How critical is this pull-up resistor to Vdd for current sensing in a Field Oriented Control of a BLDC or PMSM?
Right now, I don't have that pull up...I'm running the shunt resistor voltage straight into the non-inverting pin. Because of this, I'm only getting half-wave rectified current measurements...I'm wondering whether I really need those or not if using SVM modulation. I should also mention that I think what I'm doing is wrong, since I'm unable to get a value for the unmeasured third phase, since for 120 degrees, there is no current flowing from the shunt-resistor phases to ground...So, that's obviously incorrect.
In fact, here's a picture of what my current waveforms look like over a full 360 degree cycle (I'm measuring phase A & B...I'm using KCL to infer phase C):
. Notice that for the first and last 60 degrees, there is no current in either low side leg...So, leaning on KCL is hopeless and it looks like I'm doing a garbage measurement...
Does this help put 0A at mid-rail or something?
What's the purpose of this? It's not like the current going through any of the legs ever goes below zero in the first place, like in an audio application. Why bother pulling it up to Vdd?