PSI System Controls and Diagnostics
F3200E User Manual
F3200E_UM_180726 Page 23 of 107
The F3200E should be located as close to the source of the signal as reasonably possible. Long
signal cables increase the chances of seeing unwanted signals and noise, particularly at the
highest bandwidths. A maximum length of 10 m and maximum capacitive load per channel of
2000 pF is advised. Longer cables may be used up to a maximum of 50 m, but the lowest
detectable current will be increased.
Other signal sources can include diode detectors, ionization chamber electrodes or any sensors
which produce currents in the nA to mA range.
9.3.2 Signal cables
25-way screened cable with the minimum practicable capacitance should be used, terminated in
good-quality 25-way D subminiature plugs at the F3200E end. Two such cables are required to
connect up all thirty-two channels. Measurements at the bottom of the dynamic range may
benefit from the use of low-noise cable. This cable is made with semi-conductive coatings on
insulators to inhibit the generation of free charge (the triboelectric effect). Low current
measurements can be made with standard cable, but you must take particular care that the cable
cannot move or vibrate for several minutes before or during measurements.
The cable screen should be connected to chassis through the connector hood at the F3200E, and
similarly to the sensor housing or vacuum vessel at the other end. In some cases you may get
better noise performance if this screen is connected at one end only. The optimum arrangement
can be found by experiment.
You may use any of the AGnd pins on the signal connectors for the current return path, or the
cable screen, or a separate path to the chassis.
9.3.3 Signal current path
The currents measured by the F3200E must be allowed to return to their point of generation. If
there is no return path, then you will see no current, or get erratic readings. Usually there will be
a return path via the common ground of the F3200E chassis and the sensor enclosure, but this is
not always true, so if you don’t see the currents you expect, you should look carefully at the
circuit.
The currents you are measuring pass along the cable inner conductors to the F3200E inputs. The
current flowing into the I-V amplifier inputs are balanced by current in the feedback which is
supplied by the power supply to the amplifiers. Thus the measured current effectively flows
between the terminals of the input amplifiers to the local circuit ground (AGND), due to the
operational amplifier action.
Figure 4 shows a typical current path for conventional current in an application where the current
originates in an ion source and is formed into a particle beam by a high voltage supply. The
supply may be remote from the place where the current measurement is being made, for example
the current measured by an electrode in a charged particle beamline actually originates in the ion
or electron source, which could be many metres away.
