Where to get high speed ADC's and DACs

If you have 1 LSB hum at 50/60 Hz it will definitely not spread nicely around the Nyquist interval. Adding dithering noise artificially or natural thermal noise about 1 LSB, then the quantization noise will spread out.

The thermal noise voltage from a 50 ohm resistor at room temperature at 50 MHz BW would be 6 uV. If the ADC range is only +/-1V, that would be about 19 clean bits.

The OP claimed 22.5 bits with current system, I might even believe him, if we are talking about measuring a steady state DC signal. The thermal noise will dither the quantization noise and after heavy low pass filtering (BW

I would like to see them too. Do they differ from a system where the lower few bits are generated by random noise? A sigma-delta ADC might have a 40MHz clock, so internally there is a 40MHz 1-bit ADC, that with sufficient processing turns into a 24-bit result. But that does not make a 40MHz 24-bit ADC either.

The noise bandwidth will be a bit wider than that, because the input structure has to settle to full accuracy to get a clean conversion. So to be generous let's say 100 MHz bandwidth, i.e. 9 uV for a 50-ohm resistor.

If you want the bit to be vaguely clean, even assuming that the mean input voltage is right in the centre of the interval, you need an LSB of, say, 6 times the RMS.

That gets you a noise transition (false alarm) rate of

FAR = 2 B /sqrt(3)*exp(-0.5*[(0.5 LSB)/Vrms]**2) .

For Vrms = LSB/6, the exponent is -4.5, which for a 100 MHz bandwidth comes out to a 1.3 MHz FAR, assuming that the mean value is exactly halfway between transitions, which is the best case. (The formula is from Rice, with an additional factor of 2 because there are two equidistant thresholds. I had to go through this stuff in absolutely gory detail some years back in my particle detection work.)

Even with a false alarm rate like that (more than 1% of the samples will be wrong), you wind up with a 55 uV LSB, which is 15.2 bits at +-1V.

Thank you for your kind offer. For me, I've NEVER gotten Dropbox to work nor most of those 'public post your picture' sites. Can't even pick up pictures from Dropbox half the time.

will do.

don't know.

On a sunny day (Tue, 03 Sep 2013 10:09:51 -0700) it happened RobertMacy wrote in :

just email it to p a n t e l t j e @ y a h o o . c o m (n ospaces) and I will put them up and give a link.

Using a noisy ADC and filtering is worse than using a delta sigma, because the noise isn't shaped away from the region you're interested in.

But the same precision vs. bandwidth tradeoff is there.