The performance of an ADC depends on the quality of the input signals and
power supplies. The following points should be taken into consideration for improving
the accuracy of ADC
measurements:
Note: Some points mentioned below might not
be applicable for all megaAVRs based on their feature set. Refer to
the
device data
sheet
for further information.
- Understand the ADC, its features,
and how they are intended to be
used
- Understand the application
requirements
- Ensure that the source impedance
is not too high compared to the sampling rate
used.
If the
source impedance is too high, the internal sampling capacitor
will not be charged to the correct level and the result will
be
inaccurate.
- It is important to take great
care of the analog signal paths like analog reference (VREF) and analog power
supply (AVCC). Use filtering if the analog power supply is connected to
a
digital power supply. That is, the AVCC pin on the device
should be connected to the digital supply pin (VCC) via an LC network. For more
information about this LC network, refer to the respective device data
sheet.
- Keep analog signal paths as short
as possible. It is also important that the impedance of the PCB tracks
is
not too high, as it will result in a longer
charging
period for the sample/hold capacitor of the ADC.
- Ensure that the analog tracks run
over the analog ground
plane
- Avoid having the analog signal
path close to digital signal
paths
with high switching noise, such as communication
lines
and
clock
signals
- Consider decoupling of the analog
signal between signal input and ground for single-ended
inputs
- Avoid toggling of port pins while
the ADC conversion is in progress in order to prevent the
switching
noise affecting the accuracy. The ADC is most sensitive to switching of the I/O
pins powered by AVCC (PORTC).
- Disable digital input on the
corresponding ADC channel to minimize the power
consumption
- Switch off all unused peripherals
by writing the corresponding bit in the Power Reduction register (PRR) to
'1'
- Use
the
ADC noise reduction mode to get more accurate results
- Wait until the ADC,
reference,
or sources are stabilized before sampling, as some sources (for example,
bandgap) need time to stabilize after they are
enabled
- Before triggering an ADC
conversion, wait until the ADC completes any ongoing conversions. Ensure that
enough time is given for the reference and input source to be
stabilized.
For example, the bandgap voltage needs a certain amount of time to stabilize
when it is selected as ADC
input.
- Apply offset and gain calibration
to the
measurement
- Use oversampling to increase
resolution and eliminate random
noise
- AVCC must not differ more than
±0.3V from VCC
- The reference voltage can be made
more immune to noise by connecting a capacitor between
the
AREF pin and ground
- For differential
signals,
the decoupling has to be between the positive and negative inputs. The
decoupling capacitor value depends on the input signal. If the signals are
switching fast, the decoupling capacitor must be lower.
- Whenever the input MUX setting or
reference voltage selection is modified, it is recommended to discard the first
conversion result
- When switching to a differential
channel (with gain settings), the first conversion result may have a poor
accuracy due to the required settling time for the automatic offset cancellation
circuitry.
Thus,
it is better to discard the first sample result.
- Linear interpolation methods such
as
one-point
(offset) calibration and
two-points
(offset and gain) calibration method can be used based on the
application’s
needs