AD5121 datasheet(21/25 Pages) AD

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Data Sheet

AD5123/AD5143

THEORY OF OPERATION

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Rev. C | 21 of 25

Table 13. Control Register Bit Descriptions

Bit Name

Description

Linear setting mode/potentiometer mode

0 = potentiometer mode (default)

1 = linear gain setting mode

Burst mode

0 = disabled (default)

1 = enabled (no disable after stop or repeat start condition)

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 11).

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 11).

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 inde-

pendent control of each string resistor, RAW, and RWB. To enable

linear gain setting mode, use Command 16 (see Table 11) to set

Bit D2 of the control register (see Table 13).

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 mode enables a second input and an RDAC register per chan-

nel, as shown in Table 12; however, the actual RDAC contents re-

main unchanged. The same operations are valid for potentiometer

and linear setting gain modes. The devices 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 11); by loading new data in an RDAC register, which includes

increment/decrement operations; or by entering shutdown mode,

Command 15 (see Table 11).

Table 14 and Table 15 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 14. Top Scale Truth Table

Linear Gain Setting Mode

Potentiometer Mode

RAW

RWB

RAW

RWB

RAB

RTS

RAB

Table 15. 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 11) 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 resistance RDAC position. This com-

mand 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 11). 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 16).

Incrementing the wiper position by +6 dB essentially doubles the

RDAC register value, whereas decrementing the wiper position by

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