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ST MC workbench Current sense

Question asked by walck.chris on Sep 24, 2015
Latest reply on Oct 5, 2015 by Gigi
Hi,

I am using ST MC Workbench to design a BLDC FOC motor driver, and am running into a problem.  I am using three shunt resistors to sense current (one per channel) and am trying to use "embedded PGA" as an amplification method.

Here are my pertaining specs
-Nominal current of 25A
-Processor nominal voltage of 3.3V
-I am using 0.04 ohm shunt resistor.  (actually a 0.001 ohm shunt resistor with external amplification gain of 40 such that the circuit output resembles a 0.04 ohm shunt)
-Internal gain of 2
-overall network gain of 2.0
Vout(polorization) of 0.0V

When I click "generation" I get the following error message.

"Nominal Current Higher than maximum supported by sensing network (20.625A)"

I am confused by this because at 25A, the voltage across a 0.04 shunt should be 1V. Then with a 2.0 PGA gain, the ADC's would see 2V at my nominal current.  This shouldn't be a problem because the embedded PGA is a rail-to-rail amplifier and should be able to operate with this voltage range.  The suggested max sensing range of 20.625 suggests a max PGA input voltage of 0.825V (1.65V at the ADC, i.e. half of its range at 3.3V supply)

I'm just wondering, am I missing some setting in the software that's only allowing use of half the ADC voltage?  Does anyone know why MC Workbench may not like this configuration?

thanks,
Chris

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