Datasheet

Data Sheet AD5123/AD5143
Rev. A | Page 21 of 28
ADVANCED CONTROL MODES
The AD5123/AD5143 digital potentiometers include a set of user
programming features to address the wide number of applications
for these universal adjustment devices (see Table 15 and Table 17).
Key programming features include the following:
• Input register
• Linear gain setting mode
• Low wiper resistance feature
• Linear increment and decrement instructions
• ±6 dB increment and decrement instructions
• Burst mode (I
2
C only)
• Reset
• Shutdown mode
Input Register
The AD5123/AD5143 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 using Command 3 (see Table 15).
This feature allows a synchronous and asynchronous update of
one or all of 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 15).
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 AD5123/AD5143 allows the
independent control of each string resistor, R
AW
, and R
WB
. To enable
linear gain setting mode, use Command 16 (see Table 15) to set
Bit D2 of the control register (see Table 17).
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 setting gain modes. The parts
restore in potentiometer mode after a reset or power-up.
Low Wiper Resistance Feature
The AD5123/AD5143 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 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 15); by loading new data in an RDAC register, which
includes increment/decrement operations; or by entering
shutdown mode, Command 15 (see Table 15).
Table 11 and Table 12 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 11. 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 12. 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 15) 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 resistance RDAC position. This
command can be executed in a single channel or multiple channels.