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AD7689 - Garbage conversion results

Category: Hardware
Product Number: AD7689CCPZ


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.

BR, Fabian

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