Datasheet

AD7142
Rev. A | Page 20 of 72
05702-020
NOTES
1. SEQUENCE CONVERSION TIME
t
CONV_LP
= t
CONV_FP
+ LP_CONV_DELAY
2.
t
CALDIS
= t
CONV_LP
× LP_PROXIMITY_CNT × 4
3.
t
RECAL_TIMEOUT
= t
CONV_FP
× LP_PROXIMITY_RECAL
4.
t
RECAL
= 2 × t
CONV_LP
CALIBRATION ENABLED
t
RECAL_TIMEOUT
16 30
70
t
CONV_LP
RECALIBRATION TIME-OUT
PROXIMITY DETECTION
(INTERNAL)
CALIBRATION
(INTERNAL)
C
DC CONVERSION SEQUENCE
(INTERNAL)
RECALIBRATION
(INTERNAL)
05702-020
USER APPROACHES
SENSOR HERE
USER LEAVES SENSOR
AREA HERE
MEASURED CDC VALUE > STORED AMBIENT
BY PROXIMITY_RECAL _LVL
t
RECAL
t
CALDIS
CALIBRATION DISABLED
Figure 31. Low Power Mode Proximity Detection with Forced Recalibration Example with LP_PROXIMITY_CNT = 4 and LP_PROXIMITY_RECAL = 40
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. FF_SKIP_CNT is used to normalize 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 13 shows
how FF_SKIP_CNT controls the update rate to the fast FIFO. Recommended value for this setting when using all 12 conversion stages on
the AD7142 is FF_SKIP_CNT = 0000 = no samples skipped.
Table 13. FF_SKIP_CNT Settings
FAST FIFO Update Rate
FF_SKIP_CNT DECIMATION = 128 DECIMATION = 256
0 1.536 × (SEQUENCE_STAGE_NUM + 1) ms 3.072 × (SEQUENCE_STAGE_NUM + 1) ms
1 3.072 × (SEQUENCE_STAGE_NUM + 1) ms 6.144 × (SEQUENCE_STAGE_NUM + 1) ms
2 4.608 × (SEQUENCE_STAGE_NUM + 1) ms 9.216 × (SEQUENCE_STAGE_NUM + 1) ms
3 6.144 × (SEQUENCE_STAGE_NUM + 1) ms 12.288 × (SEQUENCE_STAGE_NUM + 1) ms
4 7.68 × (SEQUENCE_STAGE_NUM + 1) ms 15.36 × (SEQUENCE_STAGE_NUM + 1) ms
5 9.216 × (SEQUENCE_STAGE_NUM + 1) ms 18.432 × (SEQUENCE_STAGE_NUM + 1) ms
6 10.752 × (SEQUENCE_STAGE_NUM + 1) ms 21.504 × (SEQUENCE_STAGE_NUM + 1) ms
7 12.288 × (SEQUENCE_STAGE_NUM + 1) ms 24.576 × (SEQUENCE_STAGE_NUM + 1) ms
8 13.824 × (SEQUENCE_STAGE_NUM + 1) ms 27.648 × (SEQUENCE_STAGE_NUM + 1) ms
9 15.36 × (SEQUENCE_STAGE_NUM + 1) ms 30.72 × (SEQUENCE_STAGE_NUM + 1) ms
10 16.896 × (SEQUENCE_STAGE_NUM + 1) ms 33.792 × (SEQUENCE_STAGE_NUM + 1) ms
11 18.432 × (SEQUENCE_STAGE_NUM + 1) ms 36.864 × (SEQUENCE_STAGE_NUM + 1) ms
12 19.968 × (SEQUENCE_STAGE_NUM + 1) ms 39.936 × (SEQUENCE_STAGE_NUM + 1) ms
13 21.504 × (SEQUENCE_STAGE_NUM + 1) ms 43.008 × (SEQUENCE_STAGE_NUM + 1) ms
14 23.04 × (SEQUENCE_STAGE_NUM + 1) ms 46.08 × (SEQUENCE_STAGE_NUM + 1) ms
15 24.576 × (SEQUENCE_STAGE_NUM + 1) ms 49.152 × (SEQUENCE_STAGE_NUM + 1) ms