AD7147A Analog Devices, AD7147A Datasheet - Page 23

• Manufacturer Part Number: AD7147A
• Manufacturer: Analog Devices

Specifications of AD7147A

• Resolution (bits): 16bit
• # Chan: 13
• Sample Rate: 111SPS
• Interface: I²C/Ser 2-Wire,Ser,SPI
• Analog Input Type: Capacitive
• Ain Range: ± 8 pF (Delta C)
• Adc Architecture: Sigma-Delta
• Pkg Type: CSP

ENVIRONMENTAL CALIBRATION
The AD7147A provides on-chip capacitance sensor calibration
to automatically adjust for environmental conditions that have
an effect on the ambient levels of the capacitance sensor. The
output levels of the capacitance sensor are sensitive to tempera-
ture, humidity, and, in some cases, dirt.
The AD7147A achieves optimal and reliable sensor performance
by continuously monitoring the CDC ambient levels and compen-
sating for any environmental changes by adjusting the values of
the STAGEx_HIGH_THRESHOLD registers and the STAGEx_
LOW_THRESHOLD registers as described in the Threshold
Equations section. The CDC ambient level is defined as the
output level of the capacitance sensor during periods when the
user is not approaching or in contact with the sensor.
After the AD7147A is configured, the compensation logic runs
automatically with each conversion when the AD7147A is not
being touched. This allows the AD7147A to compensate for
rapidly changing environmental conditions.
The ambient compensation control registers provide the host
with 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 for each stage.
Figure 34 shows an example of the ideal behavior of a capaci-
tance sensor, 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,
and are based on the threshold sensitivity settings and the
ambient value. These values are sufficient to detect a sensor
contact and result in the AD7147A asserting the INT output
when the threshold levels are exceeded.
THRESHOLD EQUATIONS
On-Chip Logic Stage High Threshold
On-Chip Logic Stage Low Threshold
STAGEx
⎛
⎜
⎜
⎜
⎜
⎝
STAGEx
⎛
⎜
⎜
⎜
⎜
⎝
⎛
⎜
⎝
⎛
⎜
⎝
STAGEx
STAGEx
_
_
HIGH
LOW
_
_
OFFSET
OFFSET
_
_
THRESHOLD
THRESHOLD
_
_
HIGH
LOW
16
−
16
−
=
=
STAGEx
STAGEx
STAGEx
STAGEx
_
_
OFFSET
_
_
OFFSET
SF
4
SF
4
_
_
AMBIENT
AMBIENT
_
_
LOW
HIGH
⎞
⎟
⎠
+
⎞
⎟
⎠
+
⎞
⎟
⎟
⎟
⎟
⎠
⎛
⎜
⎝
⎞
⎟
⎟
⎟
⎟
⎠
⎛
⎜
⎝
×
STAGEx
×
NEG
STAGEx
POS
_
_
THRESHOLD
THRESHOLD
_
_
OFFSET
OFFSET
Rev. B | Page 23 of 68
4
4
_
_
LOW
_
HIGH
_
SENSITIVIT
SENSITIVIT
⎞
⎟
⎠
⎞
⎟
⎠
+
CAPACITANCE SENSOR BEHAVIOR WITHOUT
CALIBRATION
Figure 35 shows the typical behavior of a capacitance sensor when
calibration is not applied and the ambient levels drifting over time
as environmental conditions change. As a result of the initial
threshold levels remaining constant while the ambient levels
drift upward, Sensor 2 fails to detect a user contact in this example.
The Capacitance Sensor Behavior with Calibration section
describes how the AD7147A adaptive calibration algorithm
prevents such errors from occurring.
+
Y
Y
CHANGING ENVIRONMENTAL CONDITIONS
CHANGING ENVIRONMENTAL CONDITIONS
Figure 34. Ideal Sensor Behavior with a Constant Ambient Level
Figure 35. Typical Sensor Behavior Without Calibration
NOT ASSERTED
SENSOR 2 INT
SENSOR 2 INT
ASSERTED
SENSOR 1 INT
SENSOR 1 INT
ASSERTED
ASSERTED
t
t
STAGEx_HIGH_THRESHOLD
CDC AMBIENT VALUE
STAGEx_LOW_THRESHOLD
STAGEx_HIGH_THRESHOLD
CDC AMBIENT
VALUE DRIFTING
STAGEx_LOW_THRESHOLD
AD7147A
(1)
(2)