AD7142 Analog Devices, AD7142 Datasheet - Page 16

• Manufacturer Part Number: AD7142
• Description: Programmable Controller for Capacitance Touch Sensors
• Manufacturer: Analog Devices

Specifications of AD7142

• Resolution (bits): 16bit
• # Chan: 14
• Sample Rate: 250kSPS
• Interface: I²C/Ser 2-Wire,Ser,SPI
• Analog Input Type: Diff-Uni
• Ain Range: ± 2 pF (Delta C)
• Adc Architecture: Sigma-Delta
• Pkg Type: CSP

AD7142
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.
SRC
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.
8-ELEMENT SLIDER
CIN10
CIN1 1
CIN12
CIN13
CIN0
CIN1
CIN2
CIN3
CIN4
CIN5
CIN6
CIN7
CIN8
CIN9
Figure 25. Sequencer Setup for Sensors
Figure 24. CDC Conversion Stages
AD7142 SEQUENCER
STAGE0
STAGE1
STAGE2
STAGEX
+
–
STAGEX
+
–
STAGEX
+
–
STAGEX
+
–
STAGEX
+
–
STAGEX
+
–
STAGEX
+
–
STAGEX
+
–
CDC
CDC
CDC
CDC
CDC
CDC
CDC
CDC
STAGE3
STAGE4
STAGE5
STAGE6
STAGE7
16-BIT
STAGE8
ADC
STAGE9
Σ-Δ
STAGE10
STAGE11
SRC
BUTTONS
S1
S2
S3
AD7142 SEQUENCER
STAGEX
+
–
STAGEX
+
–
CDC
CDC
Rev. A | Page 16 of 72
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
SEQUENCE_STAGE_NUM
0
1
2
3
4
5
6
7
8
9
10
11
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 Table 10.
Figure 26 shows a simplified timing diagram of the full power
CDC conversion time. The full power mode CDC conversion
time, t
CONVERSION
CONV_FP
Figure 26. Full Power Mode CDC Conversion Sequence Time
CDC
, is set using Table 10.
NOTES
1.
t
CONV_FP
SEQUENCE N
CONVERSION
t
CONV_FP
= VALUE SET FROM TABLE 10.
SEQUENCE N+1
DECIMATION
= 128
1.536
3.072
4.608
6.144
7.68
9.216
10.752
12.288
13.824
15.36
16.896
18.432
CONVERSION
Conversion Time (ms)
SEQUENCE N+2
CONVERSION
DECIMATION
= 256
3.072
6.144
9.216
12.288
15.36
18.432
21.504
24.576
27.648
30.72
33.792
36.864