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How do ADCs tackle negative value analog input ?

Discussion in 'Electronic Design' started by [email protected], Jan 10, 2013.

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  1. Guest

    Could some electronics guru please shed
    some light on this ? First of all, may
    I point out that my work so far has
    involved either pure analog or pure digital
    circuits, with hardly any analog-digital
    conversion or vice-versa. So my questions
    might sound silly to some experts. Here we
    Suppose I am sampling an audio signal, whose
    raw voltage levels range from +2.5 Volts to -
    2.5 Volts. I understand that a sigma-delta
    encoder could tackle this, but can a flash
    ADC or successive approximation ADC tackle
    this ? If not, is there any viable modification
    so that the last two ADCs can handle it ?
    Thanks in advance for your comments/thoughts
    on this.
  2. Rocky

    Rocky Guest

    Depends on the specific ADC. If it has an input range from +2.5v to -2.5v then it will work fine. If not you will have to scale your signal and possibly add an appropriate offset.
  3. Guest

    Whether a particular ADC chip can handle negative voltages is not a function of the method it uses internally to do the conversion: it is a function of the voltage range of the chip itself. There are a number of SAR and flash ADCs that are designed to work with inputs that swing below digital ground, and there are a number of sigma-delta ADCs (mostly the ones that come attached to microprocessors and less expensive audio ones) with inputs that are constrained to not go below 0V.

    Look at data sheets. If you get onto the websites of the major semiconductor companies (TI and Analog Devices spring to mind) you should find selection guides that list, among other things, the chips' input voltage ranges.
  4. Guest

    Sure. Simple.

    In the case of the SAR, you simply need a bipolar DAC with the
    appropriate encoding in the feedback of the comparator. The sign bit
    is the MSB (tested first).

    A flash ADC would simply have the resistor chain between +2.5V and
    -2.5V, with the comparators tapped off it. The output will be a
    "thermometer code" but it's not a huge deal to convert that to
    whatever code you want (same problem as unipolar, really).
  5. Guest

    Good Lord, you're an asshole, Fields.
    Yes, really. Any second-year student would have studied this sort of
    A CPLD, if I had an idiot boss who asked for such a dumb thing. IT's
    a simple matter of logic. Maybe more than one would want to hand wire
    with 555s (all you know how to use), but simple nonetheless. The
    trivial solution is 250ish 2in muxes; a trivial CPLD or FPGA exercise.
    Kinda a dumb problem though. Not surprising coming from you.
  6. mike

    mike Guest

    First thing is to specify the problem.
    "audio signal" is way to vague.
    If you really mean "audio signal"
    couple it with a capacitor sufficient to
    pass the lowest frequencies and be done with it.

    Even if you did build some kind of DC level shifter,
    you still have the problem of deciding where zero is.

    The solution is easier if you know what you're
  7. mike

    mike Guest

    Well, we now know what YOU mean by audio.
    That may or may not be what the OP means by audio.
    I've seen many an engineer lead himself astray with the words
    "generally regarded". Why not just ASK for actual data?Well, I'd start with a cap and two resistors.
    But that's just me.
    Unless you're the OP, YOUR edification is not what's required here.

    Last thing we need is to jump to conclusion and apply tunnel vision.
    If your problem definition is correct, your solution is overkill.
  8. Uwe Hercksen

    Uwe Hercksen Guest


    audio may be the range from 300 Hz to 3,4 kHz, or from 20 Hz to 16 kHz,
    or 10 Hz to 20 kHz. It depends on telephone voice only or music and the
    desired quality.

  9. Syd Rumpo

    Syd Rumpo Guest

    Probably Mike means a high-pass, with a resistive divider to set the
    mid-point. R1 +Vref to Ain, R2 Ain to -Vref (which may well be Agnd)
    and C1 fron signal to Ain. R1 = R2 with C1 and R1//R2 setting the LF
    rolloff. Don't forget the ADC input impedance if it's unbuffered.

  10. mike

    mike Guest

    Did you just say that the question is unimportant because you have the
    I know a number of product development teams that would be delighted to
    hire that talent.
    You post a lot here. My conclusion is that you're a smart guy and this
    is just an exercise in being contrary. You're going way out of your way
    to be dense.

    There aren't many ways to hook three components together to level-shift an
    AC signal. You could have tried them all in less time than it took you
    to bitch at me.

    The more you know about the requirements, the better chance you have to
    create an optimal solution. Being a hammer can be good...unless you
    mistake a finger nail for a carpentry nail.

    Ability to run a sim has value.
    Knowing what to sim...priceless.
  11. Guest

    I answered the question, moron. All of this stuff is on the web if
    you know how to look for it. I gave a *lot* more information than you
    have. A level shifter (LOL). A typical Fields hack.

    You were the one who put parameters on the project that made it a dumb
    solution. He asked if it could be done. Certainly it can. Question
    asked and answered.
    But that wasn't the question asked, dumbshit.Is the one trick pony getting sensitive to the truth?
    A *really* dumb solution that no one doing this stuff professionally
    would have dared offer.
    You really are proud of your Fields' hairballs, aren't you?
    Because it's *much* easier to do a web search, given the right
    keywords. It took me less than 30 seconds. Here, I know you're a
    hacker, so maybe you can learn something (doubtful).
  12. P E Schoen

    P E Schoen Guest

    wrote in message
    so that the last two ADCs can handle it ?
    If your source impedance is very low (such as the output of an op-amp), it
    is fairly easy to shift the level with two equal value precision resistors,
    one of which goes to the Vref. For instance, I'm using a 5V PIC with a 2.5
    VDC reference, so when the signal is zero, the ADC sees 2.5/2 = 1.25V. At
    the extremes of +/- 2.5V, the ADC sees 2.5V to 0V. Just make sure the
    resistors present an impedance low enough for the ADC sampling to acquire a
    solid reading. A small capacitor helps. I use two 3.01k resistors and a 10
    nF capacitor which is good for most audio signals (I'm reading 60 Hz).

    It is easy to convert the resulting 10 bit unsigned digital result to a
    signed result by subtracting 512 counts. I use this for an instrument that
    calibrates to at least 0.25% which is about the limit of a 10 bit signed
    integer value.

  13. Guest

    Only because you cannot read. ...or understand.
    Another Fields' lie. BTW, how's your twin, DimBulb?
    Level shifter? The whole concept is a hack, here. Where is "zero",

    Another Fields' lie. Keep going...
    That wasn't the question asked. But you don't care. You're only here
    to prove how "smart" you are. Really, all you do is show what a hack
    you are by publishing your Fields brand of hairballs. What a loser.

    Actually, I did. Because you can't read, it's no surprise you don't
    get it.
    Yes. The truth is not an ad hominem. It's the truth. Sorry if it
    hurts your sensitive ego.
    Of course you would. It's obvious that you're not an engineer.
    Question answered.
    Simple. Because you can't read.
    Obviously you don't understand the article. It *is* trivial. A
    little hierarchy and it's a coupe of minutes work.
  14. Jasen Betts

    Jasen Betts Guest

    if 0V is of special signifigance you may need an input of 0V
    or use a bipolar ADC.
  15. josephkk

    josephkk Guest

    Keith, if you were much of a human you would be ashamed to have posted
    nearly pure ad hominems. Have fun with the strawman.

  16. Guest


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