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ADS7947: Questions regarding the Vref and need for an op amp when used with an R820T2 RF front end

BIDAULT Hoel
Intellectual 330 points
Part Number: ADS7947
Other Parts Discussed in Thread: OPA365

Hello,

We plan to use the ADS7947 at the IF output of a tuner (R820T2), we want to use it at maximum speed (2MSPS). i have two questions regarding the implementation:

-Is it absolutely required to use an op amp in our case? as of now the differencital output of the R820T2 goes directly to the differencial channel 0 of ADS7947, with one 5R termination resistor on each side and a 470pF capacitor between each side. As per datahseet the R820T IF output is 1.0V Peak to Peak nominal (2.0V P-P max) and has a differential impedance of 2kΩ//5pF .

-which voltage reference level is advised and is a dedicated voltage reference required? I wonder wether we could directly use our existing 1.2V or 2.5V rails (used by the FPGA). The REF and REF_GND decoupling has been done as per datasheet recomendations.

Thank you.

  • 0
    TI__Mastermind 25020 points

    Hello Bidault,

    You will need an op amp. At high sampling rates, the input source usually cannot support the switch cap circuit within the SAR ADC.

    Attached is a TINA file using the OPA365 with a 2V input and ADC sampling at 2MSPS using a simplified version of the ADS7947

    This circuit was based on couple of assumptions, but those were worst case scenarios, thus it should work given your use case.

    As for the reference. I would also suggest using a buffer or reference with an integrated buffer. At high sampling rates, the reference experiences a lot of current demand, yet a steady voltage rail. This can put a large strain on your power rail.

    ADS7947_Input_OPA365 2V Max scale.TSC

    Regards

    Cynthia

  • 0 in reply to Cynthia
    Intellectual 330 points

    Hello,

    Thank you for the return, could you please advise on changes to handle a 3.3V ADC Vref and i.0V PP output at the IF.

  • 0 in reply to BIDAULT Hoel
    TI__Mastermind 25020 points

    Hello,

    Apologies for the late reply.

    The ref6033 is a good choice for reference voltage supply.

    As for how to drive the tuner output, the OPA365 is a good choice, be sure to size the RC correctly.

    I would also to share with you a depository of reference designs that TI has available, I made a quick search for Tuner and came up with some good results. You can continue to narrow it down and find useful information to your use case

    Regards

    Cynthia