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wrong conversion with AD7691


we have a frustrating issue with the ADC AD7691:

we use an FPGA that reads the ADC at 200 kHz (but we tested also at 100 kHz and it's the same) in "CS MODE" (3-WIRE WITHOUT BUSY INDICATOR).

The ADC seem to works fine for almost all the input analog range (we are using it as full differential range with the ADA4941), but in the region where the input value is converted as 100000 (decimal representation comp-2) the ADC is returning "always" the same value 100384. It seem that for a "wide" (some mV) input voltage range the ADC gives the same code: 100384. That cannot be due to the noise.

I attach some screen shots pick-up with an Oscilloscope with SPI decoder: as you can see the 100384 occurs very very often even if the input voltage (CH4, AC-mode) is varying a lot.

We make different test relaxing the reading timing, changing the AD7691 chip, but we still get that damned code:100384.

Please someone can help us?