EVAL-AD7143-1EBZ Analog Devices Inc, EVAL-AD7143-1EBZ Datasheet - Page 14

• Manufacturer Part Number: EVAL-AD7143-1EBZ
• Description: BOARD EVALUATION FOR AD7143-1
• Manufacturer: Analog Devices Inc
• Type: Capacitive Touch and Proximity Sensorr

Specifications of EVAL-AD7143-1EBZ

• Contents: *
• For Use With/related Products: AD7143-1
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

AD7143
CAPACITIANCE-TO-DIGITAL CONVERTER
The capacitance-to-digital converter on the AD7143 has a Σ-Δ
architecture with 16-bit resolution. Eight possible inputs to the
CDC 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 PWR_CONTROL register located at Address 0x000
and listed in Table 7.
Table 7. CDC Decimation Rate
Decimation Bit
Value
00
01
10
11
1
The decimation process on the AD7143 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× the
decimation rate. Therefore, 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 AD7143 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 21, C
capacitance of the input sensors, while C
between layers of the sensor PCB. C
on-board DACs.
Do not use this setting.
1
1
PLASTIC OVERLAY
SENSOR BOARD
Figure 21. Capacitances Around the Sensor PCB
CAPACITIVE SENSOR
Decimation
Rate
256
128
–
–
BULK
can be offset using the
BULK
CDC Output Rate
Per Stage
3.072 ms
1.525 ms
–
–
C
C
is the capacitance
IN
BULK
IN
is the
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A simplified block diagram in Figure 22 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.
CONVERSION SEQUENCER
The AD7143 has an on-chip sequencer to implement
conversion control for the input channels. Up to eight
conversion stages can be performed in sequence. Each of the
eight conversion stages can measure an 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 sensor S1 can be assigned to
STAGE1 and sensor S2 assigned to STAGE2.
The AD7143 on-chip sequence controller provides conversion
control beginning with STAGE0. Figure 23 shows a block diagram of
the CDC conversion stages and CIN inputs. A conversion sequence is
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 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 is set to 5 if the CIN
inputs are 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 22. 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
POS_AFE_OFFSET
NEG_AFE_OFFSET
16