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Reading the article resistor noise and knowing that resistor noise is in the form of Voltage (is that thr right expression?!) leaves me wondering that this generated voltage will have only eefect through the circuit tracks and enters the components in its way, or if it is in form of EMI and can have effect on nearby sensitive components even if they are not connected to the noisy resistor?

If the second one is correct, would remedy be capping the resistors with a metal shield (like the cpa in the analog section of old PC video cards or TV cards?)

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    \$\begingroup\$ Resistor noise can be expressed in the form of either a voltage or a current, via the Thevenin/Norton equivalent circuits. Expressed as a voltage, it will look like a small voltage source in series with the resistor. Putting a cap over the resistors will have no effect on the noise, except maybe to make the resistors heat up more and thus produce more noise. \$\endgroup\$
    – The Photon
    Jan 10 '12 at 21:37
  • \$\begingroup\$ Out of curiosity I'd be very interested to know the specific application in which this becomes an issue. \$\endgroup\$
    – Argyle
    Jan 11 '12 at 5:35
  • \$\begingroup\$ Tha application will be measuring capacitor current leakage in pA range. There is my other question related here : electronics.stackexchange.com/questions/24852/… \$\endgroup\$
    – Sean87
    Jan 11 '12 at 9:02
  • \$\begingroup\$ Just a note: Your link to aikenamps.com/ResistorNoise.htm is dead, probably you mean aikenamps.com/index.php/resistor-types-does-it-matter at the same site. \$\endgroup\$
    – dominecf
    Mar 30 '20 at 19:43
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The linked article discusses three types of noise:

Thermal noise: Thermal noise is the noise that we most often refer to when we talk about resistor noise. Another name is Johnson noise. Thermal noise results from the random motion of electrons through the resistor, and is given by sqrt(4kBTR), as stated in the article. Most critically, the rms thermal noise is proportional to the square root of the temperature (Kelvin scale) and to the square root of the bandwidth of whatever is measuring the noise.

Contact noise: It's not exactly clear what the article is discussing here, but he distinguishes this noise by its 1/f characteristic. Noise with 1/f dependence could arise from the current passing through the barriers between the carbon grains in a carbon resistor (which would be why the article recommends other types) or by current passing through the boundary between the metal leads of the resistor and the carbon resistive material. This is the only type of noise that the original article claims will be reduced by using a higher-wattage resistor. This could be explained by the higher-wattage resistor having more parallel paths through different grain or interface boundaries, resulting in noise contributions averaging out.

Shot noise: I wouldn't normally characterize this noise source as being generated by a resistor. Shot noise is fundamental in any circuit, resistive or not, when current is detected. It results from the fact that current doesn't arrive in an absolutely continuous stream, but one electron at a time. Shot noise is likely to be a problem only in extremely sensitive circuits where other noise sources have been very carefully eliminated, or when high current gains are used.

Either thermal noise or shot noise can be characterized as either a voltage noise or a current noise, based on the Thevenin and Norton equivalent circuits:

Thevenin and Norton equivalent noise sources

In any case the resistor noise is injected into the circuit nodes connected to the resistor. Because it is generally a very low level signal, it's unlikely to be emitted as EMI and cause problems in unconnected parts of your circuit, unless, of course, your circuit is amplifying it.

To answer a question from the comments, "white" noise is noise that has constant power density over frequency. For example, if a white noise source produces 2 nV/sqrt(Hz), and we measure it with 1 Hz bandwidth around 100 kHz or with 1 Hz bandwidth around 100 GHz, we'll measure 2 nV rms noise in either case. Thermal noise is a white noise source, while "contact noise", as mentioned above, is not white because it has has 1/f frequency dependence.

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  • \$\begingroup\$ Just a side note, do you think that this noise can effect my leakage measurement circuit?! (My other question that you answered) \$\endgroup\$
    – Sean87
    Jan 11 '12 at 10:02
  • \$\begingroup\$ This noise shouldn't matter because it will be very small compared to your 30 V bias voltage, and your picoammeter will measure with very low bandwidth. \$\endgroup\$
    – The Photon
    Jan 11 '12 at 17:19
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There are different types of noise. One of the most prominent is thermal noise. As the name suggest, this is thermal, so it's linear to temperature (as the equation shows there).

This means that if a resistor gets hotter, it will have more noise. Metal shielding is not very effective against this kind of noise, metal shielding is typically used to suppress EMI getting into the circuit (or getting out). For example, video circuits can work in regions of MHz. Noise of 100MHz is everywhere around us (FM radioband), so if those kind of signals would get into sensitive, (high impedance) circuits, it will introduce interference.

Noise can propagate through circuits because of their frequency (some components can't suppress very high frequency noise that well) or just because the noise happens to be on a signal you want to amplify. If you amplify the signal by a hundred, the noise will be hundred times large too. An amplifier will add noise too, but doesn't have to be thermal noise, can also be flicker noise or of different types specific to semiconductors.

The signal-to-noise ratio says something about how big the signal is compared to the noise. This is expressed in decibels, and the higher the better. If you pass the signal through more amplifiers and circuits, this value will only drop further.

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  • \$\begingroup\$ Thanks for info, can you please clearify me, so I should consider the resistor thermal noise in a given temprature as a constant voltage added to my signal voltage? or it is some sort of waveform with certain frequenty? \$\endgroup\$
    – Sean87
    Jan 10 '12 at 21:55
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    \$\begingroup\$ @Sean87 the noise that thermal noise creates is white noise, but as my comment above states, its not really a big issue. \$\endgroup\$
    – Kellenjb
    Jan 10 '12 at 21:56
  • \$\begingroup\$ I am not good with terms :P What does 'white noise' means then? \$\endgroup\$
    – Sean87
    Jan 10 '12 at 21:59
  • \$\begingroup\$ electronics.stackexchange.com/search?q=white+noise \$\endgroup\$
    – Kellenjb
    Jan 10 '12 at 22:01
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    \$\begingroup\$ I decided to delete my original comment. The bigger issue here is the large gain that is being applied to a very high impedance input. I have personally found that the noise in a resistor is almost nothing compared to the noise created by active components. \$\endgroup\$
    – Kellenjb
    Jan 10 '12 at 22:07
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...will have only eefect through the circuit tracks and enters the components in its way, or if it is in form of EMI and can have effect on nearby sensitive components even if they are not connected to the noisy resistor?

Resistor noise is inherent inside the resistor itself. You cannot reduce it or filter it out; you can only learn to deal with its consequences -- one of which is that for ultra-low-noise circuits, you may need to pick resistor values that are relatively low values.

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  • \$\begingroup\$ In the article, author denotes that using higher power rated resistors can help, is this true? \$\endgroup\$
    – Sean87
    Jan 10 '12 at 22:02
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    \$\begingroup\$ @Sean87 the way this would help is a higher power rated resistor generally wont heat up as fast as a lower wattage. Since there is less heat, there is less noise. But even in the authors extreme example, you are looking at a .03V change in noise going from room temperature to boiling. Take a more real life example and you won't even see that much. \$\endgroup\$
    – Kellenjb
    Jan 10 '12 at 22:08
  • \$\begingroup\$ @Sean87 you are right that (at the same temperature), a higher-power resistor should have the "1/f" component of noise reduced. However Kellenjb is right that the "Johnson" noise can not be overcame with a choice of resistor (just with cooling it). \$\endgroup\$
    – dominecf
    Mar 30 '20 at 20:23

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