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10-Ohm Series Reisistor on ADS5474 EVM

Tuan To
Prodigy 210 points
Other Parts Discussed in Thread: ADS5440, ADS5400, ADS5474

On ADS5440/44/63/74 EVM card, there are two 10-ohm series resistors R19 and R20. What is the function of these series resistors? Are they just series terminations? Refer to TI User's Guide SLAU194A, revision November 2006, page 19 for schematic of these resistors.

Tuan.

  • 0
    TI__Mastermind 29220 points

    Hi,

    On many of our EVMs with CMOS un-buffered inputs we will often have a small series resistor at the input and maybe also a differential R-C-R filter to help remove some of the kickback noise from the input sampling circuit.  The series resistors also help provide some isolation to keep the kickback noise contained without dropping too much of the input signal.  But the ADS5474 has a buffered input, so kickback noise is not an issue.  So i would say the 10 ohm series resistor is not necessary.  For example, our ADS5400 also is an ADC with a buffered input, and we did not put series 10 ohm resistors on that input. 

    i don't think this is part of the termination circuit, unless we are considering very high input frequencies where the input impedance of the ADC itself begins to be a concern.  But with the input circuit of the ADC presenting about 250 ohms (figure 48 of the datasheet) an extra 10 ohms on each side would not have much effect at lower input frequencies.  But the approximately 2pF of parasitics in the ADC input circuit might begin to look like a lower input impedance at very high input frequencies, so maybe the series 10 ohms might begin to keep the ADC input impedance from throwing off the proper termination too much.

    Regards,

    Richard P