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How bad can a $2.97 ADC be?

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Re: How bad can a $2.97 ADC be?

#81

Post by londons_explore »

cycomanic wrote: post #53
magicalhippo wrote: post #39

Earlier quoted context omitted.

> To get something faster we switched to ADS7953. It has 16 channels and runs 10 times faster. I recall reading about a project at CERN to design a 12bit ADC chip that could sample at tens of GHz, maybe 50 or more. I was perplexed at how they could achieve this. Turned out it was the same we programmers do. Parallel processing. They had taken a 12bit SAR unit which ran at like MHz rates, and just cloned it many times…

You can buy ADCs at over 100 GS/s (keysight, teledyne and tektronix make oscilloscopes using them), however typical ENOBs are more around 5 bits for these. For people interested in this stuff, I there is a video of someone taking apart one of the high speed keysight scopes (I think signal path is the YouTube channel?).

An oscilloscope is IMO just a user interface for an ADC. The adc is where the hard engineering lies, and the bit which should command 90% of the cost.

Unfortunately this isn't the case - the company designing the plastic case and buttons gets the lions share of the money.

Re: How bad can a $2.97 ADC be?

#82

Post by londons_explore »

throwup238 wrote: post #22
q3k wrote: post #14

Earlier quoted context omitted.

Interesting, most of the counterfeits that affect me (eg. FTDI, STM32 clones) have been just straight up clones developed from scratch, not excess inventory / ghost shifts / packaged rejects. I guess it might be a digital/mixed-signal split, with the two worlds having different issues? (also interestingly the STM32 clones I've seen had stacked die flash because they didn't fab them in a technology that could also do…

I remember the mess with FTDI clones back when I was still a hobbyist and buying stuff from eBay, but ever since I’ve started doing EE professionally I rarely run into anything that bad. You’re not going to make a clone Marvell processor for example, but I’ve run into several ghost shift runs from a distributor. I don’t usually buy from electronics markets in Shenzhen either so that probably helps.

Buy in bulk from the Shenzhen markets and sellers will be pretty clear that you're getting a clone, and will give you samples of that specific clone so you can QA your product with them. (Some popular devices have multiple clone suppliers).

I now always buy clones where possible - whilst not all features are implemented and some specifications won't be met, the devices seem to match the original for reliability, and sometimes even come with their own cloned modded datasheet.

Re: How bad can a $2.97 ADC be?

#83

Post by londons_explore »

hex4def6 wrote: post #26
jamesbowman wrote: post #18

Earlier quoted context omitted.

Author here. I did consider this, as others have reported getting ADS1015 marked as ADS1115. If it were an ADS1015 the readout would be truncated at 12 bits. These parts definitely delivered 16 bits of readout.

I worry about the demo boards being radically different in terms of layout etc. Even if you're using the same interface and power supply, the PCB may be affecting performance. Getting full spec performance out of an ADC requires having good layout power supply routing etc. I would transplant the chips from PCB A to PCB B and vice versa. See if the performance follows the chip or the PCB. Also check power consumption…

It doesn't take much crosstalk from some clock signal into the signal you're measuring to really mess stuff up...

Many of these Dev boards are not produced with as much care or knowledge as the chip design itself.

Re: How bad can a $2.97 ADC be?

#84

Post by hinkley »

Hasz wrote: post #42
q3k wrote: post #2

Should be easy to compare - sand down both the cheapo and a legit one (with a similar datecode) with fine sandpaper, clean with isoprop and shove under an optical microscope - you can then tell how similar/dissimilar the dies are, you don't even have to look for markings or anything super legible, or even sand them to the same metal layer. Clones tend to be vastly different - different technology node, architecture,…

I have done this a bunch of different times, mostly to resolve sourcing disputes. Dissolving in boiling sulphuric acid/nitric acid will make quick work of most epoxy packaging IMO, I have mostly seen mislabeling, rebinning, and passing off obvious QC rejects. example from many years ago: https://www.youtube.com/watch?v=e6DfBuPwAAA

…I wanted to look at this chip… so first I put it in a jar of hot nitric acid.

Re: How bad can a $2.97 ADC be?

#85

Post by Aurornis »

cushychicken wrote: post #49

OP is making comments about poor performance single cycle of a sigma delta ADC. Single cycle readings defeat the point of sigma delta ADC setups. You're taking many high noise samples and averaging them over time to get a better picture of the average voltage.

> Single cycle readings defeat the point of sigma delta ADC setups.

The ADC's internal delta-sigma ADC takes a lot of samples at a much higher modulation frequency and presents them as a single output value.

You do not get the direct delta-sigma output from an ADC like this. The internal logic handles that for you. It's okay to take single samples of the output.

Re: How bad can a $2.97 ADC be?

#86

Post by Junk_Collector »

londons_explore wrote: post #81
cycomanic wrote: post #53

Earlier quoted context omitted.

You can buy ADCs at over 100 GS/s (keysight, teledyne and tektronix make oscilloscopes using them), however typical ENOBs are more around 5 bits for these. For people interested in this stuff, I there is a video of someone taking apart one of the high speed keysight scopes (I think signal path is the YouTube channel?).

An oscilloscope is IMO just a user interface for an ADC. The adc is where the hard engineering lies, and the bit which should command 90% of the cost. Unfortunately this isn't the case - the company designing the plastic case and buttons gets the lions share of the money.

Keysight at least, has a fab where they make their own ADCs. Those are something like ENOB 6, 10 bit raw up to 120GHz and are used in their oscilloscopes but can also be purchased standalone.

Oscilloscopes also have a significant amount of additional front end conditioning, probe control, channel timing, and analysis software built into them. Most of the math functions on oscilloscopes use custom ASICs that work off the raw bits coming from the 120GHz digitizer which is non-trivial even just to receive. Calling it a plastic case around a digitizer is disingenuous.

Re: How bad can a $2.97 ADC be?

#87

Post by Junk_Collector »

cycomanic wrote: post #53
magicalhippo wrote: post #39

Earlier quoted context omitted.

> To get something faster we switched to ADS7953. It has 16 channels and runs 10 times faster. I recall reading about a project at CERN to design a 12bit ADC chip that could sample at tens of GHz, maybe 50 or more. I was perplexed at how they could achieve this. Turned out it was the same we programmers do. Parallel processing. They had taken a 12bit SAR unit which ran at like MHz rates, and just cloned it many times…

You can buy ADCs at over 100 GS/s (keysight, teledyne and tektronix make oscilloscopes using them), however typical ENOBs are more around 5 bits for these. For people interested in this stuff, I there is a video of someone taking apart one of the high speed keysight scopes (I think signal path is the YouTube channel?).

I think you are thinking of this video: https://www.youtube.com/watch?v=DXYje2B04xE

Re: How bad can a $2.97 ADC be?

#88

Post by BertoldVdb »

Terr_ wrote: post #61
jacquesm wrote: post #54

Earlier quoted context omitted.

I like it. I've used sandpaper to solve some interesting problems in the past as well with great success. I suspect we are only seeing the beginnings here in a trend of high-tech applications of fine grit paper.

I expect it's much easier to acquire fine sandpaper, yet my inner child yearns for laser decapping. (Or I suppose more than "decapping", depending on the depth.)

Laser decapping will destroy the die, except if you have a rare package that does not mix glass in the epoxy.

You could still compare the internal structure of the package and bonding, but the die itself is mostly destroyed.

Re: How bad can a $2.97 ADC be?

#89

Post by monocasa »

bsder wrote: post #74
NoiseBert69 wrote: post #58

Earlier quoted context omitted.

But no real competition for on-chip Wifi/BLE around. Only ugly two-chip solutions or hyper exotic stuff with no community.

> But no real competition for on-chip Wifi/BLE around. No competition or no market? I can't imagine Espressif is selling much volume of these chips. The fact that they can't even fix their SPI module would tend to indicate that their engineering staff is very thin.

They've sold a billion of the things.

https://www.espressif.com/en/news/1_Billion_Chip_Sales

Re: How bad can a $2.97 ADC be?

#90

Post by torginus »

Why would it be bad for that price? Highly accurate ADCs are pretty easy to make, provided you don't need a high sample rate, thanks to the magic of signal processing. Delta-sigma ADCs, and ramp ADCs can use a single bit of digital input + some pattern hold circuitry to do incredibly accurate measurements, it's just they take some time.

If you want a flash ADC that can do 16 bit (and can do 16 bit at 100Mhz), however you'll have to probably mortgage your house.

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