AD7151BRMZ Analog Devices Inc, AD7151BRMZ Datasheet - Page 11

• Manufacturer Part Number: AD7151BRMZ
• Description: IC CAP CONV 1CH ULT LP 10MSOP
• Manufacturer: Analog Devices Inc
• Type: Capacitance-to-Digital Converterr

Specifications of AD7151BRMZ

• Input Type: Voltage
• Output Type: Digital
• Interface: 2-Wire Serial
• Current - Supply: 120µA
• Mounting Type: Surface Mount
• Package / Case: 10-MSOP, Micro10™, 10-uMAX, 10-uSOP
• Resolution (bits): 12bit
• Sampling Rate: 100SPS
• Input Channel Type: Single Ended
• Data Interface: 2-Wire, I2C, Serial
• Supply Voltage Range - Analog: 2.7V To 3.6V
• Supply Current: 70µA
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

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).
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).
INPUT CAPACITANCE
CAPACITANCE
CAPACITANCE
THRESHOLD
THRESHOLD
THRESHOLD
THRESHOLD
NEGATIVE
POSITIVE
NEGATIVE
OUTPUT
OUTPUT
POSITIVE
OUTPUT
Figure 25. In-Window (Adaptive) Threshold Mode
INPUT
INPUT
Indicates Negative Change in Input Capacitance
Indicates Positive Change in Input Capacitance
Figure 24. Negative Threshold Mode
Figure 23. 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 27).
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 register.
See Figure 28 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 28. Data Average Response to Data Step Change
(
NEGATIVE
POSITIVE
Figure 26. Out-Window (Adaptive) Threshold Mode
N
OUTPUT
OUTPUT
)
=
Average
Figure 27. Adaptive Threshold
FAST CHANGE
OUTPUT ACTIVE
OUTPUT ACTIVE
(
INPUT OUTSIDE THRESHOLD WINDOW
) 0
+
Change
DATA AVERAGE RESPONSE
1 (
SLOW CHANGE
−
e
N
/ TimeConst
TIME
AD7151
TIME
TIME
)