Part Number:TMS320F28335
This has been a puzzle for a few days.
so on our custom board we have two adc channels next to each other, i manually short them to have them share the same cap very close to the input pins (tqfp 176 package, pin 41 and 42). the oscilloscope measured variation on the actual signal (ac coupled) is less than 2mV peak to peak. Measured external reference (2.048v) is stable within 2mV. AGND is pretty clean too. below graph shows the raw counts from the two channels for 1024 contiguous sampling at the same time (triggered by pwm 10khz). as seen, blue channel (A0) has a jitter around 6 counts worst case but the red line (A1) has almost 30 counts. This is under pretty slow clock 3.75mhz and 800us/channel sampling window. i've tried 3 boards they are all behaving the same.. if this is a hardware issue, i'd expect to see some abnormally at the input pins or on the ref but in this case i do not see anything more than a few mv on the scope..
both channels are fed with 100ohm and 0.1uF at the input.
any hints where to look next?
