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Best ADC conversion method and clamping?

Question asked by MrStive on Jun 14, 2012
Latest reply on Jun 15, 2012 by fm
Hi Guys,
I'm designing a 3-phase DTC induction motor control which needs the 3-phase voltages and 2 of the currents. I am using the STM32F4 Discovery board.
The control will have a small sample time (5uS). The controller may issue a new switch state every 5uS, and therefore the 5 ADC must take place every 5uS. Assuming due to the inductance of the motor the voltages/currents will need, lets say, 2.5uS to settle. Therefore i have only 2.5uS to do these 5 ADC's.
The control loop is highly sensitive to these 5 parameters, therefore the more accurate the readings the better. Also, it is best to do these readings as close (in time) to each other as possible. I see this STM32 chip has many ADC setups, so what is the best method to go about this? Ideally it would be good to have a synchronized trigger just before my main control loop to read all the ADC values into DMA, then I can just read the values without interrupting the flow of the code.
Is this going to be too hard to get them sampled within a reasonable time? Another alternative is to just use the DC bus voltage and switch states to estimate the 3-phase voltages. Although this is mathematically less accurate, perhaps just having 3 ADC conversions (DC bus voltage, and 2 currents) will end up a more accurate solution?

Also, my UVW phase voltages will be 0~400V . I was thinking of scaling the voltages 130:1 (max 468V) and clamping using the following schematic. Thoughts? Im not sure what value i should use for R_o.

ADC%20protection.JPG

Thanks for your help in advance!

Stefan

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