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THS3217EVM: What is the significance of the extra 10kohm or 20kohm resistor between the test point and trace？

Zenan Shi
Prodigy 20 points
Part Number: THS3217EVM
Other Parts Discussed in Thread: THS3215EVM, , THS3217

Hello, engineers:
I would like to ask a question, in the application THS3215EVM and THS3217EVM (SBOU161A), I see a lot of test points are not directly added to the trace, but connected to the trace through 10kohm or 20kohm resistors. What is the significance of doing this?
I found in the test, when testing a 1kHz sine signal with THS3217 amplification, if the signal is not directly measured through the resistor (ordinary single-ended probe directly touching the pads), although you can also see the shape of the sinusoid, but the signal appears to be banded, amplifying the signal to see a lot of oscillation. While the signal is completely normal when tested through the test point. Is this related to the resistance attached to this test point? Or is it a problem related to my layout design?
Looking forward to your reply!

  • +1
    TI__Genius 9800 points

    Hi Zenan,

    The resistor is likely isolating the circuit from any parasitic capacitance effects from probing the device. By measuring directly on an input or output you could be adding parasitic capacitance resulting in instability especially with high bandwidth devices like the THS3217. There is a Ti Precision Labs video that covers this idea. The video series can be found here, and the video relating to this isolation idea can be found under stability and is called isolation resistor.

    Best Regards,

    Ignacio

  • 0 in reply to Ignacio Vazquez Lam1
    Prodigy 20 points

    Thank you very much. I will watch these videos.