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AD7148

Rev. 0 | Page 14 of 56

The AD7148 on-chip sequence controller provides conversion

control, beginning with STAGE0. Figure 21 shows a block diagram

of the CDC conversion stages and CINx inputs. A conversion

sequence is defined as a sequence of CDC conversions starting

at STAGE0 and ending at the stage determined by the value that

is programmed using the SEQUENCE_STAGE_NUM bits in

the PWR_CONTROL register (Address 0x000[7:4]). Depending

on the number and type of capacitance sensors that are used,

not all conversion stages are required. Use the SEQUENCE_

STAGE_NUM bits to set the number of conversions in one

sequence, depending on the sensor interface requirements. For

example, these bits are set to 0005 if the CINx inputs are mapped

to only six stages. In addition, set the STAGEx_CAL_EN register

according to the number of stages that are used.

STAGE0

STAGE1

STAGE2

STAGE3

STAGE4

STAGE5

STAGE6

STAGE7

Σ-Δ

16-BIT

ADC

CIN0

CIN1

CIN2

CIN3

CIN4

CIN5

CIN6

CIN7

Figure 21. CDC Conversion Stages

The number of required conversion stages depends completely

on the number of sensors attached to the AD7148. Figure 22 shows

how many conversion stages are required for each sensor and

how many inputs to the AD7148 each sensor requires.

A button sensor generally requires one sequencer stage; however,

it is possible to configure two button sensors to operate differen-

tially. 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 (see

Figure 22, B2 and B3).

A wheel sensor requires eight stages, and a slider requires two

stages. The result from each stage is used by the host software to

determine user position on the slider or wheel. The algorithms that

perform this process are available from Analog Devices, free of

charge, on signing a software license.

CDC CONVERSION SEQUENCE TIME

The time required for one complete measurement for all eight

stages by the CDC is defined as the CDC conversion sequence

time. The SEQUENCE_STAGE_NUM and DECIMATION bits

determine the conversion time, as shown in Table 9.

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

if the SEQUENCE_STAGE_NUM bits are set to 0005 for the

conversion of six stages in a sequence, the conversion sequence

time is 9.216 ms.

CDC

–

CDC

–

– CDC

CDC

–

– CDC

CDC

–

– CDC

CDC

–

CDC

AD7148

SEQUENCER

CDC

–

CDC

–

WHEEL

STAGE1

STAGE0

STAGE4

STAGE3

AD7148

SEQUENCER

AD7148

SEQUENCER

STAGE0

STAGE1

Figure 22. Sequencer Setup for Sensors

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