AD7150 datasheet(12/29 Pages) AD

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AD7150

Rev. 0 | Page 11 of 28

COMPARATOR AND THRESHOLD MODES

The AD7150 comparators and their thresholds can be

programmed to operate in several different 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 AD7150 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

DATA AVERAGE

The adaptive threshold algorithm is based on an average calculated

from previous CDC output data. The response of the average to an

input capacitance step change (more exactly, response to the change

in the CDC output data) is an exponential settling curve, which can

be characterized by the following equation:

)

(

)

(

)

(

/ TimeConst

Change

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 can be selected in the range between 2 and 65,536, in

steps of power of 2, by programming the ThrSettling bits in the

setup registers.

See Figure 30 and the Register Descriptions section.

INPUT CAPACITANCE

(CDC DATA) CHANGE

DATA AVERAGE RESPONSE

TIME

Figure 30. Data Average Response to Data Step Change

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