The input [1.2] should be briefly short circuited before each
voltage measurement in order to check the zero point. With
the input short circuited the instrument connected to the ou-
tput should indicate zero. If this is not the case, the output
voltage must be set to zero with the control knob 12.
(Zero-point balancing in the 10nA current measurement
range (see above) also produces a sufficiently accurate
zero point for the V operating mode.)
Voltage measurements are also possible in the charge
measurement operating mode (see section 2.6). With this
type of voltage measurement, the response time is delayed,
which can be useful for suppressing interference signals.
2.6 Charge measurement
This operating mode corresponds to a large extent to the
voltage mode with the difference that a high quality measu-
ring capacitor of 100nF is connected in parallel to the input.
In this way the charge measurement ranges of
1000nAs...0.1nAs are created from the voltage ranges of
10V...1mV.
For the measurement of the charge on a capacitor, the ca-
pacitor is connected to the previously discharged input (but-
ton 11) of the amplifier. The capacitance should now have
been disconnected from the voltage source used for the
charging process. The charge can be read off directly.
The measurement method assumes that the charge is com-
pletely transferred from the capacitor C, which is under in-
vestigation, to the measuring capacitor C0 in the input of the
amplifier. This is the case for all electrostatic experiments
involving conductors where C << C0. If this condition is not
satisfied, then the actual charge Q can be calculated from
the measured charge Q as follows:
C
Q = ( 1 + ) Q.
C0
Important: The maximum voltage 250Vdc on the amplifier
input should also not be exceeded in this operating mode.
A particular risk is present with electrostatic experiments in-
volving high voltage power supplies. It is essential that the
conductor to be measured is isolated from the voltage
source used for charging before it contacts the amplifier
input. Otherwise damage to the instrument cannot be avoi-
ded.
The charge measurement operating mode can also be used
for the measurement of voltages. Depending on the cha-
racteristics of the experimental circuit, the response time is
delayed due to the parallel capacitor. This may be desirable
for the suppression of interference. Furthermore, the inter-
nal resistance is greater by a factor of 50 in the Q opera-
ting mode.
The charge measurement ranges 1000nAs...0.1nAs corre-
spond to the voltage measurement ranges of 10V...1mV.
(The most sensitive voltage measurement range 100µV is
only available in the voltage measurement operating
mode.)
9 Button
for increasing the measurement range in all operating
modes.
10 Light emitting diode matrix
for indicating the operating mode and the measure-
ment range.
11 Button
for discharging the input in the operating mode Q.
The input remains short-circuited for approximately
one second after the button is released.
12 Control knob
for the electrical adjustment of the zero point.
The adjustment should be carried out with an open
input (10nA range) in the I operating mode and with
a short-circuited input [1.1] (button 11 pressed) in the
Q operating mode. In the V operating mode the
input [1.2] should be short-circuited. When changing
to more sensitive ranges, the zero-point adjustment
should be corrected if necessary.
2.3 General
Measurements in the more sensitive ranges may be subject
to interference from electrostatic charges and induction vol-
tages. The following precautions can reduce these effects:
The use of screened leads.
Experiments are conducted on a conductive, earthed
base.
Avoid movement in the complete area around the expe-
rimental apparatus during the measurement.
The buttons 8 and 9 for the selection of the measurement
range only respond when they are briefly held down. With
continuous pressure on the button (> 0.5s) the measure-
ment range changes consecutively and continuously.
2.4 Current measurement
The instrument has an extremely low voltage drop of only
0.5mV on all current measurement ranges. When connec-
ting the measurement circuit, it should be ensured that the
outer pole of the BNC socket is grounded. The instrument
cannot, for example like a portable multimeter, be connec-
ted at any point in a circuit.
The resistance of the external circuit must have a certain
minimum value so that the amplifier operates precisely. The
minimum values depend on the selected measurement
range and are given under Technical data. If values below
these are present, then a zero offset is produced and the
stated measurement accuracy is not achieved.
In spite of the low internal resistance, the amplifier is very
effectively protected against overload even in the current
measurement operating mode. Provided the voltage drop-
ped across the input does not exceed 250Vdc, then no da-
mage will occur.
2.5 Voltage measurement
In the voltage measurement operating mode the input [1.2]
has an extremely high resistance of more than 10
11
Ω
. For
precise measurements it is assumed that the source resi-
stance is at least a factor 1000 lower, i.e. 10
8
Ω
or less.
With an open-circuit input it is unavoidable that the input will
charge up as a result of the minimal amount of current flo-
wing out of the input of the operational amplifier. An instru-
ment connected to the output will therefore show a reading.
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