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Anti Aliasing Filter

Discussion in 'Electronic Design' started by [email protected], Jan 5, 2009.

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  1. Guest

    It's the first time that i use anti aliasing filter, so i have some
    doubts:
    1)My signal has infinite frequencies, but greater frequencies have
    lower harmonics.Is it correct to put fs/2 (fs=sampling frequency of
    ADC) at frequencies where dynamic range of fitered signal is equal or
    greater than dr of adc? In this way, i can use frequencies lower than
    frequencies where filter attenuation is equal or greater than dr of
    adc.
    2)About noise,what should i consider?I want that noise, at frequencies
    greater than fs/2, has Vpp lower than lsb. But (if noise has gaussian
    distribution) Vpp=6.6Vrms: in what frequency range i should calculate
    Vrms?
    3)About slew rate, i have found this link
    http://www.edn.com/article/CA6339244.html?industryid=2281
    but i don't understand how slew rate is calculated....how can i obtain
    SR from SR=dV/dt|max?
    Thanks for the help and for any suggestion
     
  2. MooseFET

    MooseFET Guest

    Usually the speed of the ADC is set by considerations other than the
    filter's parameters and then the filter is designed to do what is
    required. The "what is required" part of that depends a lot on the
    application.
    This part gets very tricky. If you imagine the desired signal not
    being there then the RMS is the standard deviation of the signal.
    This doesn't really tell you much about individual samples. When the
    RMS goes below 1/sqrt(12) of the LSB, it starts to disappear into the
    quantization noise.

    You need to consider what exactly you require of the digitized
    signal. The aliasing folds the spectrum over so that the frequencies
    in the result are falling as the frequencies of the input are rising.
    This means that the fall off of the filter appears as a bias towards
    more noise at high frequencies.

    You usually don't calculate the slew rate. It is usually a
    consideration of the op-amps in the filter. You need to select op-
    amps that won't distort the signal and noise that they are processing.
     
  3. Guest

    Thanks for the infos, i have another question...i have some problems
    to calculate the right opamp's slew rate to obtain right adc accuracy.
    I have chosen as adc AD7819
    http://www.analog.com/en/analog-to-digital-converters/ad-converters/ad7819/products/product.html
    i don't understand very well its datasheet
    1)acquisition time: is it equal to sampling capacitor charge time plus
    settling time?Is it fixed to 100ns?
    2)In the settling time associated to sampling circuit, which effect
    are considered?
    3)If adc's analog input is connected to opamp's output, what is its
    slew rate?
    In datasheet, Tcharge became significant with R2 (input impedance)
    equal to 2K: in this case Tcharge shoud be equal to 56ns. So, if input
    is equal to Vref, opamp should have Vref/56ns as slew rate....is this
    right?
    4)last question: at the beginning of sampling process, sampling
    capacitor voltage is equal to Vdd/3?If so, why equivalent sampling
    circuit (datasheet, figure 6) has C1 connected to ground?
    Thanks for the help
     
  4. MooseFET

    MooseFET Guest

    It depends a lot on how the sample and hold circuit is done. As the
    internal switch turns off, there is a span of time over which changes
    in the input voltage will effect the held voltage.
    The simplest answer here is "all of them". Many years back, I worked
    on a design where the power supply current changed when the sample and
    hold was tripped. This caused a change in the supply voltage. The
    change in power supply voltage effected things via the power supply
    rejection ratio of an op-amp. We had to consider how much to lower
    the supply impedance to bring this under one LSB of the converter.

    The ADC is likely to contain its own internal buffer. If it does, the
    slew rate is the slower of the two. If not, the op-amps slew rate
    when working in to that much load should dominate.
    No, the slew rate of an op-amp is only the case for the output moving
    due to the inputs changing. It doesn't consider the effects of the
    loading. You need to check the op-amp's bandwidth and open loop
    output impedance.
     
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