AD7147 datasheet(24/68 Pages) AD

24 / 68 page

AD7147

Rev. 0 | Page 24 of 68

CAPACITANCE SENSOR BEHAVIOR WITH

CALIBRATION

The AD7147 on-chip adaptive calibration algorithm prevents

sensor detection errors such as the one shown in Figure 35.

This is achieved by monitoring the CDC ambient levels

and readjusting the initial STAGEx_OFFSET_HIGH and

STAGEx_OFFSET_LOW values according to the amount of

ambient drift measured on each sensor. Based on the new

stage offset values, the internal STAGEx_HIGH_THRESHOLD

and STAGEx_LOW_THRESHOLD values described in

Equation 1 and Equation 2 are automatically updated.

This closed-loop routine ensures the reliability and repeatable

operation of every sensor connected to the AD7147 when they

are subjected to dynamic environmental conditions. Figure 36

shows a simplified example of how the AD7147 applies the

adaptive calibration process, resulting in no interrupt errors

even with changing CDC ambient levels due to dynamic

environmental conditions.

SENSOR 1 INT

ASSERTED

STAGEx_HIGH_THRESHOLD

(POSTCALIBRATED

CHANGING ENVIRONMENTAL CONDITIONS

1INITIAL STAGEx_OFFSET_HIGH REGISTER VALUE.

2POSTCALIBRATED REGISTER STAGEx_HIGH_THRESHOLD.

3POSTCALIBRATED REGISTER STAGEx_HIGH_THRESHOLD.

4INITIAL STAGEx_LOW_THRESHOLD.

5POSTCALIBRATED REGISTER STAGEx_LOW_THRESHOLD.

6POSTCALIBRATED REGISTER STAGEx_LOW_THRESHOLD.

CDC AMBIENT

VALUE DRIFTING

STAGEx_LOW_THRESHOLD

(POSTCALIBRATED

SENSOR 2 INT

ASSERTED

Figure 36. Typical Sensor Behavior with Calibration Applied on the Data Path

SLOW FIFO

As shown in Figure 33, there are a number of FIFOs

implemented on the AD7147. These FIFOs are located in

Bank 3 of the on-chip memory. The slow FIFOs are used by the

on-chip logic to monitor the ambient capacitance level from

each sensor.

AVG_FP_SKIP and AVG_LP_SKIP

In Register 0x001, Bits [13:12] are the slow FIFO skip control

for full power mode, AVG_FP_SKIP. Bits [15:14] in the same

AVG_LP_SKIP, and determine which CDC samples are not

used (skipped) in the slow FIFO. Changing the values of the

AVG_FP_SKIP and AVG_LP_SKIP bits slows down or speeds

up the rate at which the ambient capacitance value tracks the

measured capacitance value read by the converter:

•

Slow FIFO update rate in full power mode = AVG_FP_SKIP ×

[(3 × Decimation Rate) × (SEQUENCE_STAGE_NUM + 1) ×

(FF_SKIP_CNT + 1) × 4 × 10−7].

•

Slow FIFO update rate in low power mode = (AVG_LP_SKIP

+ 1) × [(3 × Decimation Rate) × (SEQUENCE_STAGE_NUM

+ 1) × (FF_SKIP_CNT + 1) × 4 x 10−7]/[(FF_SKIP_CNT + 1)

+ LP_CONV_DELAY].

The slow FIFO is used by the on-chip logic to track the ambient

capacitance value. The slow FIFO expects to receive samples from

the converter at a rate between 33 ms and 40 ms. AVG_FP_SKIP

and AVG_LP_SKIP are used to normalize the frequency of the

samples going into the FIFO, regardless of how many conversion

stages are in a sequence.

Determining the AVG_FP_SKIP and AVG_LP_SKIP values is

required only once during the initial setup of the capacitance

sensor interface. The recommended values for these settings

when using all 12 conversion stages on the AD7147 are as follows:

•

AVG_FP_SKIP = 00 = skip three samples

•

AVG_LP_SKIP = 00 = skip zero samples

SLOW_FILTER_UPDATE_LVL

The SLOW_FILTER_UPDATE_LVL controls whether the most

recent CDC measurement goes into the slow FIFO (slow filter).

The slow filter is updated when the difference between the

current CDC value and the last value of the slow FIFO is greater

than the value of SLOW_FILTER_UPDATE_LVL. This variable

is in Ambient Control Register 1 (AMB_COMP_CTRL1)

(Address 0x003).

AD7147 Datasheet(PDF) 24 Page - Analog Devices

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