ADC¶
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 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 techniques must be used to get good precision and accuracy.
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. Some basic tools to accomplish this are resistor dividers and Zener diodes. However, operational amplifiers and other powered components can also be used if greater precision is required.
