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ADS4149EVM: ADC driving circuit

Part Number: ADS4149EVM
Other Parts Discussed in Thread: ADS4149,

The driving circuit on the ADS4149EVM contains two back-to-back transformers as recommended in the ads4149 datasheet. Looking at the schematic, the input drives the first transformer's (T1) secondary coil, with the center tap connected to ground through a capacitor (C33), rather than driving the primary coil.  Is there a particular reason for this?  Also it appears T1, T2, and T5 have been removed from their package (as opposed to T4) and soldered directly to the board - is that correct? If so, is that to reduce parasitics?



  • Hi,

    it looks like the EVM ships with a Coilcraft transformer for the analog input, similar to what the original bill of materials calls out for the transformer in front of the optional amplifier path.  So no, these are not the insides removed from the housing of an ADT 1-1 package.  It is just the transformers themselves.  T5 is shown in the schematics as a WBC1-1, and that is what it is and what I think T1 and T2 also are.  Only the clock input remained on an ADT transformer.

    The footprint on the circuit board is made to accommodate a number of different transformers or baluns, depending on what type of magnetics we choose to install at a given time.  There is no single choice of transformer that would satisfy all applications.  Some applications might want more high frequency in the passband, others more low frequency, etc.  So we make the EVM to accommodate as wide a variety of transformer as we can.  The footprint has pads for center taps for both primary and secondary, although the Coilcraft that is on the board only has a center tap on one side.  We have an AC cap to ground to help filter out any ripple on the common mode that might be due to phase or amplitude imbalance from the transformer.  In this case, the transformers are installed with the center taps inwards between the two transformers, mirror imaged.  I don't think we want the lead transformer turned the other way from the way we have it if the center tap has an AC path to ground since we are using the lead transformer to do a single ended to differential conversion that has one side of the winding ground as well.  We want the full signal swing across that winding from where the signal comes in to ground, and an AC cap to ground in the middle of that wouldn't work, I think. 


    Richard P.

  • Thank you Richard, that clears it up.

    The design files package I downloaded ( seemed to be the only one available. Is there a more recent one that I missed by any chance, i.e. has the original bill of materials been updated?

    Thanks again!