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ADS4225: Output impedance of DA[0..11] and DB[0..11] when they are used in CMOS Mode

Part Number: ADS4225
Other Parts Discussed in Thread: SN74AVC16T245,

I want to use ADS4225 with 120MHz sampling rate and CMOS Mode. I'm afraid of output impedance mis-match of data bus. It is not easy to use it with LVDS Mode. Therefore I'm considering impedance matching by using external resistors with CMOS Mode. However there are no descriptions about output impedance of data bus on the data sheet of the device. Does it make sense to consider the impedance matching with CMOS Mode? Or do you think I'd better not use CMOS Mode with 120MHz sampling rate? I also think that I should shorten the data line on my PCB design as much as possible without considering impedance matching.  Would you please tell me your opinions?

  • Hi User5059038,

    We are looking into your question and will get back to you soon.

    Regards,
    Neeraj
  • User,

    CMOS at 120MHz should not be a problem if you use the following guidelines:  Route the single-ended signals as 50 Ohms impedance traces with a series dampening resistor (22 Ohms) placed as close as possible to the ADC. I would suggest avoiding long parallel traces, try to provide  some separation between traces to avoid crosstalk, keep the traces matched length, and short as possible. If you need to use long traces, I would suggest you use a buffer, such as the SN74AVC16T245, which is used on the ADC4225EVM. See the attached file for more tips and guidelines.

    Regards,

    Jim

    CMOS Schematic and Layout tips.ppt

  • Dear Jim,

    Thank you for your kind reply. I think your suggestion is very useful. But I slightly feel strange only about your comment that I should route the single-ended signals as 50 Ohms impedance traces with a series dampening resistor (22 Ohms) . Do you mean that the output impedance is about 28 (= 50 - 22)Ohms? I think the value means the output driver has capability of sourcing 64mA current when the digital supply voltage is 1.8V. That impedance value is too small! Would you please point me out if i am not right?

    Regards,

  • User,

    This does not mean the output impedance will be as you suggested. This is a transmission line impedance, not a load impedance. The 22 Ohms was just a suggestion. In many cases, this is not needed or a lower value like 10 Ohms would work just fine. The main concern is how long will your traces be and what is the load of the device you are driving. This will determine if you should use a buffer or not. If you go the ADS4225EVM product folder, you can download the board design package which may provide you with more suggestions for your design.

    Regards,

    Jim

  • Dear Jim,

    Thank you very much for your comprehensive explanation.  I understood that the important thing is to shorten traces between ADS4225 and the receiving devices by placing ADS4225 in contiguity with these devices. I will be able to place them proximately, then I will take care of cross-talk by considering the traces.

    I appreciate your replies.

    Sincerely.