for my Bachelor's Thesis I am working on a project that uses the AD7689 ADC, controlled by a PIC32MM micro.
However, I am observing strange readouts from the ADCs, see oscilloscope images below (all fotos taken after a couple of conversions):
- CNV: Channel 1, Yellow
- SCK: Channel 2, Red
- SDI: Channel 3, Blue
- SDO: Channel 4, Green
The behaviour is the same on all channels, though the values are quite different every time (not only the differences one would label as noise, but sometimes a MSB of the result is set where other times it is not, and so on), almost like random values.
0V @ IN2:
0.215V @ IN2:
The Config bits used were following:
- CFG: 1
- INCC: 110
- INx: 010
- BW: 0
- REF: 110
- SEQ: 00
- RB: 1
The Config Readback returns Garbage as well (same configuration except readback enabled, 0V present on IN2):
The schematic looks like this:
VREF, VDD and VIO are all stable and show no problems, JP2 is connected to GND (= Digital Ground). CNV, SDO, DIN and SCK Lines are connected to test pads and the uC, nothing else.
Note: The Project was planned with the AD7699 Part, however it was not available and was substituted with an AD7689.
VREF is generated by a nADR4540 voltage reference (4.096V, buffered with an OP-Amp), the Input-Channels of the ADC are driven by AD8656 Op-Amps.
Am I missing somethin (obvios) here, has anyone an Idea what the culprit might be? I do not understand why alle the channels show this behaviour, and I do no believe the part is damaged, because the digital interface seems to work.