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by lisper
1842 days ago
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One of my first jobs, which I got while I was still an undergrad (in the mid-80s), was designing amplifiers for fiber-optic sensors. I pretty much had no clue what I was doing so I just started futzing around with op-amps and realized very quickly that my signal-to-noise-ratio was much higher than was acceptable. I figured there was some hardware design trick that they hadn't taught me in my EE curriculum, but one day I decided to do the math on resistor noise and discovered that that was in fact my limiting factor and the only way were were going to get it to work was to either cool the first-stage resistor or to use a ridiculously high value because the gain goes up linearly with the resistor value but the noise only increases with the square root. We ended up with a ten gigaohm resistor, which was just enough to get the S/N ration we needed to make it work. |
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Depending on the voltage across a resistor like that, you may calculate less than one electron passing through the resisitor per second.
Without ceramic or teflon standoffs, the circuit board can often conduct better than the resistor, plus dust can also accumulate on the outside of the resistor and conduct better eventually, which is why they are often encased in glass, so they can be effectively cleaned during a maintenance cycle.