Electronic – Low-pA TIA Design and Noise Floor

amplifiernoise

I am currently an EE/Physics undergrad doing scanning tunneling microscopy research at my local university. For future experiments, it would be helpful to employ a higher gain preamp than we currently have, and given that other groups have built them, I was looking into doing the same.

We're hoping to measure current signals from about 0.05-10pA from DC to a few kHz (though more bandwidth with a low noise floor is certainly welcome), requiring a transimpedance gain on the order of 0.1-1 teraohm. This certainly seems possible as it was done at the University of Florence (circuit below), as well as in a number of other papers. A few other topologies are presented here.

schematic

simulate this circuit – Schematic created using CircuitLab

I was able to perform a noise analysis of the circuit in LTspice, but I'm a little uncertain of how to translate the spectrum of uV/rtHz vs frequency into a average value for noise floor as it seems to blow up at low frequency with 1/f noise.

Additionally, the LTC6268 seems to have a very low input bias and reasonable noise characteristics, so I was considering using it in place of the above for the first stage.

Best Answer

Looking at the low frequency voltage noise (0.1 to 10 Hz) from the AD549J and the LTC6268 I would say the AD549 is superior at 4 uVp-p. The LTC6268 has a noise of 13 uVp-p. So this is something to be wary of if you are considering a change.

For the input current noise at low frequencies, nothing appears to be stated that is much below 10 MHz for the LTC6268 and that would be a concern. The AD549 is 0.7 fA RMS from 0.1 to 10 Hz by comparison.

I think the LTC6268 is better suited for high speed photodiode applications and not low frequency applications like ion beam measurement.