AD5124 Datasheet, PDF (23/36 Pages) Analog Devices – The AD5124/AD5144/AD5144A potentiometers provide a nonvolatile solution for 128-/256-position adjustment applications, offering guaranteed low resistor tolerance errors of ±8% and up to ±6 mA current density in the Ax, Bx, and Wx pins.

Data Sheet
THEORY OF OPERATION
The AD5124/AD5144/AD5144A digital programmable
potentiometers are designed 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 register acts as a scratchpad register
that allows unlimited changes 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 the 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
potentiometer wiper. For example, when the RDAC register is
loaded with 0x80 (AD5144/AD5144A, 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 14).
The contents of the RDAC register can be stored to the EEPROM
using Command 9 (see Table 14). 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 14).
Alternatively, the EEPROM can be written to independently
using Command 11 (see Table 20).
INPUT SHIFT REGISTER
For the AD5124/AD5144/AD5144A, the input shift register is
16 bits wide, as shown in Figure 4. The 16-bit word consists of
four control bits, followed by four address bits and by eight
data bits.
If the AD5124 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 14 and
Table 20.
AD5124/AD5144/AD5144A
SERIAL DATA DIGITAL INTERFACE SELECTION, DIS
The AD5124/AD5144 LFSCP provides the flexibility of a selectable
interface. When the digital interface select (DIS) pin is tied low,
the SPI mode is engaged. When the DIS pin is tied high, the I²C
mode is engaged.
SPI SERIAL DATA INTERFACE
The AD5124/AD5144 contain a 4-wire, SPI-compatible digital
interface (SDI, SYNC, SDO, and SCLK). The write sequence
begins by bringing the SYNC line low. The SYNC pin must be
held low until the complete data-word is loaded from the SDI
pin. Data is loaded in at the SCLK falling edge transition, as
shown in Figure 6. When SYNC returns high, the serial data-
word is decoded according to the instructions in Table 20.
To minimize power consumption in the digital input buffers
when the part is enabled, operate all serial interface pins close
to the VLOGIC supply rails.
SYNC Interruption
In a standalone write sequence for the AD5124/AD5144,
the SYNC line is kept low for 16 falling edges of SCLK, and the
instruction is decoded when SYNC is pulled high. However, if
the SYNC line is kept low for less than 16 falling edges of SCLK,
the input shift register content is ignored, and the write sequence is
considered invalid.
SDO Pin
The serial data output pin (SDO) serves two purposes: to read back
the contents of the control, EEPROM, RDAC, and input registers
using Command 3 (see Table 14 and Table 20), and to connect the
AD5124/AD5144 in daisy-chain mode.
The SDO pin contains an internal open-drain output that needs an
external pull-up resistor. The SDO pin is enabled when SYNC is
pulled low, and the data is clocked out of SDO on the rising
edge of SCLK, as shown in Figure 6 and Figure 7.
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