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Impedance matching of op-drived path in ADS42XX EVM

rongjun Lu
Prodigy 140 points


In ADS42XX_58C28EVM-SCH_C.pdf(p.2), there is an op-driver path to the ADC. I have two questions about this path: 1) R36(R18) is 68ohm, how does this value match the input impedance of ADC (R5 and R6, 25ohm)?Should the differntial line pairs be layouted as 50ohm diffrential transmition lines?  2) If DC coupling is desired, C13(C66) will be replced by 0ohm resistor, Is it neceessary  to apply the CM to R5 and R6? how to re-calculate the impedance matching then?

Tahnks a lots.

  • 0
    TI__Genius 17360 points

    Hi Rongjun,

    I'm not sure where these values came from. My guess is that if the amp is used, then the 25 Ω termination resistors should be removed, otherwise there will be quite a bit of loss in this signal. You could use 50 ohm series resistors at the amplifier output and 50 ohm termination resistors at the ADC to create a 100 Ω differential impedance match. In this case, the traces should be 50Ω signal-ended traces or 100Ω differential traces. If you do not have matched impedances, then you can't define the trace impedance.

    For DC coupling, you'll need to consult the amplifier team through their E2E forum. The CM input would need to be used to match the common-mode voltage of the ADC, which is about 0.95 V. The amplifier will have trouble with an output common-mode of 0.95V using typical GND to 5V supplies. In this case, it is common to offset the supplies for instance VCC- is -1.5V and VCC+ is 3.5, to place the output common-mode in the middle of the supply voltage range.

    Regards,
    Matt Guibord