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  1. #1
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    fast ADC and fifo or DSP

    Hello all,

    I want to sample a signal (DC-6Mhz) and store some 500-1000 samples somewhere for later processing.
    I don't need to process the samples immediately. I can wait milli seconds for the processing and I only intend to process once every 30 seconds.
    Cost is a major factor.
    So far I have found a cheap ADC. -AD9280-EB which samples at 32Mhz. excellent!
    Now I need to store the first block of samples. I need to grab 1 block of samples every 30 seconds and process the data on a microchip lets say.
    What is the best way to do this?
    Cheap fifo?
    DSP chip? (collect and process at same time)
    How can I collect lets say 500-1000 samples in one go? or even higher?

    TI has an ADC chip that has a fifo on board but only samples at 6Mhz. Not high enough..

    •   Alt6th March 2012, 19:25

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  2. #2
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    Re: fast ADC and fifo or DSP

    Are you familiar with any microcontrollers? If so, I would suggest you use what you are familiar with and use a FIFO (and a bit of logic if necessary). You may even be able to use a timer output from the processor to clock the data.

    Keith.
    I started life with nothing and I've still got most of it left. (Seasick Steve)



    •   Alt7th March 2012, 00:00

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  3. #3
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    Re: fast ADC and fifo or DSP

    Hi Keith,
    Thanks for your response. I normally use microchips in my projects(I have always used the internal clk however). If I used an External clock to bump up the speed to lets say 100Mhz, this still would not be fast enough. This is because I need to divide this frequency by 10 for a 10 bit converter. My 100MHz clock becomes 10MSPS. The PIC needs 1 clock cycle per bit, 10 bits per symbol. However, to sample a signal from 0-6Mhz requires at least 12-15MCPS sampling rate, (12-15MCPS). So it seems, I need a fast ADC with a fifo to hold the data chunk. And finally, I need to do some processing on the data, and I suppose a Pic could do this as there is no rush here.
    This does seem a cumbersome way to do things however, and not that cheap either. Is there a better way?
    Is there a magic chip that does all this (20MCPS, integrated fifo, +processor), at reasonable cost?



    •   Alt7th March 2012, 11:10

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  4. #4
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    Re: fast ADC and fifo or DSP

    I am not sure why you need a 100MHz clock. All the high speed ADCs I have used simply require a clock at the sample rate so if you want 20Ms/s you need a 20MHz clock. I recently designed a circuit with LTC2205/4/3/2 series. They are 16 bit, up to 65Ms/s and you simply clock them with 65MHz. In your case you could use something with 32Ms/s and use a 32MHz clock which can also drive your PIC (depending on which PIC you use).

    If you really need some logic then a small CPLD/FPGA could be used, but it depends if that is a technology you are used to using. Otherwise some discrete logic would do.

    I am not sure how fast the DMA is on things like the PIC24E/H or PIC32, but you may be able to go straight into those with DMA at 16MHz+.

    Keith.
    I started life with nothing and I've still got most of it left. (Seasick Steve)



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    Re: fast ADC and fifo or DSP

    I like your ideas. I've being looking at various stuff from Linear and Analog but their demo kits are way overkill,and expensive for my project.
    The pic32 has a parallel port, DMA module and can run up to 80Mhz.
    I can get the pic to drive a clk signal for the ADC at lets say 20Mh. The ADC then produces 12 bits of data(12 bit ADC) on each clk cycle (every 20necs).
    The pic receives this data on a parallel port and using DMA clocks each 12 bit word into memory (sram)
    Am I on the right track?
    I think its time to buy components in and test..



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    Re: fast ADC and fifo or DSP

    Yes, sounds possible to me. You just need to check the DMA speed but I assume it is fast. Also, be careful when picking the port pin connections from the ADC to the PIC. I am not sure exactly what they are called - PMP I think. The pins you want to use are the ones that can clock parallel data into the DMA - legacy ports. Not all pins can do that and there are dedicated (but programmable) chip selects.

    Keith
    I started life with nothing and I've still got most of it left. (Seasick Steve)



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