Data Sheet AD5122/AD5142
Rev. C | Page 23 of 32
ADVANCED CONTROL MODES
The AD5122/AD5142 digital potentiometers include a set of user
programming features to address the wide number of applications
for these universal adjustment devices (see Table 16 and Table 18).
Key programming features include the following:
• Input register
• Linear gain setting mode
• Low wiper resistance feature
• Lineal increment and decrement instructions
• ±6 dB increment and decrement instructions
• Reset
• Shutdown mode
Input Register
The AD5122/AD5142 include one input register per RDAC
register. These registers allow preloading of the value for the
associated RDAC register. These registers can be written to using
Command 2 and read back from using Command 3 (see Table 16).
This feature allows a synchronous update of one or all the
RDAC registers at the same time.
The transfer from the input register to the RDAC register is
done synchronously by Command 8 (see Table 16).
If new data is loaded into an RDAC register, this RDAC register
automatically overwrites the associated input register.
Linear Gain Setting Mode
The proprietary architecture of the AD5122/AD5142 allows the
independent control of each string resistor, RAW and RWB. To
enable this feature, use Command 16 (see Table 16) to set Bit D2
of the control register (see Table 18).
This mode of operation can control the potentiometer as two
independent rheostats connected at a single point, W terminal,
as opposed to potentiometer mode where each resistor is
complementary, RAW = RAB − RWB.
This feature enables a second input and an RDAC register per
channel, as shown in Table 17; however, the actual RDAC contents
remain unchanged. The same operations are valid for
potentiometer mode and linear gain setting mode.
If the INDEP pin is pulled high, the device powers up in linear
gain setting mode and loads the values stored in the associated
memory locations for each channel (see Table 17). The INDEP pin
and D2 bit are connected internally to a logic or gate, if any or
both are 1, the devices cannot operate in potentiometer mode.
Low Wiper Resistance Feature
The AD5122/AD5142 include two commands to reduce the wiper
resistance between the terminals when the devices achieve full scale
or zero scale. These extra positions are called bottom scale, BS, and
top scale, TS. The resistance between Terminal A and Terminal W
at top scale is specified as RTS. Similarly, the bottom scale resistance
between Terminal B and Terminal W is specified as RBS.
The contents of the RDAC registers are unchanged by entering
in these positions. There are three ways to exit from top scale
and bottom scale: by using Command 12 or Command 13 (see
Table 16); by loading new data in an RDAC register, which
includes increment/decrement operations; or by entering
shutdown mode, Command 15 (see Table 16).
Table 12 and Table 13 show the truth tables for the top scale
position and the bottom scale position, respectively, when the
potentiometer or linear gain setting mode is enabled.
Table 12. Top Scale Truth Table
Linear Gain Setting Mode Potentiometer Mode
R
R
R
R
R
R
R
R
Table 13. Bottom Scale Truth Table
Linear Gain Setting Mode Potentiometer Mode
R
R
R
R
Linear Increment and Decrement Instructions
The increment and decrement commands (Command 4 and
Command 5 in Table 16) are useful for linear step adjustment
applications. These commands simplify microcontroller software
coding by allowing the controller to send an increment or
decrement command to the device. The adjustment can be
individual or in a ganged potentiometer arrangement, where
all wiper positions are changed at the same time.
For an increment command, executing Command 4 automatically
moves the wiper to the next RDAC position. This command
can be executed in a single channel or multiple channels.