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High Impedance node noise pick up

Dear Team,

I have read in many places "Noise pick up" in high impedance node will be high. For example in this document(https://e2e.ti.com/blogs_/archives/b/thesignal/archive/2012/04/11/input-bias-current-cancelation-resistors-do-you-really-need-them)

and also in many other places.

But nobody explains how noise pick up is happening in a high impedance node.

Could you please explain how noise pick up is happening in a high impedance node. May I know the physics behind it.

Regards

Hari 

  • Hi Hari,

    noise pick usually takes place via stray capacitance. In the real world every object is connected to any other object by an individual stray capacitance. This can be small, if the two objects are small and/or far away from each other, or it can be large, if the objects are large and/or close to each other. As a very rough estimation of stray capacitance the formula of the plate capacitor can be taken:

    C = er x 8.85 x 10^-12 x A / d

    As consequence, often voltage dividers are formed by these stray capacitances with ground (soil) as common reference. Imagine, for instance, that one object is a conductor carrying 230V mains voltage and the other object is a high impedance node in your circuit, then this equivalent circuit might be taken into consideration:

    Making the high impedance node low ohmic will help:

    But the best cure against stray coupling is consequent shielding and grounding:

    Kai

  • Hi Kai,

    Thank you very much.

    The above circuit is LPF, it will not allow any high-frequency noise.

    Will there be any inductive noise coupling mechanism in high impedance nodes.

    Regards

    Hari

  • Hi Hari,

    The above is HPF, see the simulation below. 

    Noises can be divided into intrinsic and extrinsic noise categories. In the TI Op Amp training video below, it talked about intrinsic noises affect op amp extensively. 

    https://training.ti.com/ti-precision-labs-op-amps-noise-1?context=1139747-1139745-14685-1138803-13232

    If you are talking about extrinsic noises coupling onto high impedance nodes, we are still able to summarize into 3 noises category,  namely induced voltage noise, induced current noise and 1/f noises. 

    In the circuit above, Kai demonstrated how capacitive coupling through high impedance node can be significant. It is a part of CdV/dt coupling, when changes in dV/dt through conductor's insulation material to a high impedance node. 

    In inductive noise coupling, the induced voltage is Ldi/dt, when external environment changes in di/dt. This is likely happened near high current or ground loop test environment.   

    If we consider the noise couplings in terms of noise power, when a small percentage of external power is coupled or "absorb" onto an EUT.  For a given amount of noise power, high impedance node will generate larger voltage noise, Vnoise = sqrt(Pnoise*Z) than lower impedance node. 

    Best,

    Raymond

  • Hi Hari,

    the circuit shown in my simulation is no low pass filter, it's a high pass filter. See also Raymonds simulation.

    Inductive noise coupling happens whenever conductors form a loop and a magnetic field (from a mains transformer, e.g.) is penetrating this loop area. Then, a voltage is induced in this loop and a current will flow in this loop which causes voltage drops across impedances in this loop.

    The best cure against inductive noise coupling is to minimize the loop area formed by any signal or supply current. Because of this, it's always necessary to keep the conductor in which the ground return current flows close nearby. No signal or supply conductor must be routed without having the conductor of ground return current close nearby.

    Kai 

  • Hi Kai & Raymond,

    Thank you very much.

    "Inductive noise coupling happens whenever conductors form a loop and a magnetic field (from a mains transformer, e.g.) is penetrating this loop area. Then, a voltage is induced in this loop and a current will flow in this loop which causes voltage drops across impedances in this loop."

    The above CKT(HPF) is forming a closed-loop ?. The" magnetic field is penetrating this loop" which means the magnetic field produced in the nearby circuits due to change in current?

    Regards

    Hari

  • Hi Hari,

    Do you still have questions? I noticed that you have three "? " in your last paragraph. 

    Kai is implying Faraday's law. V=Nd(phi)/dt. If magnetic lines of force cut the conductor (EUT), a voltage will be induced in a conductor (even N=1, if d(phi)/dt changes is large). 

    Best,

    Raymond

  • Hi Raymond,

    Thank you very much.

    Now I got some ideas about this issue.

    I would like to know more about this. Could you please guide me how I will proceed further. 

    I mean what are the subjects I need to be through for understanding this.

    If you have any books/Application notes please let me know.

    Regards

    Hari

  • Hi Hari,

    The noise coupling is fairly complicated topics. Even you understand Faraday's law and Ampere's Law in magnetic design, it is still difficult to have full understanding in how noise is coupled into a circuit. 

    I checked some videos in youtube that may guide you in these learning topics. I would suggest to look up Power Input and EMI-EMC sections of RTCA DO-160 document. DO-160 is aerospace airworthiness compliance and test requirements. It will give you ideas how aerospace is testing these issues. From there, you may try to learn other relevant topics. 

    Automotive has similar test requirements and compliance, but it is more difficult to find these in a single document. 

    https://www.youtube.com/watch?v=PACur_GcTJ0

    https://www.youtube.com/watch?v=J6gPzMYb5Z0

    https://www.youtube.com/watch?v=gBCOeYfRL5g

    Best,

    Raymond

  • Hi Hari,

    yes, even this parasitic high pass filter is forming a loop. But shielding and grounding breaks this loop. The induced current is now flowing in the shield and no longer through R1.

    Kai

  • Hi Raymond,

    Thank you very much for the info.

    I will watch these videos and also will try to get that document.

    Regards

    Hari

  • Hi Kai,

    Thank you very much.

    Regards

    Hari

  • You are welcome :-)

    Kai