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Differential ADC inputs

Discussion in 'Electronic Design' started by Chris Holmes, Sep 19, 2004.

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  1. Chris Holmes

    Chris Holmes Guest

    Hi,

    I'm currently designing a simple digital oscilloscope as a hobby
    project and am having some trouble with differential ADCs.

    I picked out an AD9283 50 MSPS converter for the 'scope. It's a 3V
    single supply part that takes a +-512mV differential input. I understand
    that differential inputs are pretty much the standard on fast ADCs in
    order to reduce noise.

    What's confusing me is that the datasheet says the common mode
    voltage can only be +-200mV maximum. I can program the common mode
    voltage on the differential amplfier I'm using as a driver (AD8138), so
    if I keep it simple and use a 0V common mode voltage, then one of the
    differential inputs will go below ground. Is that normal with these
    parts, even with a single supply? I need to have the converter DC
    coupled in order to make a 'scope, but I'm unable to provide adequate
    biasing to keep both inputs positive if the common mode voltage has to
    be less than 200mV.

    I looked around for dual supply ADCs at Analog, and they all seem to
    be slow! What gives? I must be missing something obvious here (being
    somewhat new to differential signaling).

    Thanks for any tips!

    Chris
     
  2. Pooh Bear

    Pooh Bear Guest

    Without looking at the data sheets - something sounds wrong.

    If it's single supply the common mode input clearly can't be negative
    *unless* the'res on-chip negative volts generation !

    Which it isn't.

    Ok - you got me - the analog inputs are pre-biased to approx 1 V by on-chip
    resistors.


    Analog Input
    The analog input to the ADC is fully differential and both inputs
    are internally biased. This allows the most flexible use of ac or dc
    and differential or single-ended input modes. For peak performance
    the inputs are biased at 0.3 × VD. See the specification table for
    allowable common-mode range when dc coupling the input.
    The inputs are also buffered to reduce the load the user needs to
    drive. For best dynamic performance, the impedances at AIN and
    AIN should be matched. The importance of this increases with
    sampling rate and analog input frequency. The nominal input
    range is 1.024 V p-p.


    The driver will need to do level-shifting. I assume that the AD8138/9 does
    this.

    Graham
     
  3. Mac

    Mac Guest

    Chris,

    I read through the datasheet for the ADC. Your analysis seems to be
    correct. Since the maximum allowable common-mode voltage is 0.2 V, and the
    maximum swing is +/- 0.512 V, one of the inputs MUST be driven below
    ground to get a full range swing on the differential input.

    There are a few things you could do that I can think of:

    1) Find another differential ADC where the common mode range is
    greater than or equal to 50% of the maximum differential input
    swing. This would save you from having to drive below ground.

    2) Provide +/- 5V supplies to the amplifier in front of the ADC so that
    you can put the common mode voltage at, say, 0.15 V. (The supplies
    don't have to be +/- 5 V. They could be +/- 3.3 or 2.7 or whatever.)

    3) Create a false ground at some intermediate voltage just for the ADC.
    This way, the amp in front won't have to drive below its ground.

    4) Accept that you will not get a full range swing.

    5) See if you can figure out how to reduce the voltage input range
    so that the maximum code is produced with a smaller swing. Most
    likely you could do this by tweaking VREF in. I didn't see anything
    about this in the datasheet, however. Mabye it's not even be
    possible.

    Good luck!

    --Mac
     
  4. Pooh Bear

    Pooh Bear Guest

    You read wrong.

    The inputs are pre-biased at about 1 Volt above ground.


    Graham
     
  5. Mac

    Mac Guest

    I didn't read wrong.

    Now that I've gone back and looked at the datasheet again, I do see where
    the inputs are pre-biased at 0.3 VDD, but I also see where it (still) says
    that the common-mode voltage range is +/- 0.2 V.

    In my opinion, the author of the data sheet has some explaining to do.

    --Mac
     
  6. Pooh Bear

    Pooh Bear Guest

    Yeah - the common mode range is clearly specified in an ambiguous manner.

    I wonder if they mean that the quiescent state of the Ain inputs can lie within
    Vd/0.33 (+/- 200mV) ?


    Graham
     
  7. Go back to that AD website and look up this chip.
    http://www.analog.com/UploadedFiles/Data_Sheets/67526251AD8138_e.pdf
    GG
     
  8. Mac

    Mac Guest

    Yeah, that's about the only thing that makes sense. If I were doing a
    DC-coupled design with this part, I would get an analog devices
    applications engineer on the phone to clear it all up.

    For an AC-coupled design, it wouldn't really matter, since the inputs
    are biased to a place that makes them happy.

    --Mac
     
  9. Chris Holmes

    Chris Holmes Guest

    Yep, I checked with an applications engineer who confirmed that the
    common mode range is Vd * 0.3 (+/- 200mV).


    Thanks for everyone's help on this!

    Chris
     
  10. Ian Buckner

    Ian Buckner Guest

    Looks pretty clear to me. Figure 2 in the datasheet shows
    that Ain and Ainbar are internally biassed to just over 30%
    of the rail voltage, roughly 1 volt. The schematic for the
    evaluation board on p11 shows the inputs as AC coupled,
    so the bias is set by the internal resistors.
    The +/-200mV CM range simply means that if you want
    to DC couple, overriding the on-chip bias, you need to make
    sure you finish up with 30% of the rail voltage +/-200 mV
    on the inputs for no input signal.

    Regards
    Ian
     
  11. Mac

    Mac Guest

    That has already been established.
    I said, in the message you are replying to, that I wouldn't
    worry about the AC-coupled case.
    While that is true, and has now been confirmed by the OP who contacted an
    applications engineer at AD, it is NOT what the datasheet says.

    When no other reference is specified, voltages are always with respect to
    ground. In my first reading of the datasheet, I should have realized that
    the +/- 200 mV spec was ridiculous, so I concede to being a fool.

    But your interpretation was just a guess. Calling AD was the right thing
    to do in this case, and in any case where the datasheet doesn't make sense.
    --Mac
     
  12. Pooh Bear

    Pooh Bear Guest

    I agree wholeheartedly.

    If you're not sure, you shouldn't ever be afraid to ask ! No-one should ever
    think the worse of 'you' for so doing and 'you' could avoid some embarrassing
    mistakes.

    In this instance - had I been the designer, I would have phoned AD myself to
    confirm my interpretation.

    Graham

    I only wish ( in a current project ) that the manufacturer could actually
    *support* some of the more advanced functions of the device in question !
     
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