# Amplifying small non-linear signals

#### blah2222

Joined May 3, 2010
582
Hello, I am working on a project where I am receiving a very small current signal (~100 pA - 10 nA) and I would like to convert this into a voltage signal and amplify it up to something that is more reasonable.

Firstly, to convert this into a voltage signal, is the best method just to pass the current through a resistor and take the voltage drop as the new input voltage signal?

Secondly, I will be dealing with an electrochemical signal current that is far from sinusoidal, as you can imagine. I am just curious if my previous knowledge of small-signal analysis and amplifier design will be sufficient to process this signal.

Any advice would be greatly appreciated!
JP

#### #12

Joined Nov 30, 2010
18,216
40 years ago we did this with a j-fet input stage. There is probably something better now. What's your frequency range?

#### blah2222

Joined May 3, 2010
582
Hm, from what I understand these signals are not periodic. Basically my job is going to be to produce a realtime plot of current levels with time. This electrochemical sensor can have a potential placed between its electrodes to simulate a response in the current.

The plots that I have seen from articles show current spikes based on events that occur at the electrodes, so there is no periodic nature to these signals. So I can't really give a frequency range.

J

#### t_n_k

Joined Mar 6, 2009
5,455
This might also be useful ....

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#### wayneh

Joined Sep 9, 2010
16,482
What you need is also called an instrument amplifier, one with very high input impedance. A pH meter is an example, although even a pH meter might not be able to deal with currents that small. You'll need an op-amp with an extremely high input impedance. Last time I went looking, I found the LMC6035 or LMC660.

#### #12

Joined Nov 30, 2010
18,216
I see you need a DC amplifier in the picoamp range and output recording. In today's world, recording is childs play. As for the amp, now you know what to look for.

Actually, the stuff I worked on in '73 would be perfect for you. Just get in your Wayback machine and buy a Ph meter from Unilock Inc. in Irvine Ca. It comes with a DC coupled picoamp level preamp, alarm point switching, and 4-20ma recording output...or find something similar in the 21st century. It's probably cheaper now.

#### wayneh

Joined Sep 9, 2010
16,482
Just get in your Wayback machine and buy a Ph meter ...
Don't buy one. Most labs I've worked it will throw out old meters from time to time. #12's right; everything you need is all there.