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I am thinking about how or if it is even possible to build a particularly low-noise voltage-controlled DC current source. I need currents of up to 1.7 A (DC) while keeping its output noise below 1.7 nA/sqrt(Hz) (roughly for a frequency of 10 Hz). At least, this value is slightly higher than the shot-noise limit of sqrt(2 q 1.7 A) = 0.74 nA/sqrt(Hz). However, do you think that such a value can be achieved in practice?

Many thanks for your comments!

Charly
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    Almost anything is possible given enough effort and money. I was just wondering what your application is. For a 10 Hz bandwidth, the total current noise is 5.3 nA which is 170 dB below 1.7 A. The real question is can you measure such a low value of noise even if you achieve it. – Barry Feb 24 '20 at 22:11
  • Good to hear that generally it should be possible. The application is the generation of magnetic bias fields using a coil for low noise magnetic field sensors like e.g. GMI sensors. Including the possibility to magnetically saturate them, bias fields in the range of 10 mT are needed while the sensor's detectivity can be well below 10 pT/sqrt(Hz), at least this is what I want to achieve which is why I need very low noise DC sources. Of course there is still the possibility to use permanent magnets and adjust the distance to the sample. However, anything voltage-controlled would be much simpler. – Charly Feb 24 '20 at 22:23
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    The shot noise calculation is a good start. One problem you will face at 10Hz is flicker noise (1/F noise) - component selection will be important as some (carbon comp resistors) are more prone to it than others. As Barry says, ensuring you can measure it is probably the first step. –  Feb 24 '20 at 22:23
  • @Barry From my point of view the measurement is not the problem. Since I know the impedance of my coil I can simply measure the voltage noise density across the coil (while the current flows) with an FFT signal analyzer (AC-Coupled). An additional preamplifier might be needed. But these are questions of the future. First, I need an appropriate current source. – Charly Feb 24 '20 at 22:32
  • Thanks so far! Has anyone, by any chance, ever seen a similar circuit somewhere that could be used as a base and adjusted accordingly (and of course is already "very good" with regard to its noise properties)? – Charly Feb 24 '20 at 22:36
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    Just to give you an idea of what you are up against, the Keithley Model 6621 DC Current Source (which costs about $5000) specifies a 10 HZ BW noise value of 100 uA P-P/20 uA RMS on its 100 ma range (and this unit is advertised as very low noise!). – Barry Feb 25 '20 at 00:33
  • @Barry Unfortunately, I know because I have 4 Keithley 6221 current sources in my lab that are just not low-noise enough for me. – Charly Oct 06 '21 at 11:38

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Although I have not yet built such a low-noise power source, I now know that such power sources are possible in principle. I have not found a commercial current source, but there is a very interesting paper by Carmine Ciofi et al. (http://ieeexplore.ieee.org/document/728794/) which presents a current source that has a current noise (at 1 Hz) of about 10 pA/sqrt(Hz) at an output current of 46 mA, which corresponds to a current-to-current-noise ratio of about 190 dB!

Charly
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  • You are on the right track. I have written a similar answer here: https://electronics.stackexchange.com/a/563902/237061 Now what you must do is replace the reference with a lower noise one as explained in that article. Long time stability won't be good with a battery, but noise will. – tobalt Jul 03 '22 at 04:16