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Data Sheet

AD5251/AD5252

Rev. E | Page 21 of 28

THEORY OF OPERATION

The AD5251/AD5252 are dual-channel digital potentiometers

that allow 64/256 linear resistance step adjustments. The

AD5251/AD5252 employ double-gate CMOS EEPROM

technology, which 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 nonvolatile

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; this essentially erases the current

content in the EEMEM register and allows subsequent storage

of the new content. Saving data to an EEMEM register consumes

about 35 mA of current and lasts approximately 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 limits the EEMEM restore

time during power-up. For example, Figure 20 shows a power-

up profile of the VDD where there is no decoupling capacitor and

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

the measured RDACs to midscale before restoring the EEMEM

contents. By default, EEMEM is loaded at midscale until a new

value is loaded. The omission of the decoupling capacitors

should only be considered when the fast restoring time is

absolutely needed in the application. In addition, users should

issue a NOP Command 0 immediately after using Command 1

to restore the EEMEM setting to RDAC, thereby minimizing

supply current dissipation. Reading user data directly from

EEMEM does not require a similar NOP command execution.

In addition to the movement of data between RDAC and

EEMEM registers, the AD5251/AD5252 provide other shortcut

commands that facilitate programming, as shown in Table 11.

Table 11. Quick Commands

Command

Description

NOP.

Restore EEMEM content to RDAC. Users 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 to 15

Reserved.

LINEAR INCREMENT/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 a

single RDAC or to all four RDACs.

±6 dB ADJUSTMENTS

(DOUBLING/HALVING WIPER SETTING)

The AD5251/AD5252 accommodate ±6 dB adjustments of

the RDAC wiper positions by shifting the register contents to

left/right for increment/decrement operations, respectively.

Command 3, Command 4, Command 8, and Command 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 essentially doubles

the RDAC register value, whereas decrementing the wiper

position by –6 dB halves 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 incrementing

from zero scale and decrementing from full scale, respectively.

These functions are useful for various audio/video level

adjustments, especially for white LED brightness settings in

which human visual responses are more sensitive to large

adjustments than to small adjustments.

DIGITAL INPUT/OUTPUT CONFIGURATION

SDA is a digital input/output with an open-drain MOSFET that

requires a pull-up resistor for proper communication. On the other

hand, SCL and

WPEE

are digital inputs for which pull-up resistors are

recommended to minimize the MOSFET cross-conduction current

when the driving signals are lower than VDD.

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