AD7151 datasheet(13/29 Pages) AD

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AD7151

Rev. 0 | Page 12 of 28

SENSITIVITY

In adaptive threshold mode, the output comparator threshold is

set as a defined distance (sensitivity) above the data average,

below the data average, or both, depending on the selected

threshold mode of operation (see Figure 29). The sensitivity

value is programmable in the range 0 to 255 LSBs of the 12-bit

CDC converter (see the Register Descriptions section).

NEGATIVE

THRESHOLD

POSITIVE

THRESHOLD

DATA AVERAGE

OUTPUT ACTIVE

TIME

SENSITIVITY

DATA

SENSITIVITY

Figure 29. Threshold Sensitivity

HYSTERESIS

In adaptive threshold mode, the comparator features hysteresis.

The hysteresis is fixed to one-fourth of the threshold sensitivity

and can be programmed on or off. The comparator does not

have hysteresis in the fixed threshold mode.

OUTPUT

POSITIVE

THRESHOLD

DATA AVERAGE

OUTPUT ACTIVE

TIME

HYSTERSIS

DATA

Figure 30. Threshold Hysteresis

TIMEOUT

In the case of a large, long change in the capacitive input, when

the data average adapting to a new condition may take 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 threshold) is forced to

follow the new CDC data value immediately (see Figure 31).

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 32, Figure 33, and the

DATA AVERAGE

+ SENSITIVITY

LARGE CHANGE IN DATA

DATA AVERAGE

– SENSITIVITY

TIME

TIMEOUT

Figure 31. Threshold Timeout After a Large Change in CDC Data

INPUT CAPACITANCE

THRESHOLD

OUTPUT

DATA AVERAGE

OUTPUT ACTIVE

TIME

TIMEOUT APPROACHING

Figure 32. Approaching Timeout in Negative Threshold Mode

Shortens False Output Trigger

INPUT

CAPACITANCE

LARGE CHANGE

THRESHOLD

OUTPUT

OUTPUT ACTIVE

TIME

TIMEOUT RECEDING

Figure 33. Positive Timeout in Negative Threshold Mode

Shortens Period of Missing Output Trigger

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