AD5121 datasheet(23/32 Pages) AD

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Data Sheet

AD5121/AD5141

Rev. 0 | Page 23 of 32

ADVANCED CONTROL MODES

The AD5121/AD5141 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

•

Linear increment and decrement instructions

•

±6 dB increment and decrement instructions

•

Burst mode (I2C only)

•

Reset

•

Shutdown mode

Input Register

The AD5121/AD5141 include one input register per RDAC

associated RDAC register.

This feature allows a synchronous and asynchronous update of

one or all the RDAC registers at the same time.

These registers can be written to using Command 2 and read back

from using Command 3 (see Table 16).

The transfer from the input register to the RDAC register is

done asynchronously by the LRDAC pin or 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 AD5121/AD5141 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 16; however, the actual RDAC contents

remain unchanged. The same operations are valid for

potentiometer and linear gain setting modes.

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 parts cannot operate in potentiometer mode.

Low Wiper Resistance Feature

The AD5121/AD5141 include two commands to reduce the wiper

resistance between the terminals when they 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 two ways to exit from top scale and

bottom scale: by using Command 12 or Command 13 (see

Table 16); or by loading new data in an RDAC register, which

includes increment/decrement operations and a shutdown

command.

Table 12 and Table 13 show the truth tables for the top scale

position and the bottom scale position, respectively, when linear

gain setting mode is enabled.

Table 12. Top Scale Truth Table

Linear Gain Setting Mode

Potentiometer Mode

Table 13. Bottom Scale Truth Table

Linear Gain Setting Mode

Potentiometer Mode

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 resistance segment position. This

command can be executed in a single channel or multiple channels.

±6 dB Increment and Decrement Instructions

Two programming instructions produce logarithmic taper

increment or decrement of the wiper position control by

an individual potentiometer or by a ganged potentiometer

arrangement where all RDAC register positions are changed

simultaneously. The +6 dB increment is activated by Command 6,

and the −6 dB decrement is activated by Command 7 (see Table 16).

For example, starting with the zero-scale position and executing

Command 6 ten times moves the wiper in 6 dB steps to the full-

scale position. When the wiper position is near the maximum

setting, the last 6 dB increment instruction causes the wiper to go

to the full-scale position (see Table 14).

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