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ADS58B18EVM input signal conditions

Lorenzo Columbo
Prodigy 70 points
Other Parts Discussed in Thread: ADS58B18EVM, THS4509

I have to convert a fast repetitive step-like signal applied as an input to the ADS58B18EVM in single-ended mode (through the already installed J6 SMA connector).

My first question is: what output impedance my signal source should have to match TI's EVM input impedance? I've looked to the schematics and BOM and I'm a bit confused about why J6 SMA connector has an impedance of 50 ohms, while ADT1-1WT input transformer has a characteristic impedance of 75 ohms, so I couldn't find out this information by myself.
Moreover, I couldn't find on the board user manual informations about the two DNI resistors R22 and R25: what is their function? In which case should I install them? 

I've got also another question: how high could my input frequency be for my signal to be not distorted before coming to the ADC input pins? If I needed to DC couple my input signal on J6 connector to the ADC input pins (IN_P and IN_M on the board schematics), could I simply uninstall the two transformers T1 ad T2 or what issues should I care about? What would you suggest?

Thank you in advance for any help you will give me.

Regards,
Lorenzo

  • 0
    Prodigy 70 points 
    Are my questions confused? Should I explain them better?
    Any help would be greatly appreciated.

    Hopeful regards,
    Lorenzo
  • 0
    TI__Genius 17340 points

    Lorenzo,

    Sorry for the delayed response. When you say "single-ended mode" are you referring to the signal applied to the EVM as single-ended or to the ADC. Keep in mind that there are back to back transformers on the board that convert from single-ended to differential for the ADC.

    Your signal source should be 50 ohms. The ADT1-1WT is specified as a 75 ohm transformer, but what this really means is that the specs in the datasheet were taken in a 75 ohm system (75 ohm source and 75 ohm load), but from our experience it still works well at 50 ohms. At our frequencies a transformer can mostly be considered a lumped element.

    You should never need to install resistors R22 and R25. This was a legacy thing that we've carried forward on quite a few boards. Most of our new boards will not have these placeholders anymore.

    If you want the signal to be DC coupled, you would need to remove any transformers as well as any DC blocking caps. You would then need to use an amplifier to convert from single-ended to differential and make sure that the common mode level is at the corrent voltage. Most highspeed amps have an output common-mode pin that can be tied to the ADCs VCM pin to set the correct common-mode level. There is an amplifier on the board (THS4509) but it is also AC coupled through a transformer. I believe it can be made to be DC coupled, but I'm not sure what might be involved. If you can supply a differential signal to the board through J6 and J8 at the correct common-mode level, then it should be easy to do. If you have a single-ended signal, the amp will need to be setup to do a single-ended to differential conversion.

    Regards,
    Matt Guibord