AD7142 Analog Devices, AD7142 Datasheet - Page 15

• 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

CAPACITANCE-TO-DIGITAL CONVERTER
The capacitance-to-digital converter on the AD7142 has a Σ-Δ
architecture with 16-bit resolution. There are 14 possible inputs to
the CDC that are connected to the input of the converter through a
switch matrix. The sampling frequency of the CDC is 250 kHz.
OVERSAMPLING THE CDC OUTPUT
The decimation rate, or oversampling ratio, is determined by
Bits[9:8] of the control register, as listed in Table 9.
Table 9. CDC Decimation Rate
Decimation Bit Value
00
01
10
11
1
The decimation process on the AD7142 is an averaging process
where a number of samples are taken and the averaged result is
output. Due to the architecture of the digital filter employed, the
amount of samples taken (per stage) is equal to 3 times the
decimation rate. So 3 × 256 or 3 × 128 samples are averaged to
obtain each stage result.
The decimation process reduces the amount of noise present in
the final CDC result. However, the higher the decimation rate,
the lower the output rate per stage, thus, a trade-off is possible
between a noise-free signal and speed of sampling.
CAPACITANCE SENSOR OFFSET CONTROL
There are two programmable DACs on board the AD7142 to
null any capacitance sensor offsets. These offsets are associated
with printed circuit board capacitance or capacitance due to any
other source, such as connectors. In Figure 22, C
capacitance of the input sensors, and C
between layers of the sensor PCB. C
on-board DACs.
A simplified block diagram in Figure 23 shows how to apply the
STAGE_OFFSET registers to null the offsets. The 7-bit
POS_AFE_OFFSET and NEG_AFE_OFFSET registers program
the offset DAC to provide 0.16 pF resolution offset adjustment
over a range of ±20 pF. Apply the positive and negative offsets
to either the positive or the negative CDC input using the
NEG_AFE_OFFSET register and POS_AFE_OFFSET register.
This process is only required once during the initial capacitance
sensor characterization.
Do not use this setting.
1
1
PLASTIC OVERLAY
SENSOR BOARD
Figure 22. Capacitances Around the Sensor PCB
CAPACITIVE SENSOR
Decimation Rate
256
128
–
–
BULK
BULK
can be offset using the
is the capacitance
C
C
BULK
IN
CDC Output Rate
Per Stage
3.072 ms
1.536 ms
–
–
IN
is the
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CONVERSION SEQUENCER
The AD7142 has an on-chip sequencer to implement
conversion control for the input channels. Up to 12 conversion
stages can be performed in one sequence. Each of the 12
conversion stages can measure the input from a different sensor. By
using the Bank 2 registers, each stage can be uniquely configured to
support multiple capacitance sensor interface requirements. For
example, a slider sensor can be assigned to STAGE1 through
STAGE8, with a button sensor assigned to STAGE0.
The AD7142 on-chip sequence controller provides conversion
control beginning with STAGE0. Figure 24 shows a block diagram of
the CDC conversion stages and CIN inputs. A conversion sequence is
defined as a sequence of CDC conversions starting at STAGE0 and
ending at the stage determined by the value programmed in the
SEQUENCE_STAGE_NUM register. Depending on the number and
type of capacitance sensors that are used, not all conversion stages are
required. Use the SEQUENCE_STAGE_NUM register to set the
number of conversions in one sequence, depending on the sensor
interface requirements. For example, this register would be set to 5 if
the CIN inputs were mapped to only six stages. In addition, set the
STAGE_CAL_EN registers according to the number of stages that
are used.
SRC
CIN
CIN_CONNECTION_SETUP
Figure 23. Analog Front-End Offset Control
REGISTER
(20pF RANGE)
(20pF RANGE)
–DAC
+DAC
POS_AFE_OFFSET_SWAP BIT
NEG_AFE_OFFSET_SWAP BIT
+
_
16-BIT
CDC
7
7
NEG_AFE_OFFSET
POS_AFE_OFFSET
16
AD7142