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ADS127L01: Register and Capacitor

Yuta Nishie
Genius 3040 points
Part Number: ADS127L01
Other Parts Discussed in Thread: TIDA-01471,

Hi,

I have any questions.

1. Please tell me the reason why R2, R3 and R4 described in "Schematic" of TIDA-01471 are connected.

https://www.ti.com/tool/TIDA-01471

If the reason for this is a damping resistor, please tell me the reason why it is connected to the input side of ADS127L01 (SCLK and DIN, ADC_CS) instead of the output side of the IC.

2. R2 = R3 = R4 = 499Ω, but please tell me the reason why the value is larger than 50Ω.

3. Is it correct to recognize that the role of the capacitor (C18) connected to SCLK is noise suppression?

Best Regards,

Nishie

  • +1
    TI__Mastermind 36240 points

    Hello Nishie,

    RC filters are commonly added to the digital control lines to reduce the rise time at the ADC inputs, which can couple into the ADC inputs and increase the noise floor. 

    Typically, just a resistor is added in the 10ohm to 100ohm range, which coupled with the parasitic pin capacitance of the ADC input pin, creates a low pass filter, reducing the edge rates for the digital signals.  SCLK is typically the most active digital signal, and many times a small amount of additional capacitance is added to further reduce the edge rate of the SCLK signal.

    These resistor values are not added for source termination (50ohm is typically used in this case), which is why the resistor values have such a wide range of values.  499ohm is high, and will definitely limit the maximum SCLK frequency, but in this design, f-SCLK is low enough where the 499ohm series resistor still allows reliable communications.

    1.  Resistors and capacitors are added to the digital input pins to reduce the edge rate of the digital signals, reducing the amount of digital noise coupling into the inputs.

    2.  The resistor values R2, R3, R4 are used to reduce the edge rate of the signals, not for impedance matching.

    3.  C18 adds additional capacitance to further reduce the edge rate of the SCLK signal, further reducing digital noise coupling into the ADC inputs.

    Regards,
    Keith Nicholas
    Precision ADC Applications

  • 0 in reply to Keith Nicholas
    Genius 3040 points

    Hi Keith-san,

    Thank you for your support.

    I will tell our customer about this.

    Best Regards,

    Nishie