AD5121 datasheet(32/37 Pages) AD

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Data Sheet

AD5124/AD5144/AD5144A

THEORY OF OPERATION

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Input Register

The AD5124/AD5144/AD5144A 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 17).

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

asynchronously by the LRDAC pin or synchronously by Command

8 (see Table 17).

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 AD5124/AD5144/AD5144A al-

lows the independent control of each string resistor, RAW, and RWB.

To enable this feature, use Command 16 (see Table 17) to set Bit

D2 of the control register (see Table 19).

This mode of operation can control the potentiometer as two inde-

pendent rheostats connected at a single point, the W terminal.

This feature enables a second input and an RDAC register per

channel, as shown in Table 18, but the actual RDAC contents re-

main unchanged. The same operations are valid for potentiometer

and linear gain setting modes. The EEPROM commands affect the

RWB resistance only. The parts restores in potentiometer mode after

a reset or power-up.

Low Wiper Resistance Feature

The AD5124/AD5144/AD5144A include two commands to reduce

the wiper resistance between the terminals when the devices ach-

ieve 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 into

these positions. There are three ways to exit from top scale and

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

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

increment/decrement operations; or by entering shutdown mode,

Command 15 (see Table 17).

Table 20 and Table 21 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 20. Top Scale Truth Table

Linear Gain Setting Mode

Potentiometer Mode

RAW

RWB

RAW

RWB

RAB

RTS

RAB

Table 21. 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 17) 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 17). 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 22).

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 AD5124/AD5144/

AD5144A 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 22. 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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