Detecting Pico-Amp AC signals at 100KHz

Thread Starter

will6459

Joined Aug 6, 2013
5
I'm working on a project where I will be attempting to detect a change in current (of the order of pico-amps) at ~100 KHz between a capacitor (dielectric is simply a vacuum). I'm looking for any pointers anyone may have in circuit design for measuring such low signals at high frequencies. I have modelled the system and have a strong idea of the signals I'm looking for but have little background in amplifying circuits.

Thanks in advance,

Will
 

WBahn

Joined Mar 31, 2012
29,928
Could you explain a little more clearly. I don't know what it means to detect a change in current between a capacitor. The word "between" implies two things.
 

Thread Starter

will6459

Joined Aug 6, 2013
5
Sorry my description was a little vague. Essentially I'm trying to detect ions as they rotate between two parallel plates, the ions induce an image charge on each plate as they rotate between the two. So the change in charge over time can be measured as an alternating 'image current'.
 

WBahn

Joined Mar 31, 2012
29,928
What is the capacitance?

Do you have control over the rotation? In particular, can you provide a signal that is locked to the rotation rate?

The thoughts that come immediately to mind are to look at lock-in amplifier techniques and perhaps correlated double sampling.

If the capacitance is small enough, you might be able to look at the capacitor voltage more-or-less directly. If you have 1pA on a time scale of 10μs, that's a change of roughly 10aC. Pretty small. But if the capacitance was 1pF, that would still give you a voltage change of about 10μV. So not entirely out of the question.
 

Thread Starter

will6459

Joined Aug 6, 2013
5
Well its not strictly a parallel plate system (slightly curved) which makes calculating the capacitance a little more complex, however a good approximation is ~0.05pF.

The rate of rotation is used to accurately calculate properties of the ions. However I'm trying to detect a range of frequencies (bandwidth of around 200KHz), so I'm guessing using a single signal wouldn't be suitable?
 

WBahn

Joined Mar 31, 2012
29,928
Well its not strictly a parallel plate system (slightly curved) which makes calculating the capacitance a little more complex, however a good approximation is ~0.05pF.
That would get you up into the range of 200μV, which is probably enough to work with.

The rate of rotation is used to accurately calculate properties of the ions. However I'm trying to detect a range of frequencies (bandwidth of around 200KHz), so I'm guessing using a single signal wouldn't be suitable?
[/QUOTE]

No, not to use lock-in techniques. For a lock-in approach, you would need to have a signal that can sample reliably at the different phases of the signal. There's a reason they call it a "lock-in" amplifier.

My first suggestion would be to look for a FET-input differential preamplifier with good performance specs. With a bandwidth of only a few hundred kilohertz at a gain of several hundred to a few thousand, that shouldn't be too hard to find.
 

Thread Starter

will6459

Joined Aug 6, 2013
5
Perhaps something like this? http://pdf1.alldatasheet.com/datasheet-pdf/view/56673/BURR-BROWN/INA111.html

Looking at some amplifier circuit designs I've found they use a three stage design.

Firstly with the above instrumentation amplifier (INA111) using a 10MΩ resistance at each input (apparently a gain of 10 is achieved).

Secondly a single-pole low-pass RC network with a high frequency breakpoint of 500KHz. I don't really understand what this is/does.

The third and final stage uses another INA111 to provide a further gain of 50.

Thanks for the feedback, its greatly appreciated.
 

WBahn

Joined Mar 31, 2012
29,928
That may work -- probably worth looking into. I haven't looked at the actual specs. When I tried to access the datasheet via that link I got a really strange message:

Your require pages is cannot open by blow
Reason : Connect this pages through directly deep link

That was more than enough to make me not want to click on any links there!
 

Thread Starter

will6459

Joined Aug 6, 2013
5
Yes I get the same when I attempt to follow that link! Sorry about that. I've found some good work from some groups on pre-amp circuit design for similar requirements to my own. Looks like the tricky aspect is going to be reducing noise!

Thanks very much for your help, really appreciate it.
 
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