Cancelling out Power Supply Noise out COMPLETELY

AI Thread Summary
To effectively cancel out power supply noise while measuring the noise of a 600 MΩ resistor, circuit design must consider the inherent limitations of power supplies, as complete noise cancellation is impossible. Using a high-voltage capacitor in place of a resistor may reduce noise, but achieving balance in the circuit remains challenging. The bandwidth of interest is critical; for example, a power supply that allows mains noise may be acceptable for certain frequency ranges but problematic for others. Additionally, thermal noise from the resistor can significantly impact measurements, necessitating bandwidth restrictions to mitigate this effect. Overall, careful consideration of circuit configuration and measurement parameters is essential for accurate noise assessment.
jonlg_uk
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Hi I have a HV (R2) resistor that I want to test for noise. Assuming the 2 low voltage resistors are perfectly stable and HV R1 is also perfectly stable. How could I completely cancel out the noise of the power supply if the bridge was slightly unbalanced? Does any circuit exist that can do this?

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No, there is no such thing as a PS with no noise. There are some very low noise supplies out there (use a battery) but I'd say that if the noise of the PS is that problematic you need to re-think your measurement methodology.
What is the resistance of R2? Remember that you also have the thermal noise (of R2 and the rest of the circuit) to deal with, and that can be substantial even compared to the input noise of a typical OP-amp.
Also, which bandwidth are you interested in?
 
f95toli said:
No, there is no such thing as a PS with no noise. There are some very low noise supplies out there (use a battery) but I'd say that if the noise of the PS is that problematic you need to re-think your measurement methodology.
What is the resistance of R2? Remember that you also have the thermal noise (of R2 and the rest of the circuit) to deal with, and that can be substantial even compared to the input noise of a typical OP-amp.
Also, which bandwidth are you interested in?


Hi thanks for the reply. I know it is impossible to design and build a PS with no noise. The value of R2 is 600Mohms. I am looking for some type of circuit configuration that will cancel the noise and the HV of the power supply but at the same time allowing me to measure the noise of R2.

I have thought about using a HV cap in place of R1 as they have much less noise. However balancing the circuit so that it cancels out all the power supply noise is proving difficult and I am still reading some of the PS noise.
 
But again. which BW are you interested in? If you are only interested in say the noise between 10kHz and 100kHz you might be OK with a PS that let's a lot of mains+harmonics through, but perhaps not OK with a switching supply that switches at e.g. 20 kHz and so on.
The BW is a crucial parameter when designing noise measurement experiments.

Also, a 600 Mohm resistor will give you a lot of thermal noise unless you restrict the BW of you circuit (a few mV/sqrt(Hz)).
 

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