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Diffstat (limited to 'source/adc.rst')
| -rw-r--r-- | source/adc.rst | 65 |
1 files changed, 31 insertions, 34 deletions
diff --git a/source/adc.rst b/source/adc.rst index b943808..af613cc 100644 --- a/source/adc.rst +++ b/source/adc.rst @@ -6,23 +6,12 @@ Analog-Digital Conversion is the process of reading a physical voltage as a number. The Maple has a large number of pins which are capable of -taking 12-bit ADC measurements, which means that voltages from ground -to +3.3v are read as numbers from 0 to 4095; this corresponds to a +taking 12-bit ADC measurements, which means that voltages from 0 to +3.3V are read as numbers from 0 to 4095. This corresponds to a theoretical sensitivity of just under 1 millivolt. In reality, a -number of factors introduce noise and bias into this reading and a +number of factors introduce noise and bias into this reading, and a number of techniques must be used to get good precision and accuracy. -.. compound:: - - The header pins with ADC functionality (marked as "AIN" on the - silkscreen) are: - - D0, D1, D2, D3, D10, D11, D12, D13, D15, D16, D17, D18, D19, D20, D27, D28 - - Note that pins 3, 27, and 28 are not marked AIN on the silkscreen - for Maple revisions through Rev 5, however, they **do work** as - analog input pins. - .. contents:: Contents :local: @@ -31,13 +20,15 @@ number of techniques must be used to get good precision and accuracy. Noise and Bias -------------- +.. FIXME [0.0.10, Maple-specific] + The biggest issues with analog-digital conversion are noise and bias. With the Maple, we have tried to isolate the ADC pins and traces from -strong noise sources but there are always trade--offs between noise, +strong noise sources, but there are always trade--offs between noise, additional functionality, cost, and package size. The 6 ADC pins in a bank (D15--D20) generally have the least -noise and should be used for fine measurements. If the input voltage +noise, and should be used for fine measurements. If the input voltage changes relatively slowly, a number of samples can be taken in succession and averaged together, or the same voltage can even be sampled by multiple ADC pins at the same time. @@ -47,34 +38,40 @@ voltages that the sample is being compared against. In the case of the Maple, the high reference is |vcc| and the low reference is ground. This means that noise or fluctuations on either |vcc| or ground will affect the measurement. It also means that the voltage you are trying -to sample must be between ground and 3.3V. In the case of a variable -reading, it is best if the voltage varies over the entire range of -0--3.3V; otherwise, only a fraction of the sensitivity is being -leveraged. Resistor dividers and constant voltage diodes are basic -tools which can help bring a given voltage signal into the appropriate -range; opamps and other powered components can also be used. +to sample must be between ground and 3.3V. + +.. _adc-range: + +In the case of a variable reading, it is best if the voltage varies +over the entire range of 0--3.3V; otherwise, only a fraction of the +sensitivity is being leveraged. Some basic tools to accomplish this +are `resistor dividers +<http://en.wikipedia.org/wiki/Voltage_divider>`_ and `Zener diodes +<http://en.wikipedia.org/wiki/Voltage_source#Zener_voltage_source>`_\ +. However, `operational amplifiers +<http://en.wikipedia.org/wiki/Operational_amplifier>`_ and other +powered components can also be used if greater precision is required. .. _adc-function-reference: Function Reference ------------------ -.. doxygenfunction:: analogRead - -.. doxygenfunction:: pinMode - -.. doxygenenum:: WiringPinMode +* :ref:`lang-analogread` +* :ref:`lang-pinmode` .. _adc-recommended-reading: Recommended Reading ------------------- -* `Wikipedia article on Analog-to-digital converter <http://en.wikipedia.org/wiki/Analog-to-digital_converter>`_ - -* `Arduino Analog Input Tutorial <http://arduino.cc/en/Tutorial/AnalogInputPins>`_ - -* STMicro documentation: +* `Wikipedia: Analog-to-Digital Converter + <http://en.wikipedia.org/wiki/Analog-to-digital_converter>`_ +* `Arduino Analog Input Tutorial + <http://arduino.cc/en/Tutorial/AnalogInputPins>`_ +* ST documentation: - * `Application Note on ADC Modes (pdf) <http://www.st.com/stonline/products/literature/an/16840.pdf>`_ - * `Application Note on ADC Oversampling (pdf) <http://www.st.com/stonline/products/literature/an/14183.pdf>`_ + * `Application Note on ADC Modes + <http://www.st.com/stonline/products/literature/an/16840.pdf>`_ (PDF) + * `Application Note on ADC Oversampling + <http://www.st.com/stonline/products/literature/an/14183.pdf>`_ (PDF) |