Analog Input and ADC (Analog-to-Digital Converter)
An analog input is the reading of a continuous voltage coming from a potentiometer, temperature sensor or current measurement circuit by converting it into a numeric value with an analog-to-digital converter. A 10-bit converter divides the measurement range into 1024 steps, a 12-bit one into 4096.
Resolution on its own does not mean accuracy. The main factors that determine the quality of the reading are:
- Reference voltage: the converter produces the reading relative to the reference. If the reference is shared with the supply, the reading drifts as the supply fluctuates.
- Source impedance: sampling from a high-resistance source causes the sampling capacitor not to charge fully and the reading to come out low.
- Noise: on a board where a motor is driven with PWM, the reading wanders if the analog line is not kept away from the power tracks. A hardware filter and averaging in software are used together.
- Temperature: drift of the reference and divider resistors with temperature affects long-term accuracy.
In AXI products this reading is typically used for the speed setting potentiometer, the position of the proportional lever, current feedback and temperature monitoring. Since stable behaviour is wanted at the ends of the adjustment, a deadband is usually defined in software at the end regions and the reading is evaluated over a certain averaging window.
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