Electronics > Projects, Designs, and Technical Stuff
Testing transimpedance amplifiers
new299:
--- Quote from: RoGeorge on June 16, 2020, 07:59:38 pm ---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.
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Yes, I've used some of the large Ohmrite through-hole resistors. They state "low inductance", but I'm less clear on their other specs.
--- Quote from: RoGeorge on June 16, 2020, 07:59:38 pm ---Why are you looking for a controlled current source, did you get unsatisfactory results with the series resistor method?
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The results look fine. I'm mostly concerned that they look too good. I.e. does this not model my system well, where I have a fixed bias voltage and, as I understand it, varying resistance (the tunneling junction). So in the real system, I'm measuring the current through that varying resistance.
In this test setup I have a varying voltage and a fixed resistance. So I imagine this is equivalent. But (likely due to my own poor understanding) it concerns me that there might be issues I'm unaware of.
new299:
--- Quote from: nfmax on June 17, 2020, 07:20:40 am ---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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Thanks, I like this approach and will also use it for testing.
RoGeorge:
So, if I got it right, you need the pA source not to test/measure a transimpedance amplifier, but to simulate the tunneling microscope. I don't know enough about the tunneling to help you with that, but given the fact that a capacitor is a derivator, looks possible to integrate the desired waveform, and to apply it as varying voltage through a series capacitor.
Regarding how to generate small currents, there are all kind of tricks, including linear voltage through a capacitor mentioned by nfmax, or even mechanically vibrating capacitors https://doc.xdevs.com/doc/_Metrology/femto_ampere_current_source.pdf
FreezeSSC:
One thing to keep in mind is that most transimpedance amplifiers have to deal with peaking so you'll want to try and test for that with your setup, not testing for that can lead to oscillating op amps.
new299:
--- Quote from: FreezeSSC on June 18, 2020, 02:02:02 pm ---One thing to keep in mind is that most transimpedance amplifiers have to deal with peaking so you'll want to try and test for that with your setup, not testing for that can lead to oscillating op amps.
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Thanks, yes I generally look at the Bode plots created using the built in waveform generator on my Keysight scope. I then add caps to the feedback to remove the peaking.
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