ADC or VF converter? - Page 1
Forum section: electronics / projects

author: PartialDischarge
author_rank: Super Contributor
timestamp: September 12, 2018, 12:20:26 pm
content: I'm designing a system that requires precision and linear measurement of a signal DC-400Hz. I was determined to use one of the many ADCs available but then this remembered this app note from LT where Jim claims a 7ppm linearity for a VF converter and 17bit accuracy. I could live with that and even improve the design with better (more expensive) parts. My signal is bipolar so some tweaking would be needed but thats another issue.Which one would you be inclined for?

author: ogden
author_rank: Super Contributor
timestamp: September 12, 2018, 12:27:34 pm
content: You mentioned up-to 400Hz signal measurement? That LT appnote circuit have quite slow integrator which means that it is designed for "pure" DC operation only.

author: nfmax
author_rank: Super Contributor
timestamp: September 12, 2018, 12:36:09 pm
content: My go-to parts for jobs like this have been, for many years, the Cirrus Logic CS5509 family. I see they are scheduled to be discontinued next May, with no replacements listed.For now, you can still order them.

author: PartialDischarge
author_rank: Super Contributor
timestamp: September 12, 2018, 12:38:23 pm
content: Quote from: ogden on September 12, 2018, 12:27:34 pmThus my "improve the design with better (more expensive) parts". Also the switched capacitor block could run as slow as 1kHz and that would need a tweak alsoI understand this exact circuit does not meet my needs, the question is, how do you like the idea of a nice VF custom converter vs an off the shelf ADC

author: xaxaxa
author_rank: Regular Contributor
timestamp: September 12, 2018, 01:17:59 pm
content: a delta-sigma modulator operates on similar principles and provides high linearity, plus it can work at higher frequencies. Might be worth looking into.

author: ogden
author_rank: Super Contributor
timestamp: September 12, 2018, 02:09:19 pm
content: Quote from: MasterTech on September 12, 2018, 12:38:23 pm By increasing component cost is not how you improve anything in analog electronics. Anyway improvement of Jim Williams design is quite bold target.QuoteADC wins in all categories.QuoteWell, for proper 400Hz sine sampling you need WAY, WAY higher that 1KHz switched capacitor frequency. In your improvement plans you did not even address slow integrator which is one of main contributors to linearity of that circuit. If you modify it for 400Hz sine sampling, I bet one of two happens: 1) you can't meet original specs anymore and performance (linearity) is worse than most SD ADC 2) "improved" circuit is unnecessarily complex and way more expensive than quality SD ADC.Anyway delta-sigma ADC wins here.

author: PartialDischarge
author_rank: Super Contributor
timestamp: September 12, 2018, 09:48:39 pm
content: Quote from: ogden on September 12, 2018, 02:09:19 pmI’ve always distrusted that kind of assertion and clearly shows that you don’t know that much, as there areas where a vf converter will have advantages over adcs.

author: ogden
author_rank: Super Contributor
timestamp: September 12, 2018, 10:39:18 pm
content: Quote from: MasterTech on September 12, 2018, 09:48:39 pmWell.. This is not general discussion "VF converter versus ADC". Here we are talking aboutone and very specific area. If you forgot, I can remind you:Quote from: MasterTech on September 12, 2018, 12:20:26 pmLet's dig into it. To sample up-to 400Hzsignalsyou would need at least 4000 samples/second. At max 17 bits reading (131072) VF converter shall output at least 4000*131072= 524.288 MHz. As frequency span cannot be so many decades (4000Hz to 524MHz), it shall be shifted into higher frequencies, like 100Mhz ... 700Mhz. Note that max freq of original design was 1MHz.Before you dare to judge others - grow up first. Or at least learn elementary math.

author: David Hess
author_rank: Super Contributor
timestamp: September 13, 2018, 12:09:06 am
content: Quote from: ogden on September 12, 2018, 10:39:18 pmNo, there are no aliasing or settling time issues with this frequency-to-voltage converter but see below.QuoteThe frequency shifts practically instantly but noise depends on integration time of the output; nothing precludes measuring the output frequency in a single cycle or a limited number of cycles although the highest performance practical implementations would probably use a reciprocal counter and fixed gate time. The integration time produces a very clean sin(x)/x frequency response at the input and that would determine the input bandwidth although this is not the bandwidth most people are used to because the response is non-linear.Delta-sigma ADCs also integrate their inputs and have a similar non-linear response.With some math it is possible to calculate the integration time needed to produce a 400Hz response with a given error from the sin(x)/x curve. The same calculation is needed for delta-sigma ADCs although using a different formula. Whether that time is long enough to produce the needed noise free resolution is another matter.Quote from: MasterTech on September 12, 2018, 12:20:26 pm7ppm linearity and 17 bit accuracy are the same specification; the converter is linear to 17 bits. 7ppm is 143,000 counts and 17 bits is 131,000 counts. What we do not know is the noise but I suspect he used a 1 second integration time (I'm too lazy to search for the application note right now) which is way to long to resolve 400 Hz. Even 1/60th of a second (one 60Hz power line cycle) would not be fast enough.The 1ppm resolution actually makes this a 20 bit ADC with 17 bits of linearity.QuoteAs much as I like precision voltage-to-frequency converters, I would use a instrumentation delta-sigma converter for anything above DC and probably with DC as well except in a very special application. Instrumentation delta-sigma converters are just too economical to ignore if they meet the required specifications.

author: PartialDischarge
author_rank: Super Contributor
timestamp: September 13, 2018, 08:05:11 am
content: Odgen, you are still missing other important facts about VF converters like the lack of synchronization between emitter and receiver, which makes it advantageous in fiber optic transmission. Also VF converters are self-contained, no need for CPU or clock generator if the system doesn't already have one. Also MHz clock generators, a must for an ADC are power hungry, where a VF can run in less than a 1mA, extremely desirable for a battery powered system. Of course I didn't tell u everything, you just assumed.You are assuming too much about what I've thought and not, and about what I know and I don't. Like explaining me the sampling basics, with your own set or rules, when I'm an EE specialized in digital communications, my last project in college was building in a DSP a complex IQ spread-sprectrum transmitter and receiver, with a I-Q PLL programmed in assembly language in a C54, so I can tell u something about math.

author: ogden
author_rank: Super Contributor
timestamp: September 13, 2018, 08:50:01 am
content: Quote from: MasterTech on September 13, 2018, 08:05:11 amIt's you who are missingeverything. I am talking only and only aboutyour case, said it once already. And answer is NO - for your DC-400Hz application VF is no good.I know what VF's are good for and how they are good, but this is not what we are talking about here!QuoteEvery experienced analog EE will assume that you don't know much after you say that you can improve Jim Williams design "with better (more expensive) parts".QuoteSuch as? Where I waswrong? What was baseband bandwidth of your IQ modem and what was sampling rate of ADCs? - just curious.

author: xaxaxa
author_rank: Regular Contributor
timestamp: September 13, 2018, 09:35:24 am
content: Quote from: MasterTech on September 13, 2018, 08:05:11 amBeing pedantic, but all of this applies to delta-sigma modulators as well. Asynchronous DS modulators are self-oscillating and have the same characteristics as VF converters except it can operate on signals of much higher frequency content.

author: ogden
author_rank: Super Contributor
timestamp: September 13, 2018, 10:17:19 am
content: Quote from: xaxaxa on September 13, 2018, 09:35:24 amRight. I would like add multislope converters to the list. They shall be considered very linear - if built properly. After all such are used in hi-end bench multimeters, not VF converters.