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ADS42JB49 imput impedance

Russell Wetherley1
Expert 4255 points
Other Parts Discussed in Thread: ADS42JB49

We are looking at using the ADS42JB49 ADC. We have a 1.5 volt input source and are looking for a solution to drive the 2.5 volt input of the ADS42JB49.

One possibility is to use a matching transformer. Is any information available on  the input impedance characteristics of the ADS42JB49?

I saw this post promoting the IBIS model but am wondering if you just have the raw data?

https://e2e.ti.com/support/data_converters/high_speed_data_converters/f/68/t/378534

Thank you and have a nice weekend!

Russell

  • 0
    TI__Genius 9695 points

    Hi Russell,

    Figures 112 and 113 in datasheet gives the input impedance information that you are looking for. Figure 111 in datasheet also gives the equivalent input impedance model that you can use in your simulations.

    Thanks,

    Eben.

  • 0 in reply to Ebenezer Dwobeng
    TI__Expert 4255 points
    Great! Thank you Ebenezer. I have a few additional questions.

    Figure 114 in the datasheet shows a recommended input circuit for input frequencies up to 250 MHz with the following explanation:

    “The mismatch in the transformer parasitic capacitance (between the windings) results in degraded even-order harmonic performance. Connecting two identical RF transformers back-to-back helps minimize this mismatch and good performance is obtained for high-frequency input signals.”

    I am trying to understand this circuit.

    1) Is the distortion primarily coming from:
    a) the interaction of the transformer ferrites and the non-symmetrical coupling of the single ended input signal through the interwinding capacitance of the first transformer?
    b) or is it from the interaction of the ads42jb49 input with the non-symmetrical coupling of the single ended input signal through the interwinding capacitance of the transformers?
    2) If the answer is a) then is the circuit supposed to counteract the effect of the single ended coupling across the first transformer with the single ended residual coupling across the second transformer which is wired out of phase to the first?
    3) If so, it would seem that the intermediate circuit between the two transformers should float.
    4) If the distortion is from the ads42jb49 input, then it would seem that AC grounding the inter transformer node at the centertap would be a good thing.

    We are considering using a 1 to 1 transformer for the first transformer and a 3:5 (turns ratio – not impedance ratio) transformer for the second.

    Thank you for your help!

    Russell
  • 0 in reply to Russell Wetherley1
    TI__Genius 9695 points

    Hi Russell,

    - The distortion is due to non-linearity in the signal path caused by combined effect of the transformer and internal circuit of ADC.

    - However, because HD2 is even order harmonic, it should perfectly cancel out in the ideal case if the signal is truly differential.

    - Using two identical transformers back to back helps to reduce the phase/amplitude imbalance characteristic of the transformer.

    - This helps to maintain a truly differential signal to the ADC to help cancel out all even order harmonics including HD2

    Thanks,

    Eben.