How to test the bias batteries in a 9103 picoammeter

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The 9103 USB picoammeter is often equipped with a +90 V bias option which improves the accuracy of electron and ion current measurements (by reducing the number of low energy secondary electrons that are generated by the beam from leaving the target).

The +90 V bias comprises two 45V batteries in series which are located inside the 9103 chassis.   This blog post will explain how to test and replace the batteries in a 9103 picoammeter.

1. Connect a DVM (digital volt meter) to the 9103 Input BNC connector.  A BNC to double banana cable works well.  Set the DVM to DC volts.

2. In Actuel (the 9103 software), select the input Grounded and bias On.

3. Sample the current.

4. When the bias ON is checked there will be about +90V DC on the input of the 9103.

5 .The input impedance of most DVMs when measuring DC voltage is 10 meg ohms.  The two 45 volt batteries should total 90 to 95V DC.    The bias voltage divided by the input impedance of the DVM will  equal the current.  In this case the voltage of the two 45 volt batteries totaled 94V and the current was 9.414 uA.

9.4 uA in Actuel

9.4 uA in Actuel

 

 

 

 

 

 

 

94 V DC on DVM

94 V DC on DVM

 

 

 

 

 

 

 

6. It is recommended that the bias batteries be tested every 6 months and replaced when the voltage drops below 80 volts.  It is normal for the batteries to wear out over time and with use.  Once the bias voltage drops to less than 50 V the effectiveness of preventing secondary electron emission is greatly reduced, which in turn reduces the accuracy of electron and ion beam current measurements.

The RBD part number for the 45 V battery is BAT-45-213.

Whether you have an RBD 9103 USB picoammeter or an older Keithley with a PHI model 78 bias box, you should test the batteries as part of your preventive maintenance procedure and replace them as needed.

To replace the batteries in a 9103:

Unplug the 9103 USB power and input cables.

Using the Torx wrench that was included with the 9103, remove the screws from the front and rear 9103 chassis covers.

Slide the board out from the front of the 9103.   You will need to rotate the back cover to feed it in. The back cover has a ground wire that is attached to the 9103 board. Also note which groove the 9103 board is in as you will need to put it back in the same groove.

Remove the battery support bracket (white plastic).

Carefully remove the old batteries.

Install the new batteries.You may need to adjust the contacts on the batteries to get them to fit onto the board snaps more easily.

Reattach the battery support bracket.

Carefully slide the back cover and board back into the chassis. Make sure that you put the board back in the same groove that it came out of.  If the front cover does not line up with the chassis then you are not in the correct groove.

Reattach the screws to the front and back covers.  Do not over tighten the screws!

Once you have installed the new batteries, test the voltage. You should have 90 to 95 Volts.

9103 bias batteries

9103 bias batteries

 

 

New Feature: Use the 9103 Picoammeter to Automatically Read microCMA Target Current

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The latest software releases of Actuel for the 9103 Picoammeter and CMapp for the microCMA now support the ability to automatically read target current when acquiring data.

To use this feature, you must (of course) have a 9103 Picoammeter running RBD’s Actuel software.

Run CMapp for the microCMA, select Hardware Properties from the System menu, and check the option “Use 9103 Picoammeter to read target current.” You only have to do this once.

Turn on the 9103 Picoammeter, run Actuel, and measure your target current as usual. Leave Actuel open. It doesn’t matter whether the 9103 is sampling, but keep in mind that your current settings (sample rate, etc.), will be used.

Now, whenever you take an acquisition (except for an alignment), the target current will be measured at the beginning of the acquisition and displayed with the other electron gun settings when the acquisition is complete.

If you don’t have a 9103, you can still manually enter a value for the target current in the acquisition dialog.

Download the latest release of Actuel for the 9103 Picoammeter here.

Download the latest release of CMapp for the microCMA here.

 

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.  For more information on our new 5kV floating 9103 please contact us by emailing us at sales@rbdinstruments dot com or visiting our website at www.rbdinstruments dot com.