AD7150 Analog Devices, AD7150 Datasheet - Page 11

• Manufacturer Part Number: AD7150
• Description: Ultra-Low Power, 2-Channel, Capacitance Converter for Proximity Sensing
• Manufacturer: Analog Devices

Specifications of AD7150

• Resolution (bits): 12bit
• # Chan: 2
• Sample Rate: n/a
• Interface: I²C/Ser 2-Wire,Ser
• Analog Input Type: SE-Uni
• Ain Range: 0.5 pF,1 pF,2 pF,4 pF
• Adc Architecture: Sigma-Delta
• Pkg Type: SOP

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).
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).
INPUT CAPACITANCE
CAPACITANCE
CAPACITANCE
THRESHOLD
THRESHOLD
THRESHOLD
THRESHOLD
NEGATIVE
POSITIVE
NEGATIVE
OUTPUT
OUTPUT
POSITIVE
OUTPUT
Figure 27. In-Window (Adaptive) Threshold Mode
INPUT
INPUT
Indicates Negative Change in Input Capacitance
Indicates Positive Change in Input Capacitance
Figure 26. Negative Threshold Mode
Figure 25. Positive Threshold Mode
OUTPUT ACTIVE
INPUT INSIDE THRESHOLD WINDOW
NEGATIVE CHANGE
OUTPUT ACTIVE
OUTPUT ACTIVE
POSITIVE CHANGE
TIME
TIME
TIME
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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).
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:
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.
Average
Indicates Fast Changes and Eliminates Slow Changes in Input Capacitance
INPUT CAPACITANCE
INPUT CAPACITANCE
INPUT CAPACITANCE
(CDC DATA) CHANGE
THRESHOLD
THRESHOLD
THRESHOLD
Figure 30. Data Average Response to Data Step Change
(
NEGATIVE
POSITIVE
Figure 28. Out-Window (Adaptive) Threshold Mode
N
OUTPUT
OUTPUT
)
=
Average
Figure 29. Adaptive Threshold
FAST CHANGE
OUTPUT ACTIVE
OUTPUT ACTIVE
INPUT OUTSIDE THRESHOLD WINDOW
(
) 0
+
Change
DATA AVERAGE RESPONSE
1 (
SLOW CHANGE
−
e
N
/ TimeConst
TIME
AD7150
TIME
TIME
)