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ADS62P49: Drive Circuit for High Bandwidth

Part Number: ADS62P49

We have designed and tested a board using the ADS62P49.  We are digitizing data at 200 MHz, and initial testing showed reasonable results.  While integrating this board with our system, we had some issues that caused us to look more closely at the input impedance of the the ADC drive circuit.  Our design attempted to copy the drive circuit shown in Figure 103 of the datasheet.  Unfortunately, we used 75 ohm 1:1 transformers on our board instead of 50 ohm 1:1 transformers.  We would like to change to 50 ohm 1:1 transformers, but the closest pin-compatible part we could find is 50 ohm 1:1.5.  Since the secondaries are wired together in the example drive circuit from Figure 103, would there be any issue using two 50 ohm 1:1.5 transformers?

  • Rafe,

    This should work as long as the secondary's are connected to each other. If you have the transformers used in a back-to-back configuration (as shown in the figure that you are referencing), I don't think this will be an issue since the impedance transforms back to its original impedance (1:1.5 || 1.5:1 ).

    Also, the difference between a 50 Ohm and 75 Ohm transformer is negligible enough not to matter. You could probably use the 75 Ohm one and not notice much difference along as the bandwidth of the transformer is wide enough for your application.

    Regards,

    Jim

  • Jim,

    Thank you for the response.  I forgot about one followup question.  The example drive circuit from Figure 103 appears to have a signal inversion in it.  Can you confirm?

    Thanks,

    Rafe

  • Rafe,

    There is not an inversion.

    Regards,

    Jim