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ADS42LB49: analog interface DC couple

jung fang
Prodigy 80 points
Part Number: ADS42LB49
Other Parts Discussed in Thread: LMH5401, LMH6552

hi expert,

i'm using ADS42LB49 to sample high speed ac signal,  but at the same time need to sample dc voltage combined with high speed ac signal.

from datasheet, it recommends to use ac couple in ADC's differential analog input.

i want to know whether this ADC can sample DC voltage, as DC couple may change common voltage . datasheet shows VCM typical is 1.9V.

is there any recommended design for this ADC's differential analog input working in DC couple mode.

thanks very much!

Br

jung

  • 0
    TI__Mastermind 26730 points

    Hi Jung,

    Yes, you can DC couple with the ADS42LB49. I would recommend using the LMH5401 for the interface. 

    If you want I can forward you to the amplifier group and they can help you work thru any recommendations there.

    Regards,

    Rob

  • 0 in reply to Rob Reeder
    Prodigy 80 points

    hi Rob

    thanks very much。pls forward this DC couple question to amplifier group.

  • 0 in reply to jung fang
    TI__Mastermind 25505 points

    Hello Jung,

      You can use a fully differential amplifier (FDA) to pass both DC and AC signals to the ADC. FDAs make it easy to interface with a differential input ADC, and you can drive the FDA's VCM pin from the ADC reference + buffer. 

      What is your frequency range and gain requirements?

    Thank you,
    Sima 

  • 0 in reply to Sima Jalaleddine
    Prodigy 80 points

    hi sima,

    frequency range is DC to 500Mhz, and I'm using a FDA LMH6552.

    DC couple schematic as follows.    U1 is LMH6552, U2 is ADS42LB49.

    1/ when in DC couple mode, U1's VCM pin must connected to U2's VCM ouput , right? a buffer between U1 VCM and U2 VCM is much better ?  When float U1's VCM pin , i measured common voltage in R9.R10 is 0V.

    2/ ADS42LB49's datasheet recommend R3/4/5/6=25ohm, and U1's load resistor is 25*4=100ohm.

    U2 max input voltage range is 2Vpp, so U1 need output 4Vpp and load current is 4Vpp/100ohm=40mA.

    to LMH6552, 100 ohm load maybe to small? as datasheet shows LMH6552's performance tested under 800ohm load and 100ohm load maybe degrade it's some performance?   

  • 0 in reply to jung fang
    Prodigy 80 points

    hi Rob, 

    DC couple schematic as follows.    U1 is LMH6552, U2 is ADS42LB49. as U1 has worse THD performance under load=100ohm.

    can below R3/4/5/6 change to 50ohm or 75ohm or 100ohm , any impact to U2's performance? maybe bandwidth? i need U2 can sample signal above 250Mhz.

    thanks!

  • +1 in reply to jung fang
    TI__Mastermind 25505 points

    Hello Jung,

      Correct you can drive the VCM pin from U2's VCM output. A buffer might be needed if the current drive is not enough. Usually, you could float the VCM of the FDA, and it will default to mid-supply, but for the LMH6552, this pin must be driven by a low impedance reference and must be bypassed to ground with a 0.1 µF ceramic capacitor. Any unwanted signal coupling into the VCM pin is passed along to the outputs, reducing the performance of the amplifier. So, this pin must not be left floating. 

       You can increase the load to improve performance if the design can handle the drop in output voltage. You probably want to switch over to a faster ADC since your operating frequency is above 250MHz. 

        I am moving this thread back to ADC team for their inputs.

    Thank you,

    Sima 

  • +1 in reply to Sima Jalaleddine
    TI__Mastermind 26730 points

    Hi Jung,

    As long as the output common mode of the LMH6552 can operate at 1.9V, the ADC's input common mode, this circuit should be fine.

    The are other ADC with higher input BW, but if you need to capture a signal at 500MHz, this should work.

    R25/26 is 25ohms each. Is there a reason for this? I would set them 500ohms each. This will match the load from the ADC's internal input impedance to what the LMH6552 desirable output load resistance that was used to characterize the part.

    Regards,

    Rob