Hi
I need a bandpass filter with cut-offs : 100 Hz & 7KHz
and my problem is that my signal amplitude is very small : 5nA
Can you help me?

 

5 na is a measure of current, not voltage. That gives little sense of the actual magnitude. Is there some reason why the signal can't be amplified?

Here is a link to a calculator - http://www.wa4dsy.net/robot/bandpass-filter-calc

 

the importance of current for me is for that the sensed information is converted to current variations by detector,and i'm afraid of destroying of response(that has very low amplitude) by presence of noises.and i think that if i amplify the response before filtering,segregation of noise and attenuation of it,will be more hard.
can i have your idea about this?

 

I've been measuring voltages in the microvolt region and currents in the picoamp region for many years. I have to amplify these signals by up to 10,000 X before they give useful information.

I do not know what the nature of you signal is (mine are biological in origin), nor why you think that amplification will destroy anything. Can you say more about the signal?

 

my signal is biological too.it is sensed by an optical system that i,m designing it.

 

a big problem that we have,is that many of devices that we need,are not available,and we should design and fabricate them ourselves,so i think they won't work as good as that are manufactured in electronic factories.so the noise is important for me!

 

This is my first big educational project and I haven't sufficient experience,please guide me.

 

A schematic would be most helpful. Also the purpose of the circuit.

 

I have a photodetector (Hamamatsu,S5981) that detects an optical signal with frequency range of 100Hz to 7KHz . Output current of Photodetector in controlled condition is in order of nA,so I should amplify it with a big order;but noise of devices(50 Hz) and other noises of environment are mixed with signal,so I should use a bandpass filter in this range.after that I will process the amplified signal.
this is theory of my project,but in practice I couldn't find a proper filter.
and can I ask that what amplifier you propose for this purpose?
Many Thanks for all your helps

 

There are noise control techniques you can employ, like a Faraday cage to isolate the experiment from electrical noise. You are better off keeping the noise out of the signal than trying to take it out later. Use an amp to buffer the signal and raise it to a level that is easy to work with (I use 3 stages of amplification to get my voltage gain of 10,000). You can send the signal to an external amplifier box with variable controls through shielded cables.

As to an amp for the photodiode - http://focus.ti.com/lit/an/sboa061/sboa061.pdf

http://www.national.com/onlineseminar/2004/photodiode/PhotodiodeAmplifers.pdf

This is a big project if you have little experience with small signals and noise elimination.