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adc12d1800 analog input balun layout issue

Randy Mummert
Prodigy 95 points

I'm designing the ADC12D1800RFIUT analog inputs. I've decided that I'm using a balun input device from Anaren, part number B0430J50100AHF. This balun has a balanced output and seems like a good device for my application. The issue is that the ADC analog inputs for the positive and negative input pins are swapped at the ADC device. Reference input pins H1=VinIP, J1-VinIN, N1=VinQP and MVinQN.

N1 - VINQP

M1- VINQN

J1 - VININ

H1 - VINIP

When performing the PWB layout the issue is that since the ADC inputs P and N and essentially swapped for the I and Q inputs and the balun interface crosses for one of the inputs. I belive that pin 6 on the balun is the high side and pin 4 is the low side of the balun outputs. I'm attempting to keep the analog input layout from the balun to the ADC identical but if I need to add vias to one balun output interface to the ADC then the circuitry is different. Can you suggest a layout that works with the Anaren baluns?

Thans,

Randy

  • 0
    Prodigy 95 points

    I found a TI app note that explains my predicament.

    4276.TI GSPS ADC Driving for High Bandwidth Applications[1].pdf

    I now see I need to add two more baluns and some caps in order to phase the inputs properly to the I and Q inputs.

    Please consider this request closed.

    Randy

  • 0 in reply to Randy Mummert
    TI__Mastermind 44680 points

    Hi Randy

    I'm glad that app note was helpful. Can you share what mode of operation you are using the ADC in (non-DES, DESI, DESQ, DESIQ, etc.)? The recommended input circuitry can change depending on the mode of operation and I would like to make sure you have considered those tradeoffs.

    Best regards,

    Jim B

  • 0 in reply to Randy Mummert
    TI__Expert 3940 points

    Hi Randy,

    Depending upon what mode you are driving, there are different front end configurations which may be useful.  See the attached presentation for some examples and recommendations.

    Kind regards,

    Marjorie

    3051.TI GSPS ADC Driving for High Bandwidth Applications Nov 2012.pdf

  • 0 in reply to Marjorie Plisch
    Prodigy 95 points

    My application is Non-DES so I need both inputs to be independant. It seems I'll have a cross over at the inputs due to the ADC inputs be flip flopped. I did contact Anaran to see if they offered another balun with a phase inversion, they don't.

  • 0 in reply to Randy Mummert
    TI__Expert 3940 points

    Hi Randy,

    Please check the Apps Note again - we actually don't recommend doing this:

    "If the negative input is not available, such as in the case of many baluns, the simple brute force way to

    swap the polarity is to achieve this via the routing. However, crossing the positive (+) and negative (–)

    input signals requires the use of a different layer, which requires the use of vias. This results in an

    impedance mismatch at one channel, as well as a mismatch between channels and is not recommended.

    A cleaner solution is to invert the digital signal of one channel once it has been captured by the FPGA (or

    ASIC). Note that the ability to invert one channel is not a feature on the ADC."

     

    Kind regards,

    Marjorie