ADC keeps outputting negative numbers, how?

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I have some digital waveforms, during the idle interval, it's zero, while in
active
mode, some patterns are output. This waveform is send through DAC (TI
DAC290x-EVM) and ADC (Analog AD9218). I am expecting a clean
zeros during the idle interval, however the ADC output are not. How can I
correct this problem?

Thanks in advance.




Re: ADC keeps outputting negative numbers, how?

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while in
I

Thank you Thad. I have a 4 channel oscilloscope and can see three ADC
pins only. My digital signal is 20MS/s, 10us idle and 30us active. I channel
of DAC is fed into oscilloscope and Q channel to ADC. From what I saw,
during idle interval, bits 4,5,6 (0~9) have level changes during the idle
and
active interval.

I am setting DFS/GAIN pin to low for ADC board, and the DAC's I channel
waveform lies between -20mV~20mV (idle) ~ 0.8V(active) on oscilloscope.




Re: ADC keeps outputting negative numbers, how?

Quoted text here. Click to load it
while in
I

Thank you Thad. I have a 4 channel oscilloscope and can see three ADC
pins only. My digital signal is 20MS/s, 10us idle and 30us active. I channel
of DAC is fed into oscilloscope and Q channel to ADC. From what I saw,
during idle interval, bits 4,5,6 (0~9) have level changes during the idle
and
active interval.

I am setting DFS/GAIN pin to low for ADC board, and the DAC's I channel
waveform lies between -20mV~20mV (idle) ~ 0.8V(active) on oscilloscope.





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