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    choosing right adc for data acquisition system

    hi all.
    i want to build a 8-channel, 1 or 2 Mega sample per second data acquisition system with 16 bit resolution.
    but i did not find any 8 channel ADC with 1 MSample for each channel!
    every part's i found like this:
    https://www2.mouser.com/ProductDetai...XWLfKGmsPeI%3d
    this part is 8 channel 1 mega sample ADC but according to the datasheetو the sampling rate split by increasing channels.
    can you introduce me an adc that works for me?
    ---------------------------------------------------------
    P.S: What do you think about other ways to implement this system?
    for example using 8 channel multiplexer and on adc ?!
    regards

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    Re: choosing right adc for data acquisition system

    Do you need simultaneous sampling?

    Generally, mux-ing at 1 MS/s with 16 bit accuracy is rather difficult, using separate ADC channels is reasonable.


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    Re: choosing right adc for data acquisition system

    Hi,

    What about two 4ch simultaneous sampling ADCs?
    Like LTC2324-16


    Klaus
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    Re: choosing right adc for data acquisition system

    Quote Originally Posted by KlausST View Post
    Hi,

    What about two 4ch simultaneous sampling ADCs?
    Like LTC2324-16


    Klaus
    thank you KlausST. this works for me.
    I've found AD9637 12 bit, 40 or 80 MSample with 8 channel. this part has good snr
    Do you think it is good for me to use this ADC and add more bits with over sampling and averaging?
    with this method i can sample data with much more sample rate in the case that Resolution isn't an important factor.

    - - - Updated - - -

    Do you need simultaneous sampling?
    actually its not an important factor!
    Generally, mux-ing at 1 MS/s with 16 bit accuracy is rather difficult, using separate ADC channels is reasonable.
    yes you're right. what a about this?!
    I've found AD9637 12 bit, 40 or 80 MSample with 8 channel. this part has good snr
    Do you think it is good for me to use this ADC and add more bits with over sampling and averaging?
    with this method i can sample data with much more sample rate in the case that Resolution isn't an important factor.



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    Re: choosing right adc for data acquisition system

    Hi,

    16 bit ADC vs 12bit ADC with oversampling:

    you need to average (or sum) at least 16 pieces 12 bit values to get a 16 bit value.
    But the 16 bit ADC has a DNL of typically +/-0.4LSB this equals to 0.025LSB at a 12 bit value.

    You wonīt achieve the same with the 12 bit ADC..
    You need to know: The "averaging" method only works good when the bit values change. And they change if there is noise or a (fast) signal. You need noise with at least 1 LSB magnitude of a 12 bit value.
    And for a good DNL value you need a linear distribution of the voltage values (almost impossible. You may generate a triangle style dither signal, or some artificial random noise with linear distribution, maybe with the use of DAC....but thatīs overkill.. )
    In your case: the more noise the better the averaging method. But who wants noise?

    If you have clean signals, then you gain nothing: worst case you read 16 times the same ADC value. And this is not unusual with a 12 bit ADC and clean input signals.
    This means the resulting DNL may be worse than +/-8LSB referred to the 16 bit value.

    If your signal is noisy, then the single 16 bit value is not useful. Then the 12Bit oversampled, digitally averaged and low pass filtered signal may give an improvement.
    My recommedation: Use the 16 bit ADC, generate clean signals and use a good anti-aliasing filter.

    Klaus
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    Re: choosing right adc for data acquisition system

    My recommedation: Use the 16 bit ADC, generate clean signals and use a good anti-aliasing filter
    Is it enough to use a RC filter as anti-aliasing filter at the input of adc?



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    Re: choosing right adc for data acquisition system

    Hi,

    it depends on your requirements. What do you do with the digital values from the ADC? (Just showing a waveform? calculating THD? ...)

    ****

    You know the transfer characteristic of a first order low pass filter? If not, then search the internet for it.

    You have to decide:
    * What is your pass band (uper frequency)
    * what max amplitude attenuation do you allow for the pass band
    * then there is the stop band (lower frequency). It usually is nyquist frequency, wich is f_sample/2
    * and what attenuation (minimum) do you need for the stop band.

    alias frequency:
    the ADC converts the analog information into digital information. This works great for frequencies up to half of the sampling frequency.
    But any frequency (frequency component. Maybe an overtone of any useful signal) will be mirrored at hlaf of the sampling frequency.

    *******
    Example:
    Sampling frequency = 2.000MHz
    ADC input voltage range 0V...5V
    Noise source: switch mode buck converter running at 1.300MHz, introduced with 50mVpp at the analog signal (for ease of calculation we treat it is pure sine shape)
    First order low pass filter with cutoff frequency of 250kHz

    --> attenuation of the 1.3MHz signal is about 0.189
    --> 50mVpp x 0.189 = 9.44mVpp (remaining amplitide)
    --> Nyquist frequecy = mirror frequency = 2.000MHz/2 = 1.000MHz
    --> alias frequency: 1.300MHz mirrored at 1.000MHz gives 0.700MHz

    The resulting alias frequency = 700kHz with an amplitide of 9.44mVpp
    One LSB of your ADC represents 5V / 2^16 = 0.0763mV.
    On the digital side the alias signal has an amplitude of about 124 LSB-pp

    After the ADC there is no way to get rid of this alias frequency. You have to do it on the analog side (Alternative: oversampling + digital filters + reduced data rate)

    Conclusion:
    If you want to do some signal analysis you will need a far higher order anti aliasing filter.
    If you only want to show a graph, then you may be happy with the first order filter.

    Klaus
    Please donīt contact me via PM, because there is no time to respond to them. No friend requests. Thank you.


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