AD7142 datasheet(15/68 Pages) AD

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AD7142

Rev. 0 | Page 15 of 68

CIN0

CIN1

CIN2

CIN3

CIN4

CIN5

CIN6

CIN7

CIN8

CIN9

CIN10

CIN1 1

CIN12

CIN13

STAGE 0

STAGE 1

STAGE 2

STAGE 3

STAGE 4

STAGE 5

STAGE 6

STAGE 7

STAGE 8

STAGE 9

STAGE 10

STAGE 11

Σ-Δ

16-BIT

ADC

Figure 22. CDC Conversion Stages

The number of required conversion stages depends completely

on the number of sensors attached to the AD7142. Figure 23

shows how many conversion stages are required for each sensor,

and how many inputs each sensor requires to the AD7142.

STAGEX

CDC

STAGEX

CDC

–

STAGEX

SCROLL BAR

8-WAY SWITCH

CDC

–

STAGEX

–

CDC

STAGEX

–

CDC

STAGEX

–

CDC

AD7142 SEQUENCER

CDC

STAGEX

–

STAGEX

–

CDC

STAGEX

–

STAGEX

–

CDC

STAGEX

–

STAGEX

–

CDC

AD7142 SEQUENCER

STAGEX

–

CDC

Figure 23. Sequencer Setup for Sensors

A button sensor generally requires one sequencer stage;

however, it is possible to configure two button sensors to

operate differentially. Only one button from the pair can be

activated at a time; pressing both buttons together results in

neither button being activated. This configuration requires one

conversion stage.

A scroll bar sensor requires eight stages. The result from each stage

is used by the host software to determine the user’s position on the

scroll bar. The algorithm that performs this process is available

from Analog Devices free of charge, on signing a software license.

Scroll wheels also require eight stages.

The 8-way switch is made from two pairs of differential buttons.

It, therefore, requires two conversion stages, one for each of the

differential button pairs. It also requires a stage to measure

whether the sensor is active. The buttons are orientated so that

one pair makes up the top and bottom portions of the 8-way

switch; the other pair makes up the left and right portions of the

8-way switch.

CDC CONVERSION SEQUENCE TIME

The time required for one complete measurement for all 12 stages

by the CDC is defined as the CDC conversion sequence time. The

SEQUENCE_STAGE_NUM register and DECIMATION register

determine the conversion time as listed in Table 10.

Table 10. CDC Conversion Times for Full Power Mode

Conversion Time (ms)

SEQUENCE_STAGE_NUM

DECIMATION

= 128

DECIMATION

= 256

1.536

3.072

6.144

4.608

9.216

6.144

12.288

7.68

15.36

9.216

18.432

10.752

21.504

12.288

24.576

13.824

27.648

15.36

30.72

16.896

33.792

18.432

36.864

For example, while operating with a decimation rate of 128,

if the SEQUENCE_STAGE_NUM register is set to 5 for the

conversion of six stages in a sequence, the conversion sequence

time is 9.216 ms.

Full Power Mode CDC Conversion Sequence Time

The full power mode CDC conversion sequence time for all 12

stages is set by configuring the SEQUENCE_STAGE_NUM

Figure 24 shows a simplified timing diagram of the full power

CDC conversion time. The full power mode CDC conversion

time tCONV_FP is set using Table 10.

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