AD7143 datasheet(19/56 Pages) AD

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AD7143

Rev. 0 | Page 19 of 56

CALIBRATION ENABLED

tRECAL_TIMEOUT

tCONV_FP

MEASURED CDC VALUE > STORED AMBIENT

BY PROXIMITY_RECAL _LVL

RECALIBRATION TIMEOUT

PROXIMITY

DETECTION

(INTERNAL)

CALIBRATION

(INTERNAL)

CDC CONVERSION

SEQUENCE

(INTERNAL)

RECALIBRATION

(INTERNAL)

USER APPROACHES

SENSOR HERE

USER LEAVES SENSOR

AREA HERE

tCALDIS

tRECAL

CALIBRATION DISABLED

NOTES

1. SEQUENCE CONVERSION TIME

tCONV_LP = tCONV_FP + LP_CONV_DELAY.

tCALDIS = tCONV_LP × LP_PROXIMITY_CNT × 4.

tRECAL_TIMEOUT = tCONV_FP × LP_PROXIMITY_RECAL.

tRECAL = 2 × tCONV_LP.

Figure 30. Low Power Mode Proximity Detection with Forced Recalibration Example with LP_PROXIMIT_CNT = 4 and LP_PROXIMITY_RECAL = 10

FF_SKIP_CNT

The proximity detection fast FIFO is used by the on-chip logic

to determine if proximity is detected. The fast FIFO expects to

receive samples from the converter at a set rate. Using

FF_SKIP_CNT normalizes the frequency of the samples going

into the FIFO, regardless of how many conversion stages are in a

sequence. In Register 0x02, Bits[3:0] are the fast filter skip control,

FF_SKIP_CNT. This value determines which CDC samples are not

used (skipped) in the proximity detection fast FIFO.

Determining the FF_SKIP_CNT value is required only once

during the initial setup of the capacitance sensor interface.

Table 11 shows how FF_SKIP_CNT controls the update rate to

the fast FIFO. The recommended value for FF_SKIP_CNT

when using all 12 conversion stages on the AD7143 is

FF_SKIP_CNT = 0000 = no samples skipped

SLOW FIFO

As shown in Figure 31, a number of FIFOs are implemented on

the AD7143. 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. These values determine which CDC samples

are not used (skipped) in the slow FIFO. Changing theses values

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 is equal to

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 is equal to

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

SEQUENCE_STAGE_NUM +1) × (FF_SKIP_CNT +1) × 4 × 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 of 33 ms to 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 value is

only required once during the initial setup of the capacitance

sensor interface. Recommended values for these settings when

using all 12 conversion stages on the AD7143 are

AVG_FP_SKIP = 00 = skip 3 samples

AVG_LP_SKIP = 00 = no samples skipped

SLOW_FILTER_UPDATE_LVL

The SLOW_FILTER_UPDATE_LVL controls whether or not

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 last value pushed into the

slow FIFO is greater than SLOW_FILTER_UPDATE_LVL. This

variable is in Ambient Control Register 1, at Address 0x003.

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