AD7156 Analog Devices, AD7156 Datasheet - Page 12

• Manufacturer Part Number: AD7156
• Manufacturer: Analog Devices

Specifications of AD7156

• Resolution (bits): 12bit
• # Chan: 2
• Sample Rate: 100SPS
• Interface: I²C/Ser 2-Wire,Ser
• Analog Input Type: Capacitive
• Ain Range: 0.5 pF,1 pF,2 pF,4 pF
• Pkg Type: CSP

AD7156
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).
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
OUTPUT
NEGATIVE
OUTPUT
POSITIVE
OUTPUT
INPUT
Figure 27. In-Window (Adaptive) Threshold Mode
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).
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).
Figure 29. Adaptive Threshold Indicates Fast Changes and Eliminates Slow
INPUT CAPACITANCE
INPUT CAPACITANCE
DATA AVERAGE
THRESHOLD
THRESHOLD
THRESHOLD
THRESHOLD
THRESHOLD
NEGATIVE
NEGATIVE
POSITIVE
Figure 28. Out-Window (Adaptive) Threshold Mode
POSITIVE
OUTPUT
OUTPUT
DATA
Figure 30. Threshold Sensitivity
Changes in Input Capacitance
OUTPUT ACTIVE
FAST CHANGE
OUTPUT ACTIVE
OUTPUT ACTIVE
INPUT OUTSIDE THRESHOLD WINDOW
TIME
SLOW CHANGE
SENSITIVITY
SENSITIVITY
TIME
TIME