R
DC Amplifier
1 PURPOSE AND DESCRIPTION
The DC Amplifier is a versatile instrument for the measurement of very low DC currents, for the measurement of charges - in particular in experiments on electrostatics - and also
for the quasi-static measurement of direct voltages.
The selection of the 12 measurement ranges is made using
buttons. A matrix of light emitting diodes indicates the active
measurement range. An output voltage of 10V corresponds
to a full scale deflection. Any voltmeter, recorder or computer measurement system (COMEX, COBRA, CENT) can be
used for the display.
2 OPERATION
2.1 Initial operation
The DC Amplifier is connected to the AC mains using the
equipment connecting lead supplied with the instrument.
Please ensure that the mains voltage agrees with the operating voltage details shown on the nameplate. The rectangular fuse holder situated above the equipment connection
plug can only be opened with the aid of a screwdriver or similar tool when the mains lead is withdrawn.
Replacement fuse: M 0.2 C.
The mains switch is located at the back of the unit. The instrument is ready for operation immediately after being switched on.
In order that the measurement accuracy can be maintained,
a warm-up time of about 5 minutes should be allowed before taking measurements.
2.2 FUNCTIONAL ELEMENTS AND CONTROLS
1.1 BNC socket INPUT I, Q
for connecting the measurement circuit in the operating modes for current and charge measurement. The
outer pole is connected with the equipment ground
and the protective earth.
1.2 BNC socket INPUT V
for connecting the measurement circuit in the operating mode for voltage measurement. The outer pole is
connected with the equipment ground and the protective earth.
2 Button I
for switching into the operating mode Current measurement.
3 Button Q
for switching into the operating mode Charge measurement.
4 Button V
for switching into the operating mode Voltage measurement.
5 Socket Ground
connected internally with the equipment ground and
the protective earth conductor.
6 Pair of sockets OUTPUT
for the connection of an instrument with a range
10Vdc range.
The internal voltage limit becomes active at an output
voltage of about 10.5V - i.e. the output voltage then
does not correspond to the measurement.
7 Button INVERT
for inverting the output signal.
8 Button
for reducing the measurement range in all operating
modes.
PHYWE SYSTEME GMBH ·Robert-Bosch-Breite 10 · D-37079 · Göttingen · Telefon (0551) 604-0 ·Telefax
(05 51)6041 07
13620.93
Operating Instructions
The unit complies
with the corresponding
EC guidelines.
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 output 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 measuring 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 capacitor is connected to the previously discharged input (button 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 completely transferred from the capacitor C, which is under investigation, 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 involving 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 avoided.
The charge measurement operating mode can also be used
for the measurement of voltages. Depending on the characteristics 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 internal resistance is greater by a factor of 50 in the Q operating mode.
The charge measurement ranges 1000nAs...0.1nAs correspond 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 measurement 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 voltages. 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 measurement 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 connecting 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 connected 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 dropped across the input does not exceed 250Vdc, then no damage 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 resistance 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 flowing out of the input of the operational amplifier. An instrument connected to the output will therefore show a reading.
13620.93
2
3 TECHNICAL DATA
General
Inputs BNC sockets
Overload protection ±250V in all operating modes
Output Pair of 4mm safety sockets
Voltage 0...10V
Output resistance 1kΩ
Overload protection short-circuit proof
Current measurement
Measurement ranges 100µA. 10µA...10pA (8 ranges)
Accuracy typ. ≤ 3%
Zero-point drift typ. ≤ 0.5%/K
Voltage drop typ. ≤ 0.5mV
Minimum external resistance
Range:
100µA 100Ω
10µA 1kΩ
1µA 10kΩ
100nA 100kΩ
10nA 1MΩ
1nA 10MΩ
100pA 100MΩ
10pA 1GΩ
Voltage measurement
Measurement ranges 10V, 1V...100µV (6 ranges)
Accuracy typ. ≤ 3% (ranges 10V...1mV)
typ. ≤ 5% (range 100µV)
Zero-point drift typ. ≤ 1%/K (ranges 10V...1mV)
typ. ≤ 2%/K (range 100µV)
Internal resistance >10
11
Ω
Charge measurement
Measurement ranges 1000nAs, 100nAs...0.1nA
(5 ranges)
Accuracy typ. ≤ 3%
Measurement capacitor 100nF ± 1%
Insulation resistance > 5 · 10
12
Ω
Self-charge rate typ. < ± 12pAs / min
4 GUARANTEE
We grant a guarantee of 6 months on equipment that we
have supplied; it does not include natural wear and tear and
faults which are the result of improper handling.
The manufacturer can only be regarded as being responsible for the proper function and safety characteristics of the
equipment if maintenance, repair and modifications have
been carried out by the manufacturer or by agents expressly authorised by the manufacturer.
3
13620.93
Loading...