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Data Sheet

AD5122/AD5142

THEORY OF OPERATION

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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 in-

dependent 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 inde-

pendent rheostats connected at a single point, W terminal, as

opposed to potentiometer mode where each resistor is complemen-

tary, 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 potentiome-

ter 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

RAW

RWB

RAW

RWB

RAB

RTS

RAB

Table 13. 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 Com-

mand 5 in Table 16) are useful for linear step adjustment applica-

tions. 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.

±6 DB Increment and Decrement Instructions

Two programming instructions produce logarithmic taper increment

or decrement of the wiper position control by an individual poten-

tiometer 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

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