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Why use amplifier with adc?

Discussion in 'Electronic Basics' started by [email protected], Oct 21, 2008.

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  1. Guest

    I don't know very well how adc works, so i have a question. Most adc
    have a Vref pin that defines Vin range of adc. Vin range of adc is
    then divider for 2^n (n=resolution) to obtain LSB. If i resize Vref,
    new Vin range (smaller) is divided for 2^n, so LSB has smaller
    value.If i have a sensor, why i shoud use amplifier to drive Vin pin
    of adc?can i resize Vref as i like (or there is a limit)?
     
  2. The larger the voltage the lower the noise floor(relative).

    A simple extreme case to show how it works: Suppose you scale the signal
    with the amplifier to 1/10000mV and set the adc. Do you think it would work?
    You think it would be accurate? Surely there is a minimum voltage that the
    adc can "work" at?

    The same idea exists in communication lines. Suppose you just send your
    signal along the line with X volts peak. Any noise on the line will distort
    the signal making it impossible(or at least difficult) to distinguish what
    is noise and what is the signal. I.e., if the noise floor is Y volts then
    when the signal is below Y volts it is impossible to tell the signal from
    noise.

    But if we first scale up the signal before transmission then we decrease the
    effects of the noise on the signal.

    Mathematically we might have somethign like s(t) = A*f(t) + e(t)

    where e(t) is the noise, A is an amplification factor, and f(t) is the
    signal pre transmission.

    s(t) is the transmitted signal but before de-amplication,

    After we de-amplify on the other side we have

    B*s(t) = B*A*f(t) + B*e(t)

    in general B = 1/A so the final signal is

    f(t) + e(t)/A

    So the point being that if we first amplify the signal then transmitt and
    then deamplify we end up lowering the noise floor by that amplification
    factor.


    It's a similar idea with the ADC. The pre-amp increases the signal so any
    "noise" by the op amp is reduced. Since we don't have to de-amplify because
    it is digital after that and we can just scale it mathematically without any
    loss.

    Note that in reality it's not quite analogous because noise is usually
    "external" in communications lines while for ADC's it is a propery of the
    design and non-ideal components used.

    The main point is that by "scaling" the signal first you reduce the effects
    of any "noise" or errors because they are not scaled. In some sense the ADC
    has an easier time to distinguish the signal over the noise because you've
    made the signal larger.

    A very simple analogy is a magnifying glass. In some sense your eyes when
    reading a ruler is like an ADC. If the notches are too close then any
    "error"(blur, say) can easily throw off the results. But if you first
    magnify then read it is much easier because the "error" doesn't scale.
    (although this is not to say that the magnifying glass doesn't introduce
    it's own error)
     
  3. Guest

    You can use Vref to scale the overall resolution into less range.

    Instead of say having 1000 5mv steps, hook Vref to 3.3 volts and you now have
    1000 3.3mv steps - up to 3.3 Volts.
     
  4. Guest

    On another aspect of this same subject, I have a resistive sensor that I can get to put out 3 to 4 volts of range. What type of signal conditioner or amplifier
    will allow me to change the 3-4 volts into say 0-3.3 volts?
     
  5. whit3rd

    whit3rd Guest

    The best answer, is a Wheatstone bridge. That's because you presumably
    have to excite the resistive sensor, and if your ADC uses the second
    leg of a bridge for its reference voltage, the pure resistor ratio is what
    gets measured (independent, or nearly independent, of the available
    exciting voltage source noise and drift).
     
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