AD7151BRMZ-REEL datasheet(11/28 Pages) AD

Teilenummer	AD7151BRMZ-REEL

Bauteilbeschribung	Ultra-Low Power, 1-Channel, Capacitance Converter for Proximity Sensing

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Hersteller	AD [Analog Devices]

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AD7151

Rev. 0 | Page 11 of 28

COMPARATOR AND THRESHOLD MODES

The AD7151 comparator and its threshold 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 AD7151 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 23 and Figure 24).

POSITIVE

THRESHOLD

INPUT

CAPACITANCE

OUTPUT

OUTPUT ACTIVE

TIME

POSITIVE CHANGE

Figure 23. Positive Threshold Mode

Indicates Positive Change in Input Capacitance

NEGATIVE

THRESHOLD

INPUT

CAPACITANCE

OUTPUT

OUTPUT ACTIVE

TIME

NEGATIVE CHANGE

Figure 24. Negative Threshold Mode

Indicates Negative Change in Input Capacitance

Additionally, for the adaptive mode only, the comparator can

work as window comparator, indicating input either inside or

outside a selected sensitivity band (see Figure 25 and Figure 26).

POSITIVE

THRESHOLD

NEGATIVE

THRESHOLD

INPUT CAPACITANCE

OUTPUT

OUTPUT ACTIVE

TIME

INPUT INSIDE THRESHOLD WINDOW

Figure 25. In-Window (Adaptive) Threshold Mode

POSITIVE

THRESHOLD

NEGATIVE

THRESHOLD

INPUT CAPACITANCE

OUTPUT

OUTPUT ACTIVE

TIME

INPUT OUTSIDE THRESHOLD WINDOW

Figure 26. 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 27).

THRESHOLD

INPUT CAPACITANCE

OUTPUT

OUTPUT ACTIVE

TIME

FAST CHANGE

SLOW CHANGE

Figure 27. 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:

)

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 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 register.

See Figure 28 and the Register Descriptions section.

INPUT CAPACITANCE

(CDC DATA) CHANGE

DATA AVERAGE RESPONSE

TIME

Figure 28. Data Average Response to Data Step Change

Ähnliche Beschreibung - AD7151BRMZ-REEL

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