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herve

Pmods to sample a 1 MHz Analog Input Signal

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Hi!

I have to sample a 1 MHz Analog input Signal, then do some computation and send the result to a digital to analog converter.

I would like to do this with digilent Pmods ( ADC and DAC) but I have to decide first which of the digilent Pmods will allow me to sample and rebuild a 1 MHz analog signal.

Could you please advise me some of your Pmods that might fit for my application?

What about Pmod AD1 and Pmod DA2/DA3?

Thank you!

Herve

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Hi Herve 

All the currently available Pmod AD modules have a sample rate of 1 million samples per second (or under that for the Pmod-AD5). So none of them are well suited for your needs. The maximum bandwidth you can get with a 1MS/s ADC is 0Hz to just under 500kHz, but that relies on a 'perfect' filter on the input that stops signals >500kHz reaching the ADC, so 0-300kHz is most likely a more achievable figure (allowing your input filter to start blocking at 300kHz, and must remove signals over 700kHz otherwise they will alias and look like a  300kHz signal).

If your signal is around 1MHz, and but only around 1MHz, you might be able to sample it slower and then work with the aliased signals - e.g. if you sample it at 750khz, it will alias down to a 500kHz signal, and you can then deduce what the original signal must have looked like. A bit ugly, but actually workable.

The Pmod DA3 can run at greater than 2MS/s, so is able to recreate a signal with a bandwidth of 0Hz to 1MHz, however you will need an appropriate reconstruction filter on the output. I've done some playing with it that you can see at http://hamsterworks.co.nz/mediawiki/index.php/Digital_Sine  The settling time for the DAC on PMOD DA3 is 1us, so that might be an issue - I assume that is the time it takes for the output to slew between the highest and lowest values.

 

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Hi!

Once again thank you for your helpful explanations.

I have more questions related to the same topic:

1- Do you have any tutorial or example about how to sample a signal with aliasing and deduce what the original signal must have looked like ?

2- Could you please suggest me some Digilent FMC cards (ADC and DAC) that I could use to sample and rebuild a 1 MHz analog input signal ?

3- Do you any idea about the price of those FMCs (ADC and DAC) ?

Thanks.

 

Regards,

Herve

 

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Dear Digilent,

I would like to know why you do not design Pmod ADC and PMOD DAC with a sampling rates higher than 2 MS/s?

Is it due to the limitation of the serial protocol? Or the number of wires? Or just for something else?

I am asking that question because I am interested to PMODs ADC/DAC, running at a sampling rate higher than 2 MS/s.

Thank you.

Kind regards,

Herve

 

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Hi Herve,

I asked some of our application engineers and someone will get back to you with an answer here.

Regards,

Bianca

Edited by Bianca

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Herve,

Unfortunately  the Pmod headers on most of our boards won't support speeds beyond the KHz range. It *could* be possible to set up an ADC that runs faster than 2MS/s on a board like the Arty, by connecting the SPI pins of an appropriately fast chip to the Chipkit header's SPI pins.

Looking at the documentation for the chip used on the AD5, for example, there is a 20ns minimum for the serial clock period. With a resolution of 24 bits, it wouldn't be possible to get a sample rate faster than something like 2.08MS/s.

To be specific, the 2MHz sample rate comes from the speed limit of the Pmod headers and from the speed limits of the chips in use on each Pmod. If you need to get to higher rates, you could try using the Chipkit headers, with different ADC chips. While the Pmod headers can't go fast enough to support full speed, the PmodR2R (resistor ladder DAC) documentation says it will support up to 25MHz.

Thanks,

Arthur

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On 7/14/2015 at 4:12 AM, hamster said:

Hi Herve 

All the currently available Pmod AD modules have a sample rate of 1 million samples per second (or under that for the Pmod-AD5). So none of them are well suited for your needs. The maximum bandwidth you can get with a 1MS/s ADC is 0Hz to just under 500kHz, but that relies on a 'perfect' filter on the input that stops signals >500kHz reaching the ADC, so 0-300kHz is most likely a more achievable figure (allowing your input filter to start blocking at 300kHz, and must remove signals over 700kHz otherwise they will alias and look like a  300kHz signal).

If your signal is around 1MHz, and but only around 1MHz, you might be able to sample it slower and then work with the aliased signals - e.g. if you sample it at 750khz, it will alias down to a 500kHz signal, and you can then deduce what the original signal must have looked like. A bit ugly, but actually workable.

The Pmod DA3 can run at greater than 2MS/s, so is able to recreate a signal with a bandwidth of 0Hz to 1MHz, however you will need an appropriate reconstruction filter on the output. I've done some playing with it that you can see at http://hamsterworks.co.nz/mediawiki/index.php/Digital_Sine  The settling time for the DAC on PMOD DA3 is 1us, so that might be an issue - I assume that is the time it takes for the output to slew between the highest and lowest values.

 

Can you help about reconstruction filter? I use 3rd- Low pass filter RC. It isn't better. 

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