In the case of an Arduino, we can assume it's in the range of 10 kiloohms to 100 kiloohms, depending on things such as the sampling rate. Different types of input will load what they're measuring to different extents this is where Radc comes in: it's a representation of the load that the ADC puts on the circuit, not a physical, discrete component. Our equations above assume that the ADC doesn't put any load on the resistor divider, but that's not correct. It's likely that our measuring device isn't perfect either.Dissipating a lot of current through our divider by using small resistors also wastes a lot of power, and produces a lot of unwanted heat.If we draw enough power from it, it will cause the input to sag, producing inaccurate results and potentially affecting the rest of the circuit. It's likely that 'In' isn't a true voltage source, capable of supplying unlimited current, but instead has its own internal resistance, which we call the output impedance.But what about the absolute values? In principle we can pick any magnitude we want, but in practice there are several important considerations: Knowing this, we can construct a divider for any ratio we want simply by determining the relative values of Ra and Rb. The voltage where Ra and Rb meet will depend on the current flowing and the value of Rb - it's Vdiv = i * Rb. We can calculate this with i = Vin / (Ra + Rb). Current will flow from IN, through Ra and Rb, to ground the amount of that current depends on the voltage at IN. Ignore 'Radc' for a moment, and assume 'IN' is connected to a voltage source. The basic operation of a resistor divider is simple. Simulate this circuit – Schematic created using CircuitLab In order to measure a wider range, up to 24 volts, we need a voltage divider, like this: You're going to want to use it to measure both the voltage across the sensor wires and the current through them if the wires are used for both power and communication, it's likely that the way it communicates is by increasing and decreasing the amount of current it consumes, in which case your most useful information will come by measuring the current waveform.Īs you observed, the Arduino can measure voltages between 0 and 5 volts on its analog ports. It seems like your first task is going to be determining what sort of signalling is being used, so what you need to start with is a "poor man's oscilloscope" in the form of a microcontroller with ADC. I would really appreciate it if those with more understanding and knowledge could offer their insights and assert my thoughts above and even elaborate on it to further my understanding and clear up some of my misunderstandings. I realise SE prefers Q&A type of posts, but to me the above is context that fits together for the larger circuit - at least that's what I think. Out of my depth, so some insight would be useful. One particular question that I have is about the fact that the ADC and the 24v sensor signal that I'm trying to interface is that they have different power sources, and apparently this is an issue because they don't share the same GND. I'm not certain why this is useful or what it achieves, but I think it seems to help address the mismatch of impedance from the 24v signal and that of the ADC? But I could be wrong.
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