Datasheet

AD5122A/AD5142A Data Sheet
Rev. A | Page 22 of 32
ADVANCED CONTROL MODES
The AD5122A/AD5142A 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
• A low wiper resistance feature
• Linear increment and decrement instructions
• ±6 dB increment and decrement instructions
• Burst mode
• Reset
• Shutdown mode
Input Register
The AD5122A/AD5142A 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 both 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 in an RDAC register, this RDAC register
automatically overwrites the associated input register.
Linear Gain Setting Mode
The patented architecture of the AD5122A/AD5142A allows
the independent control of each string resistor, R
AW
, and R
WB
. To
enable linear gain setting mode, 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, R
AW
= R
AB
− R
WB
.
This mode enables a second input and an RDAC register per
channel, as shown in Table 16; however, the actual RDAC
contents remain unchanged. The same operations are valid for
potentiometer 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 the D2 bit are connected internally to a logic OR gate; if one or
both are set to 1, the parts cannot operate in potentiometer mode.
Low Wiper Resistance Feature
The AD5122A/AD5142A include two commands to reduce the
wiper resistance between the terminals when the device achieves
full scale or zero scale. These extra positions are called bottom
scale, BS, and top scale, TS. The resistance between Termi na l A
and Terminal W at top scale is specified as R
TS
. Similarly, the
bottom scale resistance between Terminal B and Terminal W is
specified as R
BS
.
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
potentiometer or linear gain setting mode is enabled.
Table 12. Top Scale Truth Table
Linear Gain Setting Mode
Potentiometer Mode
R
AW
R
WB
R
AW
R
WB
R
AB
R
AB
R
TS
R
AB
Table 13. Bottom Scale Truth Table
Linear Gain Setting Mode Potentiometer Mode
R
AW
R
WB
R
AW
R
WB
R
TS
R
BS
R
AB
R
BS
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 in multiple channels.