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Shielding ADC with High CMRR?

Discussion in 'Electronic Design' started by Darol Klawetter, Dec 18, 2013.

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  1. I'm currently laying out a PCB, and I'm thinking that I don't need to shield my 16-bit ADC because it has a differential analog input with a CMRR of 80 dB. I'm wondering if the CMRR will desensitize the input to any expected EMI, which will be common mode because the E-field across the input pin spacing of 0.5mm should be predominantly uniform at expected frequencies. I don't expect any EMI to exceed -60 dBm so 80 dB of CMRR should bury the EMI into the ADC noise floor. Any thoughts? I suppose one problem could be the frequency-dependent degrading of CMRR, though the ADC datasheet doesn't provide an CMRR vs frequency data.

    Darol Klawetter
  2. Phil Hobbs

    Phil Hobbs Guest

    That last point is highly relevant. If it were as amazing as you want
    it to be, they'd have trumpeted it to the skies.

    Unfortunately, now that datasheets are owned by the marketing
    department, you have to read them like Kremlin communiques.


    Phil Hobbs

    Dr Philip C D Hobbs
    Principal Consultant
    ElectroOptical Innovations LLC
    Optics, Electro-optics, Photonics, Analog Electronics

    160 North State Road #203
    Briarcliff Manor NY 10510

    hobbs at electrooptical dot net
  3. Yeah, I don't know about ADC's, but for instrument amps one finds that the CMRRR drops at ~20dB/decade above some low frequency... I'd guess it follows the open loop gain, but I'm not sure.

    The pickup will also depends on the input impedance. (Your -60dBm number is for a 50 ohm input?)

    George H.
  4. The ADC follows a diff amp, whose input is connected to a balun. I'm planning to shield this input circuitry but was wondering if I really needed to extend the shielding to include the ADC. Based on the responses I've received regarding the frequency-dependent nature of CMRR, I'm now inclined to shield the ADC, but I'll first ask the manufacturer if they can provide more detailed CMRR data.
  5. "common mode GAIN" - yikes!
  6. Phil Hobbs

    Phil Hobbs Guest

    Yup. CMR and PSR are input-referred, so you can easily get gain from
    the rail to the output. Another of those traps for young players.


    Phil Hobbs

    Dr Philip C D Hobbs
    Principal Consultant
    ElectroOptical Innovations LLC
    Optics, Electro-optics, Photonics, Analog Electronics

    160 North State Road #203
    Briarcliff Manor NY 10510

    hobbs at electrooptical dot net
  7. miso

    miso Guest

    CMRR is more a matter of balance (matching) of components, plus knowing
    where to cascode devices to improve the spec.

    Now it sort of looks like it follows open loop gain only because the
    mismatch shows up as a differential signal, which in turn gets amplified
    by the open loop gain.

    The impedance of the current source feeding a long tail pair is a
    classic case where cascoding can improve CMRR simply by not modulating
    the current in the long tail pair with the common mode stimulus.
  8. RobertMacy

    RobertMacy Guest

    A lady application engineer from TI presented a seminar session on
    'proper' PCB layout. She went from terrible, using two sided board to 4
    layer bd, layout bypassing technique etc. each step she'd ask the
    attendees, "Is this enough for my 16 bit ADC?" She stopped at what looked
    like a rather simple 4 layer PCB layout, that was just done well,
    announcing that that layout gave less than 1 LSB noise.

    No mention of shielding over the ADC at all.

    For what it's worth I've seen unshielded 24 bit ADC's mounted INSIDE a PC!
    which demonstrate lower than 1-2LSB's of noise. Tht kind of makes your
    problem seem trivial, eh?
  9. Bill Sloman

    Bill Sloman Guest

    When I was working at Cambridge Instruments in the mid-1980's, we up-gradeda bunch of noise-sensitive two layer boards, that had always been fitted with solid aluminium screens under the track side, to four layer boards, with solid 5V and 0V planes on the two inner layers. The artwork changes were minimal - the +15V and -15V rails were still distributed on the outside layers of the board, though the decoupling capacitors were directly coupled toburied 0V ground plane.

    Not only could we leave off the aluminium screens but the board-to-board shielding without them was better than it had been with the screened two-layer baords. Obvious enough when you think about it - a buried layer is closerto the tracks that it is shielding than a bolted on screening plate, so all the radiating and receiving dipoles are closer together, and the radiator/receiver aerials are correspondingly shorter and less effective.

  10. Guest

    Any kind of modern DA advertised as intended for ADC apps maintains its DC CMRR out to 10MHz, with only negligible degradation of maybe 10-20dB out to1GHz. The problem is the DA is only half the equation, maintaining enough matching of the external analog stuff to any degree of accuracy better thaneven 40dB is a challenge.
  11. Guest

    140dB noise floor? At what frequency (bandwidth)? It's hard to find
    audio DACs are much better than 100dB, and that's before you put them
    in a real circuit.
  12. Massoud

    Massoud Guest

    Sorry for intrusion, I am here to learn. Why your 16 bit ACD has an analog
    input CMRR of 80 dB instead of 96 dB? 80 dB doesn't even reach quantization
    noise of a 14 bit ADC.


    --- news:// - complaints: ---
  13. Because it's low end?

  14. Glen Walpert

    Glen Walpert Guest

    Because the unintentional common mode input is supposed to be much
    smaller than the intended differential input signal, ideally zero, but
    that is not practical so some CMRR is required. To keep the effect of
    common mode below quantization noise here the common mode must be at
    least 16 dB below the differential signal at whatever frequency CMRR is
    specified at, probably more at higher frequencies. A good data sheet
    would have CMMR vs frequency data.

    Presumably Jamie forgot the smiley, sarcasm may not be what you wanted to
    learn about, although this NG is a good place for that :).

  15. miso

    miso Guest

    That is generally due to not balancing parasitic capacitance in the
    layout. Often chips have dummy devices just to keep the capacitance on
    differential nodes balanced so that no common mode signal is induced due
    to mismatch in coupling.

    There are things in the layout that do not show up in the schematic.
    That is what can be tricky in analog design.

    I bet you have seen LDOs with power supply gain too. Lots of junky
    companies out there making cheap CMOS LDOs with no clue about analog
    design. The only thing saving their asses is the capacitance on the LDO
    output hides a lot of the bad engineering.
  16. miso

    miso Guest

    On 12/18/2013 12:41 PM, Darol Klawetter wrote:

    You are far better off measuring the CMRR at high frequency yourself if
    you actually care about it.
  17. Guest

    Sure, but it's assuming a lot to get from 17ish bits S/N to 23 bits
    without some extraordinary measures. He was talking about only having
    better than 1-2 LSBs of noise "inside a PC", so I guess I assumed this
    was a sound card sort of thing. I was asking for more information
    about the application. Seems incredible.
  18. Phil Hobbs

    Phil Hobbs Guest

    Nah, you can probably do it by standing on one leg till your brain goes
    to sleep.


    Phil Hobbs

    Dr Philip C D Hobbs
    Principal Consultant
    ElectroOptical Innovations LLC
    Optics, Electro-optics, Photonics, Analog Electronics

    160 North State Road #203
    Briarcliff Manor NY 10510

    hobbs at electrooptical dot net
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