Datasheet

AD7156
Rev. 0 | Page 13 of 28
DATA AVERAGE
The adaptive threshold algorithm is based on an average calculated
from the previous CDC output data, using the following equation:
1
2
)1()(
)1()(
+
−
−
+−=
gThrSettlin
NAverageNData
NAverageNAverage
where:
Average(N) is the new average value.
Average(N − 1) is the average value from the previous cycle.
Data(N) is the latest complete CDC conversion result.
ThrSettling is a parameter, programmable in the setup registers.
A more specific case of the input capacitance waveform is a step
change. The response of the average to an input capacitance step
change (more exactly, response to a step change in the CDC output
data) is an exponential settling curve, which can be characterized
by the following equation:
)1()0()(
/TimeConstN
eChangeAverageNAverage −+=
where:
Average(N) is the value of average N complete CDC conversion
cycles after a step change on the input.
Average(0) is the value before the step change.
TimeConst = 2
(ThrSettling + 1)
ThrSettling is a parameter, programmable in the setup registers.
See Figure 31 and the Register Descriptions section for further
information.
INPUT C
A
PACITANCE
(CDC DATA) CHANGE
DATA AVERAGE RESPONSE
TIME
07726-038
Figure 31. Data Average Response to Data Step Change
HYSTERESIS
In adaptive threshold mode, the comparator features hysteresis.
The hysteresis is fixed to ¼ of the threshold sensitivity and can
be programmed on or off. The comparator does not have hyster-
esis in the fixed threshold mode.
OUTPUT
POSITIVE
THRESHOLD
DATA AVERAGE
OUTPUT ACTIVE
TIME
HYSTERSIS
DAT
A
07726-040
Figure 32. Threshold Hysteresis
TIMEOUT
In the case of a large, long change in the capacitive input, when
the data average adapting to a new condition takes too long, a
timeout can be set.
The timeout becomes active (counting) when the CDC data
goes outside the band of data average ± sensitivity. When the
timeout elapses (a defined number of CDC conversions is
counted), the data average (and thus the thresholds), is forced
to follow the new CDC data value immediately (see Figure 33).
The timeout can be set independently for approaching (for change
in data toward the threshold) and for receding (for change in data
away from the threshold). See Figure 34, Figure 35, and the Register
Descriptions section for further information.
D
A
T
A
A
V
ERAGE
+ SENSITIVITY
LARGE CHANGE IN DATA
DATA AVERAGE
DATA AVERAGE
– SENSITIVITY
TIME
TIMEOUT
07726-041
Figure 33. Threshold Timeout After a Large Change in CDC Data
INPUT
CAPACITANCE
THRESHOLD
OUTPUT
DATA AVERAGE
OUTPUT ACTIVE
TIME
TIMEOUT APPROACHING
07726-042
Figure 34. Approaching Timeout in Negative Threshold Mode
Shortens False Output Trigger
INPUT
C
A
PACITANCE
LARGE CHANGE
THRESHOLD
OUTPUT
OUTPUT ACTIVE
TIME
TIMEOUT RECEDING
0
7726-043
Figure 35. Positive Timeout in Negative Threshold Mode
Shortens Period of Missing Output Trigger