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VCM in ADS62P49

Gonzalo Asencio
Prodigy 110 points
Other Parts Discussed in Thread: ADS62P49

Hello,

I'm just designing an ADC board based on the ADS62P49. It's going to work with 250MSPS with the two channel. The previous stage is a DGA that cames from a 2.4 GHz Front-End. The DGA has a differential output and fix the common mode voltage in a range from 1.6 to 2V.

For that reason I think that, in the ADC, I would need to fix the CM externaly taking the voltage from the previous stage. The problem is that if I want to configurate the ADC in serial mode way, at the beginning, before write the external mode value in the corresponding register,  it may be configured with an internal reference and in the CM will be two references at the same time trying to fix it's value.

¿Is it really how I think or I can fix an external reference and configurate it in serial mode without any problem?

¿There is any example which uses the external CM reference?

 

Thank you very much,

  • 0
    TI__Guru* 79304 points

    Hello,

    Even though the ADS62P49 can be configured for external reference mode, the reference voltage still needs to be 1.5V+/-0.05V. Please refer to page 62 of the datasheet. Therefore, the output of the DGA may require level translation to shift the common mode to 1.5V in order to interface to the ADC input correctly.

    If the application does not require direct DC interface, AC coupling may be used to facilitate the interface between the two devices. 

    -KH

  • 0 in reply to Kang Hsia
    Prodigy 110 points

    Thank you very much for your answer.

    I was confused becouse in the page 7 it says as typical 1.5V but it doesn't add any máximum or minimum value and in the page 3 it says that the máximum rates for the VCM in external mode is between -0.3 and 2V and in the graphs (like for example in page 43m fig. 37) is shown like a variation of VCM between 1.3 and 1.7.

    So it wasn't clear for me if I could change the VCM from 1.5V. Thank you very much in advance for your answer!

    Best Regards,