ADC keeps outputting negative numbers, how?

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.

Frank wrote:

> 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?

How does it compare with the analog signal?  What is the magnitude of 
your error?  IF there an AC component to your reading during idle?  If 
the error is small, it is probably system noise.

-- 
Thad

"Thad Smith" <ThadSmith@acm.org> wrote in message
news:438b1300$0$245$892e0abb@auth.newsreader.octanews.com...
> Frank wrote:
>
> > 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?
>
> How does it compare with the analog signal?  What is the magnitude of
> your error?  IF there an AC component to your reading during idle?  If
> the error is small, it is probably system noise.
>
> -- 
> Thad
>

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.

"Thad Smith" <ThadSmith@acm.org> wrote in message
news:438b1300$0$245$892e0abb@auth.newsreader.octanews.com...
> Frank wrote:
>
> > 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?
>
> How does it compare with the analog signal?  What is the magnitude of
> your error?  IF there an AC component to your reading during idle?  If
> the error is small, it is probably system noise.
>
> -- 
> Thad
>

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.