Datasheet

AD7142
Rev. A | Page 16 of 72
CIN0
CIN1
CIN2
CIN3
CIN4
CIN5
CIN6
CIN7
CIN8
CIN9
C
IN10
C
IN11
C
IN12
C
IN13
STAGE0
STAGE1
STAGE2
STAGE3
STAGE4
STAGE5
STAGE6
STAGE7
STAGE8
STAGE9
STAGE10
STAGE11
SWITCH MATRIX
Σ-Δ
16-BIT
ADC
C
O
NV
ERS
I
O
N
S
EQ
UEN
C
E
05702-012
Figure 24. CDC Conversion Stages
The number of required conversion stages depends completely
on the number of sensors attached to the AD7142.
Figure 25
shows how many conversion stages are required for each sensor,
and how many inputs each sensor requires to the AD7142.
05702-014
BUTTONS
STAGEX
STAGEX
8-ELEMENT SLIDER
CDC
STAGEX
+
–
CDC
STAGEX
+
–
CDC
STAGEX
+
–
CDC
STAGEX
+
–
CDC
STAGEX
+
–
CDC
STAGEX
+
–
CDC
STAGEX
+
–
CDC
STAGEX
+
–
AD7142 SEQUENCER
AD7142 SEQUENCER
S
RC
SRC
S2
S3
S1
CDC
+
–
CDC
+
–
Figure 25. 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, and is shown in
Figure 25, B2 and B3.
A scroll bar or slider 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
Tabl e 10 .
Table 10. CDC Conversion Times for Full Power Mode
Conversion Time (ms)
SEQUENCE_STAGE_NUM
DECIMATION
= 128
DECIMATION
= 256
0 1.536 3.072
1 3.072 6.144
2 4.608 9.216
3 6.144 12.288
4 7.68 15.36
5 9.216 18.432
6 10.752 21.504
7 12.288 24.576
8 13.824 27.648
9 15.36 30.72
10 16.896 33.792
11 18.432 36.864
For example, 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
register, and DECIMATION register as outlined in
Tabl e 10.
Figure 26 shows a simplified timing diagram of the full power
CDC conversion time. The full power mode CDC conversion
time, t
CONV_FP
, is set using Table 10 .
NOTES
1.
t
CONV_FP
= VALUE SET FROM TABLE 10.
t
CONV_FP
CONVERSION
SEQUENCE N
CONVERSION
SEQUENCE N+1
CONVERSION
SEQUENCE N+2
CDC
CONVERSION
05702-015
Figure 26. Full Power Mode CDC Conversion Sequence Time