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Old 6th March 2012, 03:25 PM   #11
percy is offline percy  United States
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yep, the ADS1675 was the reason behind the origin of this thread. I only wish there was a chip for 17 or 18 or anything over 16 bit but a little more bandwidth. I’d gladly trade in some vertical resolution for horizontal bandwidth.
Considering an EVM was going to be my next logical step if I didn’t find any diy projects. However I see that for this chip they don’t have a traditional EVM but rather a REF (“Reference Design”) and its $500. At that price it makes you want to consider just buying used equipment or even just renting one when needed. Or who knows maybe its only a matter of time before someone here builds one around this chip, and costs far less than the evm/ref. Jack, what are your thoughts on that ? I haven't looked at that implementation in detail but do you think it can be done for much less than $500 - say $250-300 ?

Gootee, I did check some of those links and didn’t find what I was looking for so stopped there. But if you’re saying there are some in there I will go check them all. I am sure it will be worth the time and effort.

Last edited by percy; 6th March 2012 at 03:28 PM.
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Old 6th March 2012, 03:41 PM   #12
Frex is offline Frex  France
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Hi percy,

Designing an hardware interface for the ADS1675 is a big job but that would be very pleasant for me...
The main issues (for me) to get a functional system is the needed software to speak with !
If you purchase the EVM for 500$, you have the hardware and the software that allow to use it directly as a FFT spectrum analyzer.
I really doubt that you will find used hardware that could be comparable for that price.
I don't have the EVM, but i'm very tempted !

Jackinj, could speak a little about it ? How it work in real world ?
What type of analysis have you already perform with ?
Thank you !
Regards.

Frex
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Old 6th March 2012, 04:04 PM   #13
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I just did buy on eBAy a card (E-MU1820) that is capable of relevant ADC performance and use it to take some measurements. Now I wil try to get the "m" version of the dock that has even better specs for ADC. My measurements with the present dock (at 24bit/96kHz) provide some 17-18bit of real resolution up to 50kHz. I can try to get 192kHz SR measurements, but I didn't think I needed them.
Measurements

Last edited by SoNic_real_one; 6th March 2012 at 04:28 PM.
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Old 6th March 2012, 04:43 PM   #14
BFNY is offline BFNY  United States
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Quote:
Originally Posted by percy View Post
yep, the ADS1675 was the reason behind the origin of this thread. I only wish there was a chip for 17 or 18 or anything over 16 bit but a little more bandwidth. Iíd gladly trade in some vertical resolution for horizontal bandwidth.
Considering an EVM was going to be my next logical step if I didnít find any diy projects. However I see that for this chip they donít have a traditional EVM but rather a REF (ďReference DesignĒ) and its $500. At that price it makes you want to consider just buying used equipment or even just renting one when needed. Or who knows maybe its only a matter of time before someone here builds one around this chip, and costs far less than the evm/ref. Jack, what are your thoughts on that ? I haven't looked at that implementation in detail but do you think it can be done for much less than $500 - say $250-300 ?
What you are asking for, you won't find, at least not for cheap. This is pushing the envelope for chip design, i.e. way over 16 ENOB (i.e. 20-24bit) AND over 50MHz sampling / frequency. If it does exist, expect to pay dearly as it will be in great demand by those who will.

As mentioned, you can get what you want, and people do, with bandpass filtering and adjustable front end end gain / attenuation using solutions with less resolution. You just can't see the entire BW "all at once".

Also, as folks mentioned, the hardware is easy compared to the software and calibration.

Getting a display is one thing - getting an amplitude and time / frequency calibrated display is another. This has to tie in with the front end amplitude gain / attenuation ranging, and so on, and is not trivial, as this part is not built into the A/D chip
Building practical test gear is not an easy thing, especially for people that don't design and build it for their real job.

I'd suggest used, off the shelf test gear, even for more $, like a decent digital scope w/ FFT and filtering in front of it. Or a low frequency SA (40-200MHz). HP made quite a few, and there were others brands too.
Very easy to resell and likely break even once you are done playing.
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Old 7th March 2012, 10:31 PM   #15
percy is offline percy  United States
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Quote:
Originally Posted by BFNY View Post
As mentioned, you can get what you want, and people do, with bandpass filtering and adjustable front end end gain / attenuation using solutions with less resolution. You just can't see the entire BW "all at once".
I didn't quite get the 'with bandpass filtering and adjustable front end end gain / attenuation using solutions with less resolution' part. Can you please explain that me in a little more detail ? Perhaps with an example ?
Thanks.
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Old 7th March 2012, 11:30 PM   #16
benb is offline benb  United States
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I presume he was talking about my "near-off-the-shelf solution."

Furthermore, it wouldn't really be lower resolution. If the device you're testing is any good at all, ultrasonic distortion products will be (pulling out a rough figure) 60dB or more below full audio-band signal output. Once the audio band is filtered out, the remaining ultrasonic component can be amplified by 60dB and analyzed with a "standard" DSO with as little as 8 bits of resolution that then shows the range from 60dB to 110dB below the audio-band signal.
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Old 8th March 2012, 01:55 AM   #17
BFNY is offline BFNY  United States
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A typical scope has about ballpark 45dB dB range on the display screen. So this means at the current full scale input setting, you can see about -45dB from that number.

But, you can adjust the full scale input (peak to peak voltage) on most scopes from ballpark 1mV/division to 5V/ division. It's assumed there are 10 divisions, so typical full scale range is 10mV to 50V. The ratio of these 2 numbers is 74dB.
Since 10mV is full scale, it's assumed you can see 45dB below this, so in essence, by adjusting ranges, you can see 74dB + 45dB = 119dB.

As mentioned above, if you excite in the audio range at say 2Vrms (= 5.65V pk-pk), and then high pass filter with a 4 pole filter (24dB attenuation/octave) above audio (80kHz) you'll have 48dB attenuation at 20kHz. You can then set the scope range to say 20mV full scale and effectively look at the bottom 45dB of the (45+48dB=93dB) range above 100kHz.

For coverage up to 96kHz, you can use a 24/192 sound card, which is probably not good to 96kHz, depending on how the input is alias filtered, but will be usable well above 20kHz.

Last edited by BFNY; 8th March 2012 at 02:07 AM.
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Old 8th March 2012, 10:46 AM   #18
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You forgot to trow in the noise/THD factor of the scope input stage in there... I doubt that you can really "see" 119dB on a scope.
A professional sound card is way more usefull than a scope for measurements up to 50-80kHz.
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Old 8th March 2012, 04:53 PM   #19
percy is offline percy  United States
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Thanks BFNY. I wasnít aware of this approach at all. I guess I didnít realize that the mv/div setting of a scope actually amplifies/attenuates the signal itself before measuring it. I just took it has some kind of a visual zoom.
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Old 8th March 2012, 05:23 PM   #20
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It just ATTENUATES the input signal to the lowest value. That will decrease the final SNR for any higher voltages.
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