2018-03-27, 10:32
(2018-03-22, 06:06)Cutty Wrote: The DC offset is Normal for current BLUE controllers... It does NOT affect signal processing on the servers, and mayl be improved somewhat in later production boards. It is ignored by the server.
The offset is due to the reference voltage established on IC500, which is derived from the VDD source. IC500 effectively separates the Analog devices from the Digital Processing. This tends to minimize digital 'noise' in the supply for the analog amplifier chains. The Analog portion uses VDD, and VGND. The digital processing, uses 3V3 and GND, with a reference derived from 3V3 volt supply... this slight difference in circuit 'dc levels' produces the offset artifact induced in the ADCs.
Thanks for that explantion Mike. It is rather irritating and makes for "peculiar" values for the + and - trigger thresholds.
The offset on my blue is the same across all channels so if either reference could be moved slightly it could be compensated for, in theory. The IC500 VGND is used all the way along the 4 analogue amplifer/filter chains, on the basis that the more something is used the more likely there is to be knock on effects, fiddling with that may not be a good idea. To shift the VGND one or other of the 1 M resistors forming the potential divider on the input of IC500 would need a very minor tweak.
The other reference for the ADC's is Vref on pin 21 of the microcontroller. That appears to be a well filtered feed from a +3.3V supply. Perhaps altering the value of R804 would shift the ADC reference?