Re: 12 and 16-bit oscilloscopes


John Devereux wrote:
"Tom Bruhns" <k7itm@xxxxxxx> writes:
....
I'm not quite seeing the 120dB SNR in the data *** for the
AD7760--looks like 112dB to me. At 78kHz bandwidth, that's about
-160dBfs/Hz. That's good, certainly, though it's not all that
different from a 130Ms/s part that has a 78dB SNR: such a part has a
noise level at about -156dBfs/Hz.

Interesting; your implication is that one can obtain equivalent
performance at lower frequencies by signal averaging, yes? So just
pick the device with the highest sampling rate and low dBfs/Hz?

Yes, with respect to noise anyway. After all, a delta-sigma does it,
but in spades, by using feedback (the "modulator") to move the noise
from low frequencies to high, and then "averaging" (decimation
filtering, actually). ADCs designed for low frequencies are likely
going to have better distortion performance than the high speed ones.
Of course, the distortion of the high frequency ones is usually worse
at high frequencies than at low, but still, even at low, they won't be
quite as good. The advantage, of course, is that you can also look at
the higher frequencies, which is tough with a delta-sigma
narrow-bandwidth converter (except in narrow aliased bands...)

If it's really for a scope, my main interest in more bits would be to
avoid input ranging, and that's not all that difficult at low
frequencies (1MHz these days is "practically DC"...). But if it's for
spectral analysis, that's a different story. Then distortion and noise
are important (more important than the number of bits, which is largely
a red herring).

My personal interest would be in fewer bits and much faster sampling:
maybe a Virtex 5 connected to two or four 500Ms/s converters, into a
deep buffer memory.

My motivation was based on experience with a TDS3054 scope. I love it,
but one thing it does not do is improve resolution at the lower sample
rates. For one-shot events, you are stuck with the same ~7 bit
resolution at 10kHz that it uses at 500MHz. It would be nice to have
something optimized for low frequency use.

Yes, IF the 7 bits is really linear, then band-limiting filtering will
help things out considerably. The ADC in the HP3563 that Fred
mentioned is only 13 bits, but is dithered and has very good linearity,
so you can accurately measure signals well below one part in 2^13.

I suppose doing decimation on a 130Ms/s data stream with a PC real-time
might be possible, but would be pushing things. You have to deal with
getting data in, as well as implementing a multiple pole filter that
probably is going to take a few dozen instructions per sample. Much
nicer to have that with the ADC, along with a big capture buffer.

Cheers,
Tom

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