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diff --git a/source/adc.rst b/source/adc.rst index 17c672d..4e118aa 100644 --- a/source/adc.rst +++ b/source/adc.rst @@ -4,4 +4,72 @@ ADC ===== -Stub. +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 +theoretical sensitivity of just under 1 millivolt. In reality, 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 + +.. contents:: Contents + :local: + +.. _adc-noise-bias: + +Noise and Bias +-------------- + +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, +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 +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. + +An important factor when taking a voltage reading is the reference +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. + +.. _adc-function-reference: + +Function Reference +------------------ + +.. TODO function reference (figure out Doxygen first) + +.. _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 for STM32F103RB microcontroller: + + * `All <http://www.st.com/mcu/devicedocs-STM32F103RB-110.html>`_ + * `Datasheet (pdf) <http://www.st.com/stonline/products/literature/ds/13587.pdf>`_ + * `Reference Manual (pdf) <http://www.st.com/stonline/products/literature/rm/13902.pdf>`_ + * `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>`_ + + |