Electronics > Projects, Designs, and Technical Stuff
Testing transimpedance amplifiers
new299:
I've looked at transimpedance amplifiers in the past for various applications. Generally these would be used with a photodiode or another sensor (for example, used in a scanning tunneling microscope). I'm less than clear on how these amplifiers should be independently evaluated.
If I'm looking at doing measurements in the pico to nanoamp range, I've tried a sticking 100Meg Ohm resistor on the output of a function generator. This seems like a relatively common approach. It seems to work relatively well. And I assume is fine because the input impedance is low?
However I've also seen people suggesting building a current source using an LT1167 [1] for testing.
Alternatively, Keithley make programmable picoamp current sources. Like the Keithley 220 for example.
What I don't understand well is the tradeoffs between these various options. Is using a function generator just as valid as the other options? Or are there issues that it could cause?
[1] http://images.100y.com.tw/pdf_file/33-LT-LT1167.pdf figure 7.
RoGeorge:
In theory, no difference between a voltage source + a big resistor instead of a current source. There is even a way to convert from one to another: Thevenin/Norton equivalence for (Vsource with serial Rs) <===> (Isource with parallel Rp).
In practice, it depends. No component is ideal, so a resistor is expected to have stray capacitance, inductance, temperature coefficient, etc. It depends what parameters matter or not for a given test, and what value ranges are expected for a given test.
The specs for a tunneling microscope amplifier are not common knowledge. Might help answering if you briefly explain what do you want to test for a transimpedance amplifier used in a tunneling microscope. Ranges for current, noise, frequency, temp, specific peculiarities, etc.
new299:
So, typically using either a 1M, 10M or 100M feedback resistor on the transimpedance amplifier. I'm interested in picoamp or nanoamp ranges. I'd like to be able to characterize down to 10s of picoamps of noise (at worst), bandwidth maybe upto ~10KHz in the picoamp range. ~100Khz in the nanoamp range. Room temperature.
RoGeorge:
100M\$\Omega\$ is not a common value, might be hard to find, or expensive. That alone can be a good reason for a very low current source. Some resistors might be noisier than other, that could be another reason. Large impedance variations of the load (compared with the required series resistor - not your situation), maybe some well designed active source current sources can deliver highest output impedance than a simple resistor for a decently low voltage, and so on.
Why are you looking for a controlled current source, did you get unsatisfactory results with the series resistor method?
nfmax:
Drive a triangle wave through a very small capacitor into the input. This will give you a square wave current, and with readily available components you can set very small currents.
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