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ADS8638: ADS8638 SPI Signals termination

LITHESH KANNA
Prodigy 20 points
Part Number: ADS8638

Hi,

I am using ADS8638 in my design. I would like to know what termination resistor value need to be placed in the SPI lines and how to calculate the termination resistor values for the SPI lines.

In Datasheet, it is mentioned 50Ω source series termination resistor shown on the DOUT signal. In ADS8638 Ref Design it is mentioned 4.99Ω in DIN,DOUT,CLK,CS lines 

Thanks in Advance

Regards,

Lithesh

  • 0
    TI__Guru 62895 points

    Hello,

    The purpose of the resistors is to provide a known series impedance which will interact with the trace capacitance to help slow down the device rise/fall times which in turn reduces ringing, reflections, and radiated emissions.  The value is not always calculated and most designs employ values between 10Ohms and 49Ohms, but 5Ohms would likely work as well.  The value should not be so large that when interacting with the expected trace capacitance that the signal BW (1/(2*pi*R*C) does not negatively interfere with desired communications frequencies.