AD7156_17 datasheet(14/29 Pages) AD

Part #	AD7156

Description	2-Channel Capacitance Converter

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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

(

)

(

+

−

+

−

=

g

ThrSettlin

N

Average

N

Data

N

Average

N

Average

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

(

)

(

/TimeConst

N

e

Change

Average

N

Average

−

+

=

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 CAPACITANCE

(CDC DATA) CHANGE

DATA AVERAGE RESPONSE

TIME

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

DATA

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.

DATA AVERAGE

+ SENSITIVITY

LARGE CHANGE IN DATA

DATA AVERAGE

– SENSITIVITY

TIME

TIMEOUT

Figure 33. Threshold Timeout After a Large Change in CDC Data

INPUT

CAPACITANCE

THRESHOLD

OUTPUT

DATA AVERAGE

OUTPUT ACTIVE

TIME

TIMEOUT APPROACHING

Figure 34. Approaching Timeout in Negative Threshold Mode

Shortens False Output Trigger

INPUT

CAPACITANCE

LARGE CHANGE

THRESHOLD

OUTPUT

OUTPUT ACTIVE

TIME

TIMEOUT RECEDING

Figure 35. Positive Timeout in Negative Threshold Mode

Shortens Period of Missing Output Trigger

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