|
- Static Tests of the Arduino Internal ADC.
-
- Several people have asked about the DC accuracy of the Arduino ADC when used in my data logging applications at slow sample rates.
-
- Here are my results of some "hobby level" measurements of the Arduino ADC.
-
- One question is how important is the ADC clock rate. I did measurents for an ADC clock rate of 125 kHz to 2MHz.
-
- Another question is how much does Noise Reduction Mode help. I did a series of measurements using this mode.
-
- Noise Reduction Mode only reduced the mean absolute error slightly.
-
- I do calibration to remove Offset Error and Gain Error. Calibration is very important for good accuracy.
-
- These tests depend on the Arduino voltage regulator providing a stable voltage during the tests. The Arduino ADC reference voltage is Vcc for these tests. This may not be realistic for practical applications
-
- Integral Non-linearity (INL) is the main remaining source of error.
-
- Here are my results for static (DC) tests of the internal ADC for three UNOs.
-
- The Arduinos are powered by a high quality nine volt power supply.
-
- These tests measure a DC level so do not include problems due to time jitter, S/H time, and other dynamic errors.
- There are several studies of the dynamic behavior of the Arduino ADC that determine ENOB (Effective Number Of Bits).
-
- I used a shield with a 12-bit MCP4921 DAC to generate voltage levels. This ADC has an output buffer so it provides a very low impedance source.
-
- I measured the voltage of the DAC with a calibrated 18-bit MCP3422 ADC on the shield.
-
- I used DAC levels from 20 to 4075 to avoid zero offset errors at low voltages and DAC buffer problems at high voltages.
-
- Each series of measurements has 4056 data points.
-
- This is a voltage range of about 0.023 to 4.972 volts.
-
- I calibrated the Arduino ADC for each series of measurements with a linear fit of the form.
-
- v = a + b*adcValue
-
- Errors are the difference between the value measured with the 18-bit ADC and the calibrated value measured with the AVR ADC.
-
- I also show the results for no calibration, the NoCal column, using the datasheet formula.
-
- Vin = Vref*adcValue/1024
-
-
- The rows in the tables tables are.
-
- Min - minimum error in millivolts
-
- Max - maximum error in millivolts
-
- MAE - mean absolute error in millivolts
-
-
- The columns in the tables are:
-
- Ideal - results for a perfect 10-bit ADC for comparison.
-
- NoCal - datasheet formula (5/1024)*adcValue with Noise Reduction Mode.
-
- NR128 - Noise Reduction mode with Prescaler of 128 (ADC clock of 125 kHz).
-
- PS128 - analogRead with Prescaler of 128 (ADC clock of 125 kHz).
-
- PS64 - analogRead with Prescaler of 64 (ADC clock of 250 kHz).
-
- PS32 - analogRead with Prescaler of 32 (ADC clock of 500 kHz).
-
- PS16 - analogRead with Prescaler of 16 (ADC clock of 1 MHz).
-
- PS8 - analogRead with Prescaler of 8 (ADC clock of 2 MHz).
-
-
- Results for three UNO Arduinos
-
- First Arduino - Error Millivolts
-
- Ideal NoCal NR128 PS128 PS64 PS32 PS16 PS8
- Min -2.44 -2.43 -3.72 -4.01 -3.88 -4.53 -6.57 -27.18
- Max 2.44 11.69 3.74 4.24 4.15 5.17 8.69 23.21
- MAE 1.22 5.02 1.33 1.38 1.37 1.44 1.96 4.11
-
- Second Arduino - Error Millivolts
-
- Ideal NoCal NR128 PS128 PS64 PS32 PS16 PS8
- Min -2.44 -9.24 -4.87 -4.86 -5.05 -5.34 -6.52 -24.04
- Max 2.44 11.62 3.95 4.64 4.69 5.71 8.41 21.29
- MAE 1.22 5.33 1.41 1.43 1.44 1.53 2.02 4.05
-
- Third Arduino - Error Millivolts
-
- Ideal NoCal NR128 PS128 PS64 PS32 PS16 PS8
- Min -2.44 -7.88 -4.12 -4.40 -4.32 -4.41 -6.97 -26.93
- Max 2.44 12.53 3.80 4.04 4.18 5.27 8.84 24.59
- MAE 1.22 4.85 1.29 1.33 1.34 1.42 1.91 4.10
-
|
