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Intro: I am projecting a Brushless motor driver which relevant specs here are basically: use with 6v to 60v. To make it work the circuit's microcontroller needs to analyze the signal that comes from the motor phases through ADC - which can not exceed 3.3v.

The phase maximum voltage is ideally equal to the power voltage and looks like the wave form shown bellow, for the three wires: enter image description here

So that, if it uses a simple resistive divider to measure each phase:

  • when the power is 60v, the phase value at the ADC input must be bellow 3.3v.
  • When the power is 6v (10x lower than 60 volts), at the ADC input it will be 3.3v divided by 10, i.e., 0.33v.

The Problem: Notice that using resistors simply the precision was lost by 10x (more than 3 bits!). This can cause instability at lower voltages and worse control, compared to 60v on power supply.

Solutions? I was thinking about using a AGC to adjust the signal to the ADC input based on the power supply voltage. No meter what voltage on the power supply, the ADC should catch a 0-to-3.3v signal linearly similar to the phase signal. What AGC chip you guys recommend? What else could be a possible simpler solution?

Emanuel M
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    Realistically people don't build systems that work over such a wide voltage range. You'd likely to encounter many other practical problems in trying to get a design which can support high voltage to work at only 6 volts. That said, you're probably overestimating the amount of "analysis" needed - likely what you need is to find the zero crossings, then use that to discipline a *model* of the rotor position. So you can probably use some sort of clipping scheme. You might look at schematics for *actual* hobby etc ESC's. – Chris Stratton Oct 20 '20 at 16:15
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    I agree with @ChrisStratton, usually it's only the zero crossings of the BEMF waveform that you care about, so why not just use a clamp to prevent the waveform from exceeding some safe voltage. – John D Oct 20 '20 at 16:17
  • @ChrisStratton , it is a project to enable some other methods to get a precise control for the motor. Some methods use integration, which could get really wrong if the ADC precision is wasted. What do you suggest? – Emanuel M Oct 20 '20 at 16:19
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    I suggest you think more about the problem: at high RPM, a model of rotor position fed only by the zero crossings is ample. At low RPM, your *actual* back EMF in the sense of kv * rpm will also be low even if the *available* supply voltage is high. But I continue to think your desired power supply range is unreasonably wide to try to support in a single selection of components. – Chris Stratton Oct 20 '20 at 16:26

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