9103 Picoammeter bias modes
This post explains the 9103 picoammeter bias modes and also explains the concept of “floating”.
There are 4 picoammeter bias modes:
No bias – the current source is connected directly to the picoammeter input
Internal – a low noise DC supply (two 45 V DC batteries in series) provides a bias between the current source and the input of the picoammeter. This application is typically used for providing a +90V bias to a target in a vacuum chamber in order to prevent secondary electrons from leaving the sample. Adding the 90V bias results in a more accurate measurement for both electron and ion current. It is also possible to use regular 9V batteries and so lower internal bias voltages are possible. Batteries are used for the internal bias as they are very clean with no ripple.
External – An isolated low noise voltage source (provided be the user) is inserted between the current source and the input of the picoammeter. Isolated means that the voltage source is not referenced to ground. A typical voltage supply has the negative lead referenced to ground internally. If you were to measure the resistance of the supply negative lead to the chassis ground it would have a resistance of less than 1 ohm. On an isolated supply the resistance between the negative lead and the chassis ground would be open. The maximum voltage that can be applied in this mode is 600V DC (that is the rating of the BNC input on the 9103 USB picoammeter).
Floating – In this mode the picoammeter signal ground is raised from near chassis ground up to a value as high as 1500V DC. If you were to measure between the signal input and the signal ground reference the voltage would be low (under 1V typically). However measuring the DC voltage from the signal reference to chassis ground the voltage would be high, the floating supply value.
When floating the 9103 USB picoammeter there are some safety considerations. First of all, the input to the 9103 is changed from a BNC to either a SHV or MHV connector (user specified). These connectors are rated for up to 5kV. The recorder output on the back of the 9103 also floats. For example if you apply a floating voltage of 1000V the recorder output would also measure 1000V with respect to chassis ground. So if you are using the recorder output and also floating the 9103 the meter that you are measuring the recorder output with also needs to be isolated from ground. Finally, to prevent voltage spikes a current limit resistor is placed between the floating supply and the 9103 signal ground. The floating mode is typically used to measure the collector output of an electron multiplier or a Faraday cup.
The maximum float voltage with the latest rev 9103 is +/- 5000 V DC. If your 9103 was purchased before January 2019 then it can only float up to 1500V DC maximum.
The floating mode cannot be used with either the internal or external bias modes. However you really would not need either of those options as you can float the 9103 up to get what ever bias you need. For example, let’s say you want to measure an ion beam that is at 3000V DC and you wanted to retard the ions by 200V. You could simple set the bias supply to +3200 V DC and you would be retarding the 3000 V beam by 200V DC.
For all modes of operation the chassis ground of the 9103 USB picoammeter should be connected to the ground on the PC or chamber.
If you would like more detailed information please contact us via email at the link in the upper right hand corner of our website at www rbdinstruments dot com.