Electronics > Metrology

Strange asymmetric nV noise of Keithley 148

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Doing some testing with Keithley 148 unit nr 1. I haven't yet tried to go thru the internal adjustment procedure.

For these runs I placed a 0.2ohm resistor across the input. I connected a battery at 6.8V thru two 100Mohm resistors in series with the 0.2ohm resistor. This gives roughly 6.8nV at the 148 input.  The 148 is on the 10nV range.   6.8nV is at 3.4 divisions above center on the scope.  Each scope vertical division is 2nV at the 148 input.

This gets time consuming. The voltage drifted off scale for a number of runs. I'm sure this is mostly due to temperature. To get good nV readings a temperature controlled chamber would be necessary.

The 1st 25 minute run shows 0V input.

The 2nd 25 minute run shows 0V input a bit later - I have no idea why it's cleaner.

The 3rd 25 minute trace shows a typical run, except that it stayed on scale.

The 4th  5 minute trace shows a rare clean run, with the rise and fall times when connecting and disconnecting the battery.

For nanovolt measurements, with quick-and-dirty input connections and lack of temperature control, I conclude that It's only possible to record clean signals by chance. The two clean traces of my last post probably show what the K148 can do when conditions are good.


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