AD5142ABCPZ100-RL7 datasheet(24/29 Pages) AD

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Data Sheet

AD5122A/AD5142A

THEORY OF OPERATION

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Input Register

The AD5122A/AD5142A include one input register per RDAC regis-

ter. 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 13).

This feature allows a synchronous update of one or both RDAC

registers at the same time.

The transfer from the input register to the RDAC register is done

synchronously by Command 8 (see Table 13).

If new data is loaded in an RDAC register, this RDAC register

automatically overwrites the associated input register.

Linear Gain Setting Mode

The proprietary architecture of the AD5122A/AD5142A allows the

independent control of each string resistor, RAW, and RWB. To

enable linear gain setting mode, use Command 16 (see Table 13)

to set Bit D2 of the control register (see Table 15).

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

channel, as shown in Table 13; however, the actual RDAC contents

remain unchanged. The same operations are valid for potentiome-

ter 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 14). The INDEP pin and the

D2 bit are connected internally to a logic OR gate; if one or both are

set to 1, the parts cannot operate in potentiometer mode.

Low Wiper Resistance Feature

The AD5122A/AD5142A include two commands to reduce the wip-

er resistance between the terminals when the device achieves 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

13); by loading new data in an RDAC register, which includes

increment/decrement operations; or by entering shutdown mode,

Command 15 (see Table 13).

Table 16 and Table 17 show the truth tables for the top scale posi-

tion and the bottom scale position, respectively, when potentiometer

or linear gain setting mode is enabled.

Table 16. Top Scale Truth Table

Linear Gain Setting Mode

Potentiometer Mode

RAW

RWB

RAW

RWB

RAB

RTS

RAB

Table 17. 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 13) 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 in 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 13). 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 18).

Incrementing the wiper position by +6 dB essentially doubles the

RDAC register value, whereas decrementing the wiper position by

−6 dB halves the register value. Internally, the AD5122A/AD5142A

use shift registers to shift the bits left and right to achieve a ±6

dB increment or decrement. These functions are useful for various

audio/video level adjustments, especially for white LED brightness

settings in which human visual responses are more sensitive to

large adjustments than to small adjustments.

Table 18. Detailed Left Shift and Right Shift Functions for the ±6 dB Step

Increment and Decrement

Left Shift (+6 dB/Step)

Right Shift (−6 dB/Step)

0000 0000

1111 1111

0000 0001

0111 1111

0000 0010

0011 1111

0000 0100

0001 1111

0000 1000

0000 1111

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