AD5121 datasheet(21/28 Pages) AD

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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 (I2C only)

•

Reset

•

Shutdown mode

Input Register

The AD5123/AD5143 include one input register per RDAC

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, RAW, and RWB. 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, RAW = RAB − RWB.

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 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 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

RAW

RWB

RAW

RWB

RAB

RTS

RAB

Table 12. Bottom Scale Truth Table

Linear Gain Setting Mode

Potentiometer Mode

RAW

RWB

RAW

RWB

RTS

RBS

RAB

RBS

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.

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