AD5253/AD5254
THEORY OF OPERATION
The AD5253/AD5254 are quad-channel digital potentiometers
in 1 k, 10 k, 50 k, or 100 k that allow 64/256 linear resis-
tance step adjustments. The AD5253/AD5254 employ double-
gate CMOS EEPROM technology that allows resistance settings
and user-defined data 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.
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The AD5253/AD5254 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 I
2
C interface. The format of the data-words and
the commands to program the RDAC registers are discussed in
the I
2
C Interface section.
The four RDAC registers have corresponding EEMEM memory
locations that provide nonvolatile storage of resistor wiper
position settings. The AD5253/AD5254 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 V
DD
reaches its final value. As a result, any
supply voltage decoupling capacitors limit the EEMEM restore
time during power-up. Figure 20 shows the power-up profile
where V
DD
, without any decoupling capacitors connected to it, is
applied with a digital signal. The device initially resets the
RDACs to midscale before restoring the EEMEM contents.
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 registers
and EEMEM memory, the AD5253/AD5254 provide other
shortcut commands that facilitate the user’s programming
needs, as shown in Table 11.
Table 11. AD5253/AD5254 Quick Commands
Commmand
Description
0
NOP.
1
Restore EEMEM Content to RDAC. User should
issue NOP immediately after this command to
conserve power.
2
Store RDAC Register Setting to EEMEM.
3
Decrement RDAC 6 dB (Shift Data Bits Right).
4
Decrement All RDACs 6 dB (Shift All Data Bits
Right).
5
Decrement RDAC One Step.
6
Decrement All RDACs One Step.
7
Reset EEMEM Contents to All RDACs.
8
Increment RDAC 6 dB (Shift Data Bits Left).
9
Increment All RDACs 6 dB (Shift All Data Bits
Left).
10
Increment RDAC One Step.
11
Increment All RDACs One Step.
12–15
Reserved.
LINEAR INCREMENT AND DECREMENT
COMMANDS
The increment and decrement commands (#10, #11, #5, #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 AD5253/AD5254. The adjustments can be directed to a
single RDAC or to all four RDACs.
±6 dB ADJUSTMENTS (DOUBLING/HALVING
WIPER SETTING)
The AD5253/AD5254 accommodate ±6 dB adjustments of the
RDAC wiper positions by shifting the register contents to left/
right for increment/decrement operations, respectively. Com-
mands 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 AD5253/AD5254
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 white LED brightness settings where the visual
responses of humans are more sensitive to large rather small
adjustments.