Electronics > Beginners
Logarithmic Picoammeter
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staticshock:
For my Senior Design course. my team is required to design and build a Logarithmic Picoammeter. So far, we've decided on 3 op amp designs: shunt resistance, feedback, and logarithmic diode feedback. Which one is best suited for our project? Also, we need to purchase a low level current source to be used for testing. Which device would you recommend? Our budget is capped at $3000.
tggzzz:

--- Quote from: staticshock on February 08, 2018, 03:42:36 pm ---For my Senior Design course. my team is required to design and build a Logarithmic Picoammeter. So far, we've decided on 3 op amp designs: shunt resistance, feedback, and logarithmic diode feedback. Which one is best suited for our project?

--- End quote ---

That is a principal part of your coursework: stating why you chose the approach and its advantages and disadvantages relative to the specification and other approaches.

It would be improper for us to answer that - and you would be unwise to rely on our answers.


--- Quote ---Also, we need to purchase a low level current source to be used for testing. Which device would you recommend? Our budget is capped at $3000.

--- End quote ---

Now that is a perfectly reasonable question. You may find you have to make one :)

Apart from that, have a look at the Tektronix "Low level measurements handbook".
David Hess:
Does it need to support positive and negative input currents?

What kind of precision do you need?

With a low input bias current operational amplifier, say something like an LMC6001 or selected LMC6081, measurements down to 1 picoamp is possible using a simple transistor based log amplifier which converts an input current into a voltage.  I would hold the logging transistor at a constant temperature instead of using a 3300ppm/C resistor but that is a matter of taste.

At picoamp current levels, shielding will be needed to prevent AC rectification from various sources of interference.
danadak:
This might help -


http://download.tek.com/document/LowLevelHandbook_7Ed.pdf


http://www.electronicdesign.com/test-amp-measurement/whats-all-femtoampere-stuff-anyhow






Regards, Dana.
danadak:
Also there is a Metrology forum here that may be useful -


https://www.eevblog.com/forum/metrology/


Regards, Dana.
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