AD7147A Analog Devices, AD7147A Datasheet - Page 14

• Manufacturer Part Number: AD7147A
• Manufacturer: Analog Devices

Specifications of AD7147A

• Resolution (bits): 16bit
• # Chan: 13
• Sample Rate: 111SPS
• Interface: I²C/Ser 2-Wire,Ser,SPI
• Analog Input Type: Capacitive
• Ain Range: ± 8 pF (Delta C)
• Adc Architecture: Sigma-Delta
• Pkg Type: CSP

AD7147A
CAPACITANCE-TO-DIGITAL CONVERTER
The capacitance-to-digital converter on the AD7147A has a
Σ-Δ architecture with 16-bit resolution. There are 13 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 power control register (PWR_CONTROL,
Address 0x000), as listed in Table 9.
Table 9. CDC Decimation Rate
Decimation Bits
00
01
10
11
The decimation process on the AD7147A 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 number of samples taken (per stage) is equal to 3×
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; therefore, there is a trade-off
possible between the amount of noise in the signal and the
speed of sampling.
CAPACITANCE SENSOR OFFSET CONTROL
There are two programmable DACs on board the AD7147A to null
the effect of any stray capacitances on the CDC measurement.
These offsets are due to stray capacitance to ground.
A simplified block diagram in Figure 23 shows how to apply
the STAGEx_AFE_OFFSET registers to null the offsets. The
POS_AFE_OFFSET and NEG_AFE_OFFSET bits (Bits[13:8] and
Bits[5:0], respectively) program the offset DACs to provide 0.32
pF resolution offset adjustment over a range of 20 pF.
The best practice is to ensure that the CDC output for any stage
is approximately equal to midscale (~32,700) when all sensors
are inactive. To correctly offset the stray capacitance to ground for
each stage, use the following procedure:
1.
2.
3.
Read back the CDC value from the CDC_RESULT_Sx
register.
If this value is not close to midscale, increase the value of
POS_AFE_OFFSET or NEG_AFE_OFFSET (depending
on if the CINx input is connected to the positive or negative
input of the converter) by 1. The CINx connections are
determined by the STAGEx_CONNECTION registers.
If the CDC value in CDC_RESULT_Sx is now closer
to midscale, repeat Step 2. If the CDC value is further
Decimation Rate
256
128
64
64
CDC Output Rate
Per Stage (ms)
3.072
1.536
0.768
0.768
Rev. B | Page 14 of 68
CONVERSION SEQUENCER
The AD7147A 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 conversions 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. For each conversion stage,
the input mux that connects the CINx inputs to the converter
can have a unique setting.
The AD7147A on-chip sequence controller provides conversion
control, beginning with STAGE0. Figure 24 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
programmed in the SEQUENCE_STAGE_NUM bits (Bits[7:4],
Address 0x00). 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 conver-
sions in one sequence. This number depends on the sensor
interface requirements. For example, the register should be set
to 5 if the CINx inputs are mapped to only six conversion stages.
In addition, the STAGE_CAL_EN register (Address 0x001)
should be set according to the number of stages that are used.
The number of required conversion stages depends solely on
the number of sensors attached to the AD7147A. Figure 25
shows how many conversion stages are required for each sensor
and how many inputs to the AD7147A each sensor requires.
CINx
from midscale, decrease the POS_AFE_OFFSET or
NEG_AFE_OFFSET value by 1.
The goal is to ensure that the CDC_RESULT_Sx is as close
to midscale as possible. This process is required only once
during the initial capacitance sensor characterization.
CINx_CONNECTION_SETUP
Figure 23. Analog Front-End Offset Control
(20pF RANGE)
(20pF RANGE)
+DAC
–DAC
POS_AFE_OFFSET_SWAP BIT
NEG_AFE_OFFSET_SWAP BIT
+
_
16-BIT
CDC
6
6
POS_AFE_OFFSET
NEG_AFE_OFFSET
16