something that leaks current but has no noise? - Page 1
Forum section: electronics / projects

author: SArepairman
author_rank: Frequent Contributor
timestamp: April 12, 2014, 06:51:51 am
content: So for an AC coupled amplifier you typically need a resistor so the capacitor does not fill up. Is there something else instead of a resistor that you could put there that would prevent the capacitor from filling up but also not create noise? Like a diode with some leakage current?or a current sink?

author: acbern
author_rank: Frequent Contributor
timestamp: April 12, 2014, 11:49:20 am
content: resistor noise is caused by physical phenomenon of random electron movement. it can be calculated and depends on constants of nature such as the boltzman c (google for exact formula if interested). anything else that in the end behaves like a resistor (whatever it be and if it existed) will also have random electron movement, so you wont get rid of the issue.the question is rather what is your noise problem (what noise level do you have in what application and what do you want to achieve). if you explain this maybe the community can help otherwise.

author: SArepairman
author_rank: Frequent Contributor
timestamp: April 12, 2014, 07:07:09 pm
content: so you are saying pretty much if there is a drain current of nanoamps or whatever flowing then the noise will be generated regardless?I know some op-amps have internal biasing circuitry so you can use AC coupling with no pull down resistor, those are no better then a discrete resistor?I am not disagreeing with you, but this means that in general, if i were to build a precision current sink, using active elements, it would do the same to noise as a resistor?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 12, 2014, 09:05:18 pm
content: Inductor.Uh.... a very big one.You can synthesize one (or a high impedance current source/sink) with a gyrator. But you incur more noise by adding amplifiers to the chain.Tim

author: acbern
author_rank: Frequent Contributor
timestamp: April 13, 2014, 12:01:07 am
content: @sarepairman. you did post a similar question in the beginners forum. maybe my answer there helps. gererally, you cannot circumvent physics. a nanoamp input current is generated by a semiconductor, it also generates noise. actually, due to the pretty basic question you are asking, i do not believe that your requirements are so extreme that they cannot be met. you have not said what noise level you are seing and what you want to achieve.generally, without being more specific of what you want to achieve, I guess everything has been said.

author: David Hess
author_rank: Super Contributor
timestamp: April 13, 2014, 07:57:25 am
content: If the coupling capacitor is driven from a low impedance source, then the noise from the resistor is going to be attenuated by the source impedance at frequencies above the capacitor breakpoint. That is why high impedance oscilloscope inputs do not suffer from noise coming from the 100k+ protection resistor in series with their inputs which is bypassed with a capacitor.As a practical matter, that means that the noise from the high value DC restoration resistor is irrelevant in most applications.

author: AndyC_772
author_rank: Super Contributor
timestamp: April 13, 2014, 08:11:45 am
content: Bear in mind that current is carried as discrete units of charge. It's not a continuous variable, and for that reason alone can never be completely "noise free" whatever technique you may use.