AD7148 Analog Devices, AD7148 Datasheet - Page 22

• Manufacturer Part Number: AD7148
• Manufacturer: Analog Devices

Specifications of AD7148

• Resolution (bits): 16bit
• # Chan: 8
• Sample Rate: 250kSPS
• Interface: I²C/Ser 2-Wire,Ser
• Analog Input Type: SE-Uni
• Ain Range: ± 8 pF (Delta C)
• Adc Architecture: Sigma-Delta
• Pkg Type: CSP

AD7148
ENVIRONMENTAL CALIBRATION
The AD7148 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 AD7148 achieves optimal and reliable
sensor performance by continuously monitoring the CDC ambient
levels and correcting for any changes by adjusting the STAGEx_
HIGH_THRESHOLD and STAGEx_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 AD7148 is not being touched, which
allows the AD7148 to account for rapidly changing environmental
conditions.
The ambient compensation control registers give the host access
to general setup and controls for the compensation algorithm.
On-chip RAM stores the compensation data for each conversion
stage, as well as setup information specific to each stage.
Figure 31 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 caused a decrease in
measured capacitance when activated. The positive and negative
sensor threshold levels are calculated as a percentage of the
STAGEx_OFFSET_HIGH and STAGEx_OFFSET_LOW values
based on the threshold sensitivity settings and the ambient value.
These values are sufficient to detect a sensor contact, resulting
in the AD7148 asserting the INT output when threshold levels
are exceeded.
CHANGING ENVIRONMENTAL CONDITIONS
Figure 31. Ideal Sensor Behavior with a Constant Ambient Level
SENSOR 2 INT
ASSERTED
SENSOR 1 INT
ASSERTED
t
STAGEx_HIGH_THRESHOLD
CDC AMBIENT VALUE
STAGEx_LOW_THRESHOLD
Rev. A | Page 22 of 56
CAPACITANCE SENSOR BEHAVIOR WITHOUT
CALIBRATION
Figure 32 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 results in the detection of a missed user contact on Sensor 2.
This is a result of the initial low offset level remaining constant
while the ambient levels drifted upward beyond the detection
range.
The Capacitance Sensor Behavior with Calibration section
describes how the AD7148 adaptive calibration algorithm
prevents errors such as this from occurring.
CAPACITANCE SENSOR BEHAVIOR WITH
CALIBRATION
The AD7148 on-chip adaptive calibration algorithm prevents
sensor detection errors such as the one shown in Figure 32.
Error prevention is accomplished by monitoring CDC ambient
levels and readjusting the initial STAGEx_OFFSET_HIGH and
STAGEx_OFFSET_LOW values according to the amount of
ambient drift measured on each sensor. The internal STAGEx_
HIGH_THRESHOLD and STAGEx_LOW_THRESHOLD
values described in Equation 1 and Equation 2 are automatically
updated based on the new values of STAGEx_OFFSET_HIGH
and STAGEx_OFFSET_LOW. This closed-loop routine ensures
the reliability and repeatable operation of every sensor
connected to the AD7148 under dynamic environmental
conditions. Figure 33 shows a simplified example of how the
AD7148 applies the adaptive calibration process resulting in no
interrupt errors under changing CDC ambient levels due to
environmental conditions.
CHANGING ENVIRONMENTAL CONDITIONS
Figure 32. Typical Sensor Behavior Without Calibration Applied
SENSOR 2 INT
NOT ASSERTED
SENSOR 1 INT
ASSERTED
t
STAGEx_HIGH_THRESHOLD
CDC AMBIENT
VALUE DRIFTING
STAGEx_LOW_THRESHOLD