AD7148 Datasheet, PDF (14/56 Pages) Analog Devices – Programmable Touch Controller for Single Electrode Capacitance Sensors

AD7148
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.
STAGE7
STAGE6
STAGE5
STAGE4
STAGE3
STAGE2
STAGE1
STAGE0
CIN0
CIN1
CIN2
CIN3
CIN4
CIN5
CIN6
CIN7
Σ-Δ
16-BIT
ADC
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.
Figure 21. CDC Conversion Stages
WHEEL
AD7148
SEQUENCER
STAGE0
+
–
CDC
STAGE1
+
–
CDC
STAGE1
+
–
CDC
STAGE1
+
–
CDC
STAGE1
+
–
CDC
STAGE1
+
–
CDC
STAGE1
+
– ^CDC
STAGE1
+
–
CDC
AD7148
SEQUENCER
STAGE0
B1
+
–
CDC
B2
STAGE1
+CDC
B3
–
AD7148
SEQUENCER
STAGE3
+ _CDC
–
STAGE4
+
CDC
–
Figure 22. Sequencer Setup for Sensors
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