AD7147ACPZ-1REEL(Rev0) View Datasheet(PDF) - Analog Devices

Part Name

Description

Manufacturer

AD7147ACPZ-1REEL
(Rev.:Rev0)

CapTouch™ Programmable Controller for Single-Electrode Capacitance Sensors

Analog Devices 

AD7147

CAPACITIANCE-TO-DIGITAL CONVERTER

The capacitance-to-digital converter on the AD7147 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 goal is to ensure that the CDC_RESULT_Sx is as close

to midscale as possible. This process is only required once

during the initial capacitance sensor characterization.

+DAC

6

(20pF RANGE)

POS_AFE_OFFSET

The decimation rate, or oversampling ratio, is determined by

Bits [9:8] of the power control (PWR_CONTROL) register

(Address 0x000), as listed in Table 9.

POS_AFE_OFFSET_SWAP BIT

Table 9. CDC Decimation Rate

CINx

DECIMATION Bits

CDC Output Rate

Decimation Rate Per Stage (ms)

00

256

3.072

01

128

1.536

10

64

0.768

11

64

0.768

+

16-BIT

16

_ CDC

NEG_AFE_OFFSET_SWAP BIT

–DAC

6

(20pF RANGE)

NEG_AFE_OFFSET

The decimation process on the AD7147 is an averaging process,

where a number of samples are taken and the averaged result is

CINx_CONNECTION_SETUP

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

Figure 23. Analog Front-End Offset Control

CONVERSION SEQUENCER

The AD7147 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,

There are two programmable DACs on board the AD7147 to null

the effect of any stray capacitances on the CDC measurement.

the input mux that connects the CINx inputs to the converter

can have a unique setting.

These offsets are due to stray capacitance to ground.

The AD7147 on-chip sequence controller provides conversion

A simplified block diagram in Figure 23 shows how to apply the

STAGEx_AFE_OFFSET registers to null the offsets. The 6-bit

POS_AFE_OFFSET and NEG_AFE_OFFSET bits program the

offset DAC 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. Read back the CDC value from the CDC_RESULT_Sx register.

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. 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. This number

will depend 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

2. If this value is not close to midscale, increase the value of

should be set according to the number of stages that are used.

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.

The number of required conversion stages depends solely on

the number of sensors attached to the AD7147. Figure 25 shows

how many conversion stages are required for each sensor and

how many inputs to the AD7147 each sensor requires.

3. If the CDC value in CDC_RESULT_Sx is now closer

to midscale, repeat Step 2. If the CDC value is further

from midscale, decrease the POS_AFE_OFFSET or

NEG_AFE_OFFSET value by 1.

Rev. 0 | Page 14 of 68

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