Electronics > Metrology
Project for standalone use of Keithley low current preamplifier.
arlo_g:
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?
pattant:
hello,
there is lots of interesting information in this thread! I am in a different position from most it seems: I have a couple 6430 source meters, but no remote pre-amps to go with them. Recently some noise issues made me think of the pre-amps again. It seems like the 6430 remote pre-amp is hard to obtain, while the 4200 pre-amp is available...
so my big question is: has anybody tried the 4200 pre-amp with the 6430 source meter?
42Khz:
--- Quote from: arlo_g on June 11, 2021, 12:08:25 pm ---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?
--- End quote ---
Hi arlo_g,
I just spent some time staring at TiN's excellent PCB photos trying to figure out how Keithley did the current ranging in at least the 6430 preamp (primarly this one). The 1k and 10M resistors caught my attention and suddenly it hit me, they're using (a modified version of) the arrangement in their US patent 5,994,947!
The gist of the patent is that they use a nice non-linear impedance (diodes in the patent) which they can either guard to achieve extremely low leakage when the switch is supposed to be off, or include in a feedback loop to compensate for their forward voltage when the switch is on.
The photograph clearly shows that pins 1,2 and 3 of the LMC7101 are shorted together, so it cannot possibly be configured in a traditional opamp configuration (2 supply contacts and then either an input or an output?). That fact coupled with the patent description must mean Keithley are using the input protection diodes on the LMC7101 as diodes 38, 40 in the patent. I would guess that they're fairly tiny and reliably low leakage.
From that point it just takes matching the patent to the PCB, although I believe the node shared between the 176 P-JFET and Maxim switch gives away that Keithley deviated slightly from their patent circuit. I've indicated the difference in my schematic diagram below.
I hope that helps!
PCB with some scribbles:
Schematic. I didn't check whether the circuit is really the same for the 4200 preamp but I would be surprised if the current ranging was different.
Noopy:
If you are interested in the internals of the LMC662, I have posted them here:
https://www.eevblog.com/forum/projects/opamps-die-pictures/msg4651233/#msg4651233
https://www.richis-lab.de/Opamp62.htm
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