High Speed data collection (DAC and ADC)

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jrogers
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Re: High Speed data collection (DAC and ADC)

Postby jrogers » Thu Apr 29, 2010 11:08 pm

I really like the idea of the "cut and paste" expandability!

I also like the buffering/protection ideas!

How would a variety of voltage ranges be handled? For instance if the voltage being measured really is only in, say the 0 to 5 volt range than it would be nice to use just that range so that we can make use of all the bits/precision.

In other cases where larger ranges of voltage need to be measured say 0 to 10, it would be nice to be able to sacrifice some of the precision to be able to handle this range. Could this be done by some sort of "register setting" or would it be a some sort of jumper on the board. The board I am using now has a little separate board that I use to adjust the voltage down.
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lilltroll
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Postby lilltroll » Thu Apr 29, 2010 11:41 pm

jrogers wrote:I really like the idea of the "cut and paste" expandability!

I also like the buffering/protection ideas!

How would a variety of voltage ranges be handled? For instance if the voltage being measured really is only in, say the 0 to 5 volt range than it would be nice to use just that range so that we can make use of all the bits/precision.

In other cases where larger ranges of voltage need to be measured say 0 to 10, it would be nice to be able to sacrifice some of the precision to be able to handle this range. Could this be done by some sort of "register setting" or would it be a some sort of jumper on the board. The board I am using now has a little separate board that I use to adjust the voltage down.
Differtial intput:
You pick a PGA you like with a Gain table as:
1, 2, 4, 8, 16, 32, 64, 128 |
1, 2, 5, 10, 20, 50, 100, 200 |
1, 10, 100, 1000
Controlled in software from XMOS

After the PGA comes the software controlled filter.

I have been searching for anti-alias filters as well. There are filters with a groupdelay of 1.5 us and the total ADC converter time is 1.0 us. (Total < 3 us)
If you need to filter the signal before the ADC , that filter will always add some time-delay. The interesting question is , what frequency response do you need of the filter.
What is the background noise in you application ?
Probably not the most confused programmer anymore on the XCORE forum.
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jrogers
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Postby jrogers » Fri Apr 30, 2010 12:21 am

Wow... PGAs sound great! Probably either the first of the second one will be fine!

Not sure about the noise characteristics... I am writing the control app for someone else and it is kind of at the "breadboard" state now... but he may have a feel for what sort of filtering will be needed in the final design. I will discuss this with him.

Question: My current project requires a 3us ADC conversion time... which is somewhere around a 300khz sample rate. How close are we to the practical upper limit of sample rates... I realize that the higher speed parts get more expensive, etc... but I would like to have an understanding of the range of sample rates that can be done with this type of system.

Again I realize that it can become a complicated equation of filtering/conditioning/etc, so I am just looking for some "ballpark" estimates. Don't give this too much time... just looking some of your general thoughts/guesses on the subject.
JohnR
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Postby JohnR » Fri Apr 30, 2010 12:58 pm

Hi,

You need to make sure that the frequency response of the PGAs is adequate for your application. Maxim, Analog Devices and Microchip all make PGAs with Microchip being the fastest I think. The gain really drops off at high gain settings.

John.
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lilltroll
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Postby lilltroll » Fri Apr 30, 2010 1:08 pm

There is 16 bit SAR converters running at 4Msample/s with an ENOB of 15bits up to 50 kHz decreasing to 14 bits at 800kHz. They have a 180 ns conversion time, but you need to add some delaytime for the data read operation as well (around 225ns).

You can go faster as well, but then you have to accept a lower SNR, ENOB.
Probably not the most confused programmer anymore on the XCORE forum.
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lilltroll
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Postby lilltroll » Fri Apr 30, 2010 1:11 pm

JohnR wrote:Hi,

You need to make sure that the frequency response of the PGAs is adequate for your application. Maxim, Analog Devices and Microchip all make PGAs with Microchip being the fastest I think. The gain really drops off at high gain settings.

John.
And some of the PGA with high BW-products have included a LP filter for anti-aliasing, so the signal bandwith is independent on the choosen gain :)
Probably not the most confused programmer anymore on the XCORE forum.
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lilltroll
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Postby lilltroll » Mon May 03, 2010 6:07 pm

I imported the gerbers for the XC-1, XC-1A and XC-2
XCs.png
I was planning to make it compatible with at least all 3 cards, but made as a stack-module, it will not just fit any board. The ports has slightly different positions.
All cards seems to have D26-D37 on the side.
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Probably not the most confused programmer anymore on the XCORE forum.

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