Data Sheet

Page 10 ams Datasheet
Document Feedback [v1-00] 2016-Dec-23
CCS811 − Detailed Description
Refer to application note ams AN000372 for more details on
NTC configuration.
If an external temperature sensor is not available the external
NTC circuit can be used to determine the ambient temperature
and write this information to CCS811.
The NTC Register provides the voltages across the R
NTC
and the
R
REF
if fitted (mV) – to enable host system to calculate the
ambient temperature. The calculated temperature can be
written to CCS811 to compensate for gas changes due to
temperature changes.
Automatic Baseline Correction
The resistance R
S
of the sensitive layer is the output of the
sensor. However, metal oxide sensors do not give absolute
readings. The resistance R
S
varies from sensor to sensor
(manufacturing variation), from use-case to use-case, and over
time. To mitigate this problem, the output of the sensor is
normalized: R
S
is divided by R
A
. The value of R
A
is known as the
baseline. R
A
cannot be determined by a one-time calibration; it
is maintained on-the-fly in software. This process is known as
baseline correction. The air quality is expected to vary in a
typical environment so the minimum time over which a
baseline correction is applied is 24 hours.
Manual Baseline Correction
There is a mechanism within CCS811 to manually save and
restore a previously saved baseline value using the BASELINE
register.
The correct time to save the baseline will depend on the
customer use-case and application.
• For devices which are powered for >24 hours at a time:
• During the first 500 hours – save the baseline every
24-48 hours.
• After the first 500 hours – save the baseline every 5-7
days.
• For devices which are powered <24 hours at a time:
• If the device is run in, save the baseline before power
down
• If multiple operating modes are used, a separate
baseline should be stored for each
• The baseline should only be restored when the
resistance is stable (typically 20-30 minutes)
• If changing from a low to high power mode (without
spending at least 10 minutes in idle), the sensor
resistance should be allowed to settle again before
restoring the baseline