Why do some ADCs have so many bits beyond their SNR effective bits? - Page 1
Forum section: electronics / general

author: Andrew_K
author_rank: Contributor
timestamp: July 14, 2023, 01:14:15 pm
content: I'm trying to learn about the nitty-gritty of ADCs and I have a question that may be dumb...As I understand, resolution and SNR are related. Theoretical SNR = 6.02*N + 1.76, where N=bits. And for every averaging factor of 4 we can gain an additional bit of resolution.So if we have an ADC with 16 bits that should give a theoretical SNR of 98.08. But many ADCs such as theAD9655have an SNR that gives significantly fewer bits. For example, 71dBFS for the highest frequency gives around 11 bits of resolution.Why bother making it a 16-bit ADC if the effective performance would be the same as an 11 bit ADC? As I understand, you can get 16 bits from an 11 bit ADC from processing gain and a decimating factor of 1024 (4^5)

author: dietert1
author_rank: Super Contributor
timestamp: July 14, 2023, 01:39:40 pm
content: As you wrote, it depends on how you use the part. Maybe at lower sampling rate the effective number of bits is closer to the number of hardware bits. Maybe somebody can make use of the noise bits in a dithering procedure. Table 3 on page 5 has numbers up to 99 dB (analyzing a 19.7 MHz carrier). Averaging works best if you have the noise bits. Otherwise quantization errors may remain in case the input signal is low noise.Anyway, it's easy to drop some low bits if you think you won't need them..Regards, Dieter

author: Kleinstein
author_rank: Super Contributor
timestamp: July 14, 2023, 02:11:55 pm
content: If the ADC aritecture allows (especiall SD ADCs) it is good to have a few extra bits, so that the quantization noise is no longer limiting. A real ADC always has additional noise sources and these or often the difficult part. So one could argue that if SNR gets close to the quantization limit, than the ADC is giving away potential and kind of missing 1 or 2 extra bits of numerical resolution.Another point is that the noise level can depend on the signal level and maybe reading speed. So the useful numerical resolution is more about the best case, which is often slow sampling and a small amplitude.It is convenient to keep the same data format for different speeds.

author: EEEnthusiast
author_rank: Frequent Contributor
timestamp: July 14, 2023, 03:09:38 pm
content: You can try to truncate the 5 bits of a 16bit ADC and try to call that an ideal 11bit ADC. However, if you want to gain extra bits with averaging, those 11 bits alone will not help you. You still need those noisy 5 bits which still contains information. The problem of lower ENOB starts to hit you when you go higher than 12 bits. You could get an 8bit ADC with an ENOB of close to 7.9bits very easily. At higher precision, the thermal noise, DNL and other factors start to limit your SNR substantially and hence the ENOB cannot reach the actual number of bits.

author: gf
author_rank: Super Contributor
timestamp: July 14, 2023, 05:30:22 pm
content: SNR is a matter of bandwidth.NSDof this ADC is somewhere in the region -155...-160dBFS/Hz. If you renounce bandwidth, then you can also get a SNR beyond 98dBFS, and a resolution enhancement beyond 16 bits (although spurs still limit the SFDR). The analog bandwidth of this ADC is pretty wide (500MHz = 4x sample rate), so I guess this ADC is not only intended for sampling baseband signals, but also for direct RF sampling (undersampling / bandpass sampling) applications?

author: KT88
author_rank: Frequent Contributor
timestamp: July 14, 2023, 07:51:06 pm
content: Another benefit of higher sample rates is the reduced complexity and related errors of the AAF. In particular frequencies >a few MHz don't allow for an active AAF anymore becaus of additional harmonics created with an active higher order filter.Or the other way around: a shallow passive filter is still effective (enough) for high frequency noise attenuation.

author: coppice
author_rank: Super Contributor
timestamp: July 14, 2023, 08:25:40 pm
content: Extra bits only gets silly when they get seriously non-linear. If they are just awash with nice clean AWGN (additive white Gaussian noise), which is often the case, they are highly usable in many applications. For an extreme example, take the ADCs in a utility's electrical energy meters. They need to be cheap. Its a high volume consumer(ish) product. They are usually sigma-delta converters with quite a lot of AWGN, but really good linearity. So, you take a lot of samples from the mains waveform, and calculate the RMS voltage. You'll get results whose accuracy is pretty much determined by the level of AWGN. You also take a lot of samples from a current sensor in the live lead, and calculate the RMS current. Obviously this can vary over a wide dynamic range, depending on the load. At low current, the accuracy will depend on the level of AWGN. The noise power doesn't vary massively with temperature, so you can subtract a bit from the current reading to compensate for the noise, and improve the accuracy. Now,hereis the nice part. You have 2 ADCs. Their noise sources are independent. You take the voltage and current samples, multiply them together, and integrate to get the consumed energy. Now you are measuring with a correlation process. Two independent noise sources don't correlate. They contribute almost nothing to the power measurement, unless you use only a very short burst of samples. The noise just correlates away. Those noisy low cost ADCs can give you 0.01% accuracy over 1:10000 range of load current. The ultimate accuracy of the meter might be only 0.1%, because of the temperature coefficient of the ADC's reference voltage. However, the ADC noise has almost no impact, and the underlying linearity of these cheap ADCs gives results that are quite impressive for a 20 cent chip.

author: radiolistener
author_rank: Super Contributor
timestamp: July 15, 2023, 12:33:52 am
content: Any analog circuit has it's noise floor. Also ADC linearity is not ideal. All that affects ADC dynamic range.

author: ejeffrey
author_rank: Super Contributor
timestamp: July 15, 2023, 01:49:42 am
content: One ENOB number isn't nearly enough to describe a part.That ADC has an DNL of only 0.7 LSB, so it's fully monotonic for DC signals. If someone cares about that they might be unhappy if the manufacturer threw away those bits. The SNR is frequency dependent so while it only has 11 effective bits at 300 MHZ, at lower frequency there are more like 13 bits. The SFDR is quite good. If they cut off at 12 or 14 bits and you didn't have enough noise to completely dither you might get spurs from quantization that exceed the SFDR spec.Also, 16 bits is such a common word size that even if they could get basically the same performance with only 14 bits there is no reason to drop them if they have any information at all.