AD7156 datasheet(12/28 Pages) AD

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AD7156

Rev. 0 | Page 12 of 28

COMPARATOR AND THRESHOLD MODES

The AD7156 comparators and their thresholds can be

programmed to operate in two modes: fixed and adaptive

threshold modes. In an adaptive mode, the threshold

is dynamically adjusted and the comparator output indicates

fast changes and ignores slow changes in the input (sensor)

capacitance. Alternatively, the threshold can be programmed

as a constant (fixed) value, and the output then indicates any

change in the input capacitance that crosses the defined fixed

threshold.

The AD7156 logic output (active high) indicates either a positive or

a negative change in the input capacitance, in both adaptive and

fixed threshold modes (see Figure 25 and Figure 26).

POSITIVE

THRESHOLD

INPUT

CAPACITANCE

OUTPUT

OUTPUT ACTIVE

TIME

POSITIVE CHANGE

Figure 25. Positive Threshold Mode

Indicates Positive Change in Input Capacitance

NEGATIVE

THRESHOLD

INPUT

CAPACITANCE

OUTPUT

OUTPUT ACTIVE

TIME

NEGATIVE CHANGE

Figure 26. Negative Threshold Mode

Indicates Negative Change in Input Capacitance

Additionally, for the adaptive mode only, the comparators can

work as window comparators, indicating input either inside or

outside a selected sensitivity band (see Figure 27 and Figure 28).

POSITIVE

THRESHOLD

NEGATIVE

THRESHOLD

INPUT CAPACITANCE

OUTPUT

OUTPUT ACTIVE

TIME

INPUT INSIDE THRESHOLD WINDOW

Figure 27. In-Window (Adaptive) Threshold Mode

POSITIVE

THRESHOLD

NEGATIVE

THRESHOLD

INPUT CAPACITANCE

OUTPUT

OUTPUT ACTIVE

TIME

INPUT OUTSIDE THRESHOLD WINDOW

Figure 28. Out-Window (Adaptive) Threshold Mode

ADAPTIVE THRESHOLD

In an adaptive mode, the thresholds are dynamically adjusted,

ensuring indication of fast changes (for example, an object

moving close to a capacitive proximity sensor) and eliminating

slow changes in the input (sensor) capacitance, usually caused

by environment changes such as humidity or temperature or

changes in the sensor dielectric material over time (see Figure 29).

THRESHOLD

INPUT CAPACITANCE

OUTPUT

OUTPUT ACTIVE

TIME

FAST CHANGE

SLOW CHANGE

Figure 29. Adaptive Threshold Indicates Fast Changes and Eliminates Slow

Changes in Input Capacitance

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 30). The sensitivity

value is programmable in the range of 0 LSB to 255 LSB of the

12-bit CDC converter (see the Register Descriptions section).

NEGATIVE

THRESHOLD

POSITIVE

THRESHOLD

DATA AVERAGE

OUTPUT ACTIVE

TIME

SENSITIVITY

DATA

SENSITIVITY

Figure 30. Threshold Sensitivity

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