Other Parts Discussed in Thread: LAUNCHXL-F280049C, DRV8320
My question is similar to the one in this post: "TMS320F280049: Electrical characteristics difference on F280049’s analog pins"
My hardware has identical analog inputs, but for some reason, one of them is behaving differently than the others. There are inputs measuring 3 motor phase currents. The circuit is represented here as shown below where R12 represents the burden resistor of the hall effect sensor, Vref_final = 3.3V, Vdd = 3.3V and the voltage across C7 goes into the A2D input on the processor. When no current is flowing the voltage across C7 = 1.65V ideally.
So here is the issue. I have 3 channels setup with this same circuit. Those channels are connected from my hardware to the LAUNCHXL-F280049C header pins (27, 28, 29) which map as follows to the processor inputs.
Header pin 27 = Processor pin 38 (A9) and pin 16 (PGA5_IN)
Header pin 28 = Processor pin 19 (C0) and pin 20 (PGA3_IN)
Header pin 29 = Processor pin 7 (B2/C6/PGA3_OF) and pin 18 (PGA1_IN)
When the launchpad is connected, the voltage at header pin 27 is about 40mV lower than the other two pins. As best I can tell the channels are setup the same way in firmware. So for example, here is what I measured when there is zero motor phase current and the launchpad is connected.
Header pin 27 = 1.612V
Header pin 28 = 1.647V
Header pin 29 = 1.645V
And when the launchpad is disconnected from the headers, here is what I measure at the same header locations.
Header pin 27 = 1.648V
Header pin 28 = 1.647V
Header pin 29 = 1.645V
What is different about this analog input relative to the other two? What should I look at in the firmware that might affect how this channel behaves relative to the other channels. I simulated the ADC input to ensure that during sampling I get good settling with the assumptions about the sampling circuit in the datasheet, but if there is something else drawing current on these lines (such as something related to the PGAs) then I need to be able to account for that (or disable it) when selecting and RC value for these inputs. And an even more general question, what is the maximum recommended source resistance on the ADC channels? I assumed input impedance could be a few kOhm because of the relatively high source impedances used for the analog voltage feedback inputs on your DRV8320 and 3PHGAN eval kits.
Here are the simulation results I referred to above. The voltage at node "FINAL" is connected to the output of the opamp circuit above. The image quality become poor when I imported it, but you can generally see the behavior. The plot is zoomed in on a single sample of a sine wave (blue). The waveform (green) settles in within a reasonable about the time and within the sampling interval we have for the ADC.
Blue Waveform = Sine Wave measured at "FINAL" node.
Green Waveform = ADC sampled waveform measured across C17 ("Cap" node).
Thanks.