AD7143ACPZ Analog Devices Inc, AD7143ACPZ Datasheet - Page 22

• Manufacturer Part Number: AD7143ACPZ
• Description: IC, CDC, 16BIT, SMD, LFCSP-16, 7143
• Manufacturer: Analog Devices Inc

Specifications of AD7143ACPZ

• Supply Voltage Range: 2.6V To 3.6V
• Operating Temperature Range: -40°C To +85°C
• Digital Ic Case Style: LFCSP
• No. Of Pins: 16
• Svhc: No SVHC (18-Jun-2010)
• Package / Case: LFCSP
• Base Number: 7143
• Ic Function: Programmable Controller For Capacitance Touch Sensors
• Rohs Compliant: Yes
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

AD7143
ENVIRONMENTAL CALIBRATION
The AD7143 provides on-chip capacitance sensor calibration to
automatically adjust for environmental conditions that have an
effect on the capacitance sensor ambient levels. Capacitance
sensor output levels are sensitive to temperature, humidity, and
in some cases, dirt. The AD7143 achieves optimal and reliable
sensor performance by continuously monitoring the CDC
ambient levels and correcting for any changes by adjusting the
STAGE_HIGH_THRESHOLD and STAGE_LOW_THRESHOLD
register values as described in Equation 1 and Equation 2. The
CDC ambient level is defined as the capacitance sensor output
level during periods when the user is not approaching or in
contact with the sensor.
The compensation logic runs automatically on every conversion
after configuration when the AD7143 is not being touched. This
allows the AD7143 to account for rapidly changing
environmental conditions.
The ambient compensation control registers located at
Address 0x002, Address 0x003 and Address 0x004 give the host
access to general setup and controls for the compensation
algorithm. The RAM stores the compensation data for each
conversion stage, as well as setup information specific to each stage.
Figure 33 shows an example of an ideal capacitance sensor
behavior where the CDC ambient level remains constant
regardless of the environmental conditions. The CDC output
shown is for a pair of differential button sensors, where one
sensor caused an increase, and the other a decrease in measured
capacitance when activated.
The positive and negative sensor threshold levels are calculated
as a percentage of the STAGE_OFFSET_HIGH and
STAGE_OFFSET_LOW values based on the threshold
sensitivity settings and the ambient value. These values for this
example are sufficient to detect a sensor contact, resulting with
the AD7143 asserting the INT output when the threshold levels
are exceeded.
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CAPACITANCE SENSOR BEHAVIOR WITHOUT
CALIBRATION
Figure 34 shows the typical behavior of a capacitance sensor
with no applied calibration. This figure shows ambient levels
drifting over time as environmental conditions change. The
ambient level drift has resulted in the detection of a missed user
contact on Sensor 2.
This is a result of the initial STAGE_LOW_THRESHOLD
remaining constant while the ambient levels drifted upward
beyond the detection range. The Capacitance Sensor Behavior
with Calibration section describes how the AD7143 adaptive
calibration algorithm prevents errors such as this from
occurring.
CHANGING ENVIRONMENTAL CONDITIONS
Figure 33. Ideal Sensor Behavior with a Constant Ambient Level
CHANGING ENVIRONMENTAL CONDITIONS
Figure 34. Typical Sensor Behavior without Calibration Applied
NOT ASSERTED
SENSOR 2 INT
SENSOR 2 INT
ASSERTED
SENSOR 1 INT
ASSERTED
SENSOR 1 INT
ASSERTED
t
STAGE_HIGH_THRESHOLD
CDC AMBIENT VALUE
STAGE_LOW_THRESHOLD
t
STAGE_HIGH_THRESHOLD
CCDC AMBIENT
VALUE DRIFTING
STAGE_LOW_THRESHOLD