Some channels have problems with sampled output values

Question

Hi, I am using LTC2320-16 chip for some research projects. According to the instructions of the chip manual, I used FPGA to send the corresponding SCK and CNV signals to the chip, and observed the SDO signals of 8 channels. All 8 pairs of differential pins are grounded during the test. The test results show that some channels have an SDO output value of around 0x0000 (e.g., channels 1, 3, 8), some channels have an output value of around 0xFFFF (e.g., channels 2, 4, 5), and some channels have a value of around 0x0500 (e.g., channels 6, 7). 
 Attached is the waveform diagram of the test. The waveform of the oscilloscope from top to bottom is SCK, SDO, CNV 
 correct channel 
 
 wrong channel 
 
 This is my schematic（Differential signal pairs are grounded in the test） 
 
 Thank you very much for giving me some help！ 
 regards， 
 zhu

enovoa1 · Accepted Answer

Hello zhu, 
 The output data of LTC2320-16 is in 2's complement format. This means that 0x0000 (decimal 0) and 0xFFFF (decimal -1) are only 1 LSB apart, so getting values closely around those with grounded inputs looks OK. 
 The other value you mention, 0x0500, is decimal 1280, which multiplied by 125uV/LSB turns out to correspond to ~375mV at the input. This, you should not be getting with a grounded input. Could you provide screenshots for when you get such values? Are you consistently measuring a value around the 375mV in the channel? Or is it around 0V and sometimes the higher value shows up? 
 Regards, Emilio)

Some channels have problems with sampled output values

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