience

noise coupling during the EEMEM writing operation.

The EEMEM restore time in power-up or during operation is

about 300 µs. Note that the power up EEMEM refresh time

depends on how fast VDD reaches its final value. As a result, any

supply voltage decoupling capacitors limit the EEMEM restore

time during power-up. Figure 32 shows the power up profile

where VDD, without any decoupling capacitors connected to it,

is applied with a digital signal. The device initially resets the

measured RDACs to midscale before reaching their final values

during EEMEM restoration.

In addition, users should issue a NOP Command 0 immediately

after using Command 1 to restore the EEMEM setting to

RDAC, to minimize supply current dissipation. Directly reading

user data from EEMEM does not require similar NOP

command execution.

In addition to the movement of data between RDAC registers

and EEMEM memory, the AD5251/AD5252 provide other

shortcut commands that facilitate the users’ programming

needs, as shown in Table 11.

Table 11. AD5251/AD5252 Quick Commands

Commmand

Description

NOP

Restore EEMEM content to RDAC. User should

issue NOP immediately after this command to

conserve power.

Store RDAC register setting to EEMEM.

Decrement RDAC 6 dB (shift data bits right).

Decrement all RDACs 6 dB (shift all data bits

right).

Decrement RDAC one step.

Decrement all RDACs one step.

Reset EEMEM contents to all RDACs.

Increment RDAC 6 dB (shift data bits left).

Increment All RDACs 6 dB (shift all data bits left).

Increment RDAC one step.

Increment all RDACs one step.

12–15

Reserved.

LINEAR INCREMENT AND DECREMENT

COMMANDS

The increment and decrement commands (10, 11, 5, and 6) are

useful for linear step adjustment applications. These commands

simplify microcontroller software coding by allowing the

controller to send just an increment or decrement command to

the AD5251/AD5252. The adjustments can be directed to an

individual RDAC or to all four RDACs.

±6 dB ADJUSTMENTS (DOUBLING/HALVING

WIPER SETTING)

The AD5251/AD5252 accommodates ±6 dB adjustments of the

RDAC wiper positions by shifting the register contents to

left/right for increment/decrement operations, respectively.

Commands 3, 4, 8, and 9 can be used to increment or

decrement the wiper positions in 6 dB steps synchronously or

asynchronously.

Incrementing the wiper position by +6 dB is essentially

doubling the RDAC register value, while decrementing by

–6 dB is halving the register content. Internally, the

AD5251/AD5252 use shift registers to shift the bits left and

right to achieve a ±6 dB increment or decrement. The

maximum number of adjustments is nine and eight steps for

increment from zero scale and decrement from full scale,

respectively. These functions are useful for various audio/video

level adjustments, especially for white LED brightness settings

where human visual responses are more sensitive to large than

small adjustments.

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