Re: External clock for Analog to Digital Converter



John Larkin wrote:

Eeyore <rabbitsfriendsandrelations@xxxxxxxxxxx> wrote:
John Larkin wrote:
"East Hunk" <bilalamin@xxxxxxxxx> wrote:

I am working on Jitter in Analog to Digital Converters(ADCs). I am
trying to setup an experiment to see the effects of jitter in real
time ADCs. I have an ADC evaluation board with external clock input
for sampling (i.e Sampling clock).
Now I want to produce a self created jittery signal to see the effects
of jitter in ADC. Can anyone out there have any idea(s) how I can
produce a real time clock signal with "variable jitter" for the input
of external clock? Producing a simple clock signal (i.e. without
jitter) is straight forward with the help of any signal generator but
a clock signal with "variable jitter" is a problem.


Get a fast analog comparator or a differential-LVDS-to-CMOS converter
chip.

Apply your low-jitter logic-level signal generator to the + input of
the comparator. Bias the - input to about logic mid-swing and AC
couple a noise source into that. The output clocks your adc, and the
jitter will depend on the slew rate of the edge at the + input and the
ac noise level at the - input; the math here is direct. The noise
source can be any waveform you like and will determine the probability
distribution of the jitter; random, gaussian noise would be the
typical choice, or use a triangle wave for a flat probability
distribution.

To get lots of jitter, you may have to reduce the slew rate of the
clock generator. Some pulse generators have adjustable slew rate, but
if yours doesn't, a simple r-l-c lowpass filter can control edge rate
pretty accurately.

Some of the adc's around these days need femtosecond RMS clock jitter
to meet their accuracy specs.

Nice detailed explanation there John.

It's useful to note that most audio converters now simply require wordclock and
generate the bit clock from an internal PPL.

As such there's not much you can do to influence their performance.

For audio, I wouldn't think that even a microsecond of RMS jitter
would be audible.

1 part in 200 ?

Are you joking ?

Graham

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