Electronics > Metrology

Project for standalone use of Keithley low current preamplifier.

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Hello TiN,

I realize that this is an old thread, but the fantastic photos that you posted of the Keithley preamps here and on xdevs.com have me wondering how they implemented the shunt swithching.

There seems to be an LMC7101 with its inverting input connected to the switched end of each shunt, as well as an LMC662 with one of its non-inverting inputs connected to each shunt(on opposite faces of the PCB. I attached a copy of one of your photos with opamps on two shunt pins sketched in).  This pattern is also seen around the low current shunts in 2612 and 2636 SMU's.  It seems very likely that the LMC662's are used as a voltage follower to monitor the voltage on the swithched end of each shunt, and they are visibly driving guard rings.  The LMC7101's might be there as a voltage follower to drive the output current into the shunt in use.

Getting the leakage current from LMC7101's on unused shunts down into the fA range seems like a challenge.  I think that driving the SMU output (guard) potential into the switched end of unused shunts is a non-starter because the 6 decade range of shunts would lead to unacceptable leakage current in the lowest current range from even uV offset errors across the lowest value shunts. Analog switches between opamp outputs and shunts might need guarding (bootstrapping to 0V across switch channels) to achieve fA leakage.

Switching both supply pins (and likely the non-inverting input) of the LMC7101 on each unused shunt to (SMU output) guard potential would set their output transistors' channels and gates as well as ESD diodes all with 1mV or less potentials across them: this could easily drop the 7101 output currents to pA range, and maybe it's enough to get down into fA's.

Is there any chance that you traced the supply connections of any of the LMC7101's connected to shunts?


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