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| Rigol DHO800 memory depth / bandwidth upgrades and noise level! |
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| ebastler:
--- Quote from: bdunham7 on November 12, 2023, 09:41:31 pm --- --- Quote from: gf on November 12, 2023, 09:39:38 pm ---But 9Vpp (provided that is the full scale range of the ADC) is only 3.182VRMS, for a full scale sine wave. --- End quote --- Doh! I knew I had some error in there somewhere. --- End quote --- Same here... Thanks! |
| Fungus:
--- Quote from: ebastler on November 12, 2023, 09:33:20 pm ---I don't have the DHO800 myself, but was assuming that the full ADC range is not mapped to exactly 8 vertical divisions, but that there is approx. half a division of "margin" at the top and bottom. --- End quote --- I can grab a 2Vpp signal in the 200mV/div range. That means it has a whole division above/below the screen. :) --- Quote from: bdunham7 on November 12, 2023, 09:30:36 pm ---9? --- End quote --- This one goes to 10! |
| Fungus:
--- Quote from: gf on November 12, 2023, 09:39:38 pm ---But 9Vpp (provided that is the full scale range of the ADC) is only 3.182VRMS, for a full scale sine wave. Then the SNR is 20*log10(3.182/0.005) = 56.075 dBFS, or equivalently (56.075-1.76)/6.02 = 9.0224 ENOB. --- End quote --- On the 1V range: 20*log10(3.535/0.005) = 56.998dB = 9.17 ENOB --- Quote from: ebastler on November 12, 2023, 09:18:00 pm ---Hmm -- pity that you deleted the screenshots taken at 50 mV/div. Those actually had a higher ENOB --- End quote --- On the 50mV range I get ... 64.95dB = 10.5 ENOB |
| 2N3055:
--- Quote from: Fungus on November 13, 2023, 02:17:23 am --- --- Quote from: gf on November 12, 2023, 09:39:38 pm ---But 9Vpp (provided that is the full scale range of the ADC) is only 3.182VRMS, for a full scale sine wave. Then the SNR is 20*log10(3.182/0.005) = 56.075 dBFS, or equivalently (56.075-1.76)/6.02 = 9.0224 ENOB. --- End quote --- On the 1V range: 20*log10(3.535/0.005) = 56.998dB = 9.17 ENOB --- Quote from: ebastler on November 12, 2023, 09:18:00 pm ---Hmm -- pity that you deleted the screenshots taken at 50 mV/div. Those actually had a higher ENOB --- End quote --- On the 50mV range I get ... 64.95dB = 10.5 ENOB --- End quote --- Martin posted a link to article about ENOB measurements. It seems not all read it, but here we are, magic numbers are coming out of the hat. Measuring ENOB in scopes is made in accordance to IEEE Std 1057™ standard "IEEE Standard for Digitizing Waveform Recorders". There are many nice articles by various scope manufacturers on the topic. ENOB is calculated from SINAD (Signal to noise and distortion) ratio. It is measured by inputting low distortion sinewave (better than scope measured) and sweeping across a frequency BW of scope. ENOB is measured as a curve, a graph, not a single number. It will vary with with frequency. On scopes it is measured with signal that is 90% of full screen.In this case it would be 7.2V P-P for DHO800 1V/div. That curve than gets statistics treatment and some kind of average or worst case scenario number is picked by marketing. Siglent publishes worst case scenario ENOB number with some reserve included, for instance, making it very conservative number. They guarantee it will be better than number published. It is always better than that. Most manufacturers are the same in that regard. Rarely manufacturers (any of them) publish full characterisation of ENOB curves directly in datasheet. It is definitely not a ratio of scope AC RMS measurement of noise floor and theoretical RANGE of ADC. Since really measuring ENOB is out of reach of many (no wide BW signal source of sufficient quality) we can compare and talk about just noise floor levels VS full screen scale. Which is definitely not the same as ENOB.In which case it is simply SNR (signal to noise ratio). Expressed in dB. We could talk about bits equivalent of SNR but that is not usually made because it has no practical meaning. But this is not ENOB. ENOB needs signal applied and includes distortions (nonlinearities, sampling jitter...) . It is already a simplified metrics.... One another note: DHO800 has 8v P-P range on screen. That equates to 2,8369 V RMS sinewave. That is your usable full dynamic range. The rest is reserve to prevent clipping. |
| Fungus:
--- Quote from: 2N3055 on November 13, 2023, 07:38:45 am ---On scopes it is measured with signal that is 90% of full screen.In this case it would be 7.2V P-P for DHO800 1V/div. --- End quote --- That's when you're measuring THD, which we aren't. If you're measuring noise you should use the full range of the ADC for most accurate results. --- Quote from: 2N3055 on November 13, 2023, 07:38:45 am ---It is definitely not a ratio of scope AC RMS measurement of noise floor and theoretical RANGE of ADC. --- End quote --- https://en.wikipedia.org/wiki/Effective_number_of_bits The SNR tells you the ENOB when distortion is zero. As noted: Calculating THD requires a lot of fancy gear that most people simply don't have. OTOH it's a safe bet that THD is small, almost negligible. SNR will give a good approximation. It's the best number we have until more data comes along. It's also a safe bet that using the same method on different 'scopes is a good way to compare them. We measured the "benchmark" Siglent using the same yardstick and found the new Rigol has two or three more bits more. I think that's a fair assumption to make. ie. Rigol's 12 bits are a big advance and this little 'scope has proved itself to be the new reference point. |
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