ADC¶
Analog to digital conversion is the process of reading a physical voltage as a number. Maple can convert voltages between 0 and 3.3V to numbers between 0 and 4095.
ADC On Maple¶
Doing analog-to-digital conversion on the Maple is simple. Maple IDE contains a basic example. To see it, choose Analog > AnalogInSerial from the examples menu.
In order to set up your board for conversion, first connect the wire (potentiometer, etc.) with the voltage you want to measure to a pin which can perform ADC. Each pin which can do ADC has “AIN” (or “ain”) written next to the the pin number. Then, as in the example program, set the chosen pin’s pin mode to INPUT_ANALOG by calling pinMode(<your_pin>, INPUT_ANALOG). You will usually do this in your setup() function. Now you can use analogRead() to perform an ADC reading.
Function Reference¶
- analogRead()
- pinMode()
- adc.h (low-level ADC support)
Noise and Bias¶
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. In theory, this means that a change in voltage of about 1 millivolt should change the numeric voltage reading by 1. In reality, however, a number of issues introduce noise and bias into this reading, and a number of techniques must be used to get good precision and accuracy.
In order to allow for good readings, LeafLabs has tried to isolate at least some of each board’s ADC pins and traces from strong noise sources. However, there are always trade-offs between noise, additional functionality, cost, and package size. More information on these isolated pins is available in each board’s hardware documentation:
That said, there are a number of more general things you can do to try to get good readings. If your 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.
Another important factor when taking a voltage reading is the reference voltages that the sample is being compared against. For Maple, the high reference is VDDA and the low reference is ground. This means that noise or fluctuations on either VDDA or ground will affect the measurement. It also means that the voltage you are trying to sample must be between ground and 3.3 V.
In the case of a variable reading, it is best if the voltage varies over the entire range of 0 through 3.3 V; otherwise, only a fraction of the sensitivity is being used. 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.
