AD7148 Analog Devices, AD7148 Datasheet - Page 14

• Manufacturer Part Number: AD7148
• Manufacturer: Analog Devices

Specifications of AD7148

• Resolution (bits): 16bit
• # Chan: 8
• Sample Rate: 250kSPS
• Interface: I²C/Ser 2-Wire,Ser
• Analog Input Type: SE-Uni
• Ain Range: ± 8 pF (Delta C)
• Adc Architecture: Sigma-Delta
• Pkg Type: CSP

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.
CIN0
CIN1
CIN2
CIN3
CIN4
CIN5
CIN6
CIN7
Figure 21. CDC Conversion Stages
STAGE0
STAGE1
STAGE2
STAGE3
STAGE4
STAGE5
STAGE6
STAGE7
16-BIT
ADC
Σ-Δ
WHEEL
Figure 22. Sequencer Setup for Sensors
Rev. A | Page 14 of 56
SEQUENCER
AD7148
STAGE0
STAGE1
STAGE1
STAGE1
STAGE1
STAGE1
STAGE1
STAGE1
+
– CDC
+
–
+
–
+
–
+
–
+
–
+
–
+
–
CDC
CDC
CDC
CDC
CDC
CDC
CDC
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.
B1
B2
B3
SEQUENCER
SEQUENCER
AD7148
AD7148
STAGE0
STAGE1
STAGE3
STAGE4
+
–
+
–
+
+
–
–
CDC
CDC
CDC
CDC