9103 Floating Ground Reference

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The term “floating ground reference” in the title of this post refers to an electrical circuit that does not have a ground connected electrically to earth. (This type of connection is also referred to as “floating input”.)

The 9103 USB data logging picoammeter has the ability to float higher than earth ground by up to 1,500V DC. A new 9103 version coming out in early 2018 will be able to float up to 5,000V DC.

This post explains how the connections to the 9103 USB picoammeter are made and how the floating capability works.

The HV option for the 9103 uses SHV and MHV 5kV connectors instead of a BNC.  The center pins on the INPUT and HV connectors are the signal input and ground reference input, respectively, to the 9103.

9103 HV option

Step one is to build an isolated (floating) power source.  A very easy way to do this is to use a 9 volt transistor battery and a resistor. For this example, I used a 9V battery with a 10 Meg ohm resistor to get about 900nA of current. Using Ohms law you can create any current that you would like to use for this test as long as it is in the range of 1nA to about 1mA.  A 9 volt battery works well because it is small and a very clean source of DC voltage and current.

simple isolated current source

Next, I wrapped the battery in electrical tape and mounted the battery in an enclosure.

9103 float test box

9103 float test box

One end of the resistor is connected to the INPUT on the 9103 and the other end of the resistor is connected to the battery. The battery is referenced to the HV input ground on the 9103.

Finally, I connected a high voltage power supply to the ground reference via a 1 Meg ohm current limit resistor as show in the schematic below.  The current limit resistor helps to reduce noise and current surges from the high voltage supply.

Current limit resistor.jpg

After connecting the INPUT and HV leads to the 9103, I am ready to measure current. It is important to note that the 9103 Input should be set to Normal and not Grounded. (The “Grounded” Input is used to short the specimen stage to ground when not measuring current. This is useful when measuring electron or ion currents in vacuum, but when floating the 9103 you do not want to short out the input or you may damage the 9103 or your high voltage supply.)

normal not grounded

We can now measure current and can see that we are getting 913.2 nA of current.

Using the Data recorder we can monitor the current vs. time to see a graphical representation of the current.

The signal ground reference on the 9103 is tied to the high voltage supply.  As I increase the high voltage supply from 0 to 1500V DC in increments of 500V (a limitation of the high voltage supply I am using) you can see some small instabilities in the data.  This is normal; there is some capacitive coupling as the ground reference voltage is changed.  It looks like a lot of noise but in fact is only about 20 pA.

9103 graph spikes when increasing voltage

1500V DC voltage source

If you look at the data referenced to zero you can see that the instabilities are very minor and also that the output is very stable once the high voltage supply stabilizes.  If you were to measure the voltage on the 9103 HV reference to earth ground you would measure 1,500V DC.  So for this example the 9103 ground is floating by 1,500V DC.

9103 graph zero base line

In this test I changed the high voltage supply from +1500V to -1500V DC with no change in my current reading which demonstrates how well isolated the 9103 input is from earth ground.

Applications for a floating input picoammeter include measuring the output of an electron multiplier directly, as well as bias experiments with electron and ion beams.  Click here for more information on our new 5kV floating 9103.

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