AD9680 DC coupling (DAQ3)

Hi

I want to use the AD9680 for DC coupling in FMCDAQ3,  but I learnt that the interface of ad9680 in the DAQ3 is AD couple.

Can i move these Baluns and capacitances, and short the selected pins?  After that, i want to connet the singled-ended signal to the SMA. Is this way pseudo differential?  

I learnt that the Vcm of AD9680 is 2.05V, but the offset of my singled-ended signal is 0V.  After moving the Balun, Could I capture the singal accurately?If not, what should i do to meet the requirement of DC coupling?

  • 0
    •  Analog Employees 
    on Oct 14, 2020 1:12 PM 1 month ago

    Hi, 

    The Datasheet page 37 shows the following: Input Common Mode The analog inputs of the AD9680 are internally biased to the common mode as shown in Figure 116. The common-mode buffer has a limited range in that the performance suffers greatly if the common-mode voltage drops by more than 100 mV. Therefore, in dc-coupled applications, set the common-mode voltage to 2.05 V, ±100 mV to ensure proper ADC operation. The full-scale voltage setting must be at a 1.7 V p-p differential if running in a dc-coupled application.

    So if you want to try that, then you can eliminate the AC path and short the inductors as you show.

    But make sure that the input common mode of the ADC is not violated. 

    Thanks

    Charly

  • Thank you Charly

    Is that means that i should adjust the offset of analog input to 2.05V?If not, the ADC may not work properly.

    After eliminating the AC path, shorting the inductors and adjusting the offset of analog input, could i connect the single-ended signal to the SMA of DAQ3? 

    The analog input interface of AD9680 is differential. I'm not sure if connecting the VIN_N to GND has any effect.  Maybe no effect?

    Thanks

  • 0
    •  Analog Employees 
    on Oct 14, 2020 7:41 PM 1 month ago

    Can you feed in a signal differentially? maybe use a differential amplifier eval board to drive this board? 

    There will be an effect on the performance as this ADC is not pseudo diff, it is full diff input.

    Charly

  • Thank you Charly

    I learnt from the FMCDAQ3 introduction that the input of the FMCDAQ3 only has one SMA of each chanal. Even if i modify the circuit, it also cannot meet the requirements of ADC fully differential input.

    Is that means i cannot use the DAQ3 to capture signal in DC coupling?  Does ADI have any suggestions to help me to meet the needs that DC coupling in DAQ3?

    Thanks

  • +1
    •  Analog Employees 
    on Oct 15, 2020 12:16 PM 1 month ago in reply to xiasigang

    That is why I asked you if you can use a differential amplifier to go from single-ended to differential and use that diff signal at the differential side of the transformer. 

    In a way, you replace the transformer with a single-ended to a differential amplifier that can handle DC.

    Is this something you can manage to do?

    Charly