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ADS5403: Analog input impedance/balun

Krzysztof Kaczmarczyk
Prodigy 10 points
Part Number: ADS5403
Other Parts Discussed in Thread: ADS5402EVM,

Hi,

 I'm trying to design device with ADC ADS5403 but I have problem with properer driving analog inputs. As a reference I use ADS5402EVM.
There are two baluns ETC1-1-13 (T3, T4) but I don't know if you use controlled impedance (for example 50 ohm). On your reference schematic it isn't marked.
Do you have any suggestion how to prepare a proper layout for it ?
Should I use differential line 100 ohm after second balun (T4) ? 
My second question is: Why your footprint for ETC1-1-13 is different than suggested by manufacturer? 

  • 0
    TI__Genius 13471 points

    Hi Krzysztof,

    Yes, you will want to use 50 ohm impedance. Additionally, this board has a double-balun configuration, which is why it is different than the manufacturer suggestion.

    Regards, Amy

  • 0 in reply to Amy
    TI__Mastermind 24950 points

    Hi Krzysztof,

    Additionally, the traces can only 50ohm Microstrips on either side of the differential, as there is no way to closely couple them to make the trace impedance differential.

    On the balun footprint, this footprint is used to accommodate multiple baluns and transformer part numbers that were popular to use back when this ADC was released at that time. Hence the larger footprint style that doesn't match the ETC1-1-13.

    Regards,

    Rob

  • 0 in reply to Rob Reeder
    Prodigy 10 points

    Thank you for your fast response.

    You wrote that there is 50ohm microstrips before and beetwen baluns, but after second balun line are closely couple (near C9, R5 and R13). Is this differenital 100 ohm ?
    Could you confirm picture belowe?

  • 0 in reply to Krzysztof Kaczmarczyk
    TI__Mastermind 24950 points

    Hi Krzysztof,

    Yes, this is tightly coupled 100ohm differential traces. I would only recommend doing this if the interface is far away from the analog input pins of the ADC.

    Otherwise, you can use 50ohm microstrips traces after the balun to the ADC.

    Regards,

    Rob