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A LOW COST PRECISION CURRENT INTEGRATOR
P. Bhaskar *, S.R. Banerjee,
Variable Energy Cyclotron Centre, 1/AF, Bidhan Nagar, Kolkata - 700064
Abstract
Considering very high cost and non serviceability
issues, a high performance, yet low cost precision Current
Integrator having a six digit counter and comparable to
commercially available models has been developed and is
being presently used to measure beam current in the
room temperature Cyclotron at VECC. In this paper, we
present the design and development of the unit.
INTRODUCTON
Measuring low DC currents often demands a lot more
than a Digital Multi Meter (DMM) can deliver [1].
Generally, DMMs lack the sensitivity required to measure
currents less than 100µA. Even at higher currents, a
DMM’s input voltage drop of hundreds of milli volts can
make accurate current measurements impossible.
Electrometers can measure low currents very accurately,
but the circuitry needed to measure extremely low
currents, combined with functions like voltage, resistance,
and charge measurement, can increase an electrometer’s
cost significantly. The Current integrator presented in this
paper combines the economy and ease of use of a DMM
with low current sensitivity near that of an electrometer.
PRODUCT OVERVIEW
This bench top instrument is a combined ampere/coulomb
meter which has a measuring range of 1nA to 100µA full
scale (F.S) and 1nC to 100µC. These ranges can be
selected by a front panel located eleven way rotary
switch. The instrument also has a Zero and Cal control for
calibrating the analog meter to exact zero position and
perfect full scale reading respectively. It displays the
current reading via a front panel analog meter and the
charge reading by a mechanical counter which provides
10 clicks per second at meter full scale. Keeping in view
the advantage of having more digits, a six digit
mechanical counter, with reset has been provided for
charge counting purpose, as compared to five digit
counters available in commercial models. This counter
generates an audible counting signal which enables the
operator to detect even small changes in beam current
intensity without looking at the instrument. The unit
automatically displays the input polarity as well as out of
range using front panel mounted LED indicators. In
addition, an Analog output of ± 1V F.S for other
monitoring applications or interfacing purpose is
provided. Considering more flexibility and for ease of
counting purpose,
a TTL pulse output, 10Hz F.S is
provided for measuring charge counts externally using
NIM Scalars or similar counting instruments.
DESIGN OVERVIEW
The schematic of the current integrator circuit was
prepared using software PROTEUS ISIS, version 7.
Packages were prepared for the op-amps which were not
available in the components library. The schematic was
then net-listed to PROTEUS ARES and the layout of the
PCB (two layered) was prepared with DRC (Design Rule
Check).The PCB was made with one side full ground
plane. Care has been taken to isolate the analogue and the
digital part of the circuit. Each block has been decoupled
with proper decoupling capacitor and adequate grounding
techniques have been employed [2].
Circuit Description
The circuit consists of an analog part and a digital part
as shown in the block diagram in Fig.1. The analog part
Figure 1: Block Diagram
*[email protected]
receives the current and measures it in meter scale
indicating current and the digital part displays the
quantity of charge with the help of a mechanical digit
counter. As shown in Fig. 2, the circuit of this Current
Integrator is realized on a double sided glass epoxy
copper clad laminate, FR4 grade having bottom side
ground plane.
feedback section which consists of an ultra precision
operational amplifier, matched pair transistor in push pull
mode and 0.5% M.F resistances in the feedback resistor
loop. Different Op Amps have been tried in the feedback
section. Finally, the feedback Op Amp has been chosen
considering its very low typical input offset voltage of
10µV at room temperature and outstanding offset voltage
drift typically in the order of 0.1µV/0C. It also has the
highest precision performance of any op-amp currently
available and this extremely low offset voltage and ultra
low drift making it ideal for the purpose .Elaborate test
steps have been undertaken for selecting the input and
feedback Op-Amps considering their long time stability
and accuracy, before being assembled on to the board. To
maintain the accuracy required all the feedback resistors
are 50 ppm metal film having tolerance of 0.5%.
Analog Output, Out of Range and Meter Section
Figure 2: The assembled PCB
Analog Section: Input
The input section includes a micro power, high
performance ultra low drift operational amplifier which is
used here as an integrator [3]. This Op Amp has been
chosen for its very low typical input offset voltage of 3µV
and offset voltage drift of 10nV/0C. The input current is
integrated on a stable low leakage capacitor. Its output is
an integrated value of the input current. At full scale the
output of this section is ± 1V, i.e., when current input is
± 1nA in the selected range of 1nA. This section also
incorporates protective measures to protect the circuitry
from any sudden spark or spurious current.
Feedback section:
The resultant voltage of the input section is fed back to
The output of Input section fed to inverting input of
very low noise JFET input Op Amp with unity gain to
get a ± 1V F.S output in the front panel BNC connector
for further monitoring purpose. Similarly this output is
also fed to different subsections such as the Input Polarity
section, Input Over Range section and Analog Meter
section. The instrument displays the input polarity and
input over range via light emitting diodes in the front
panel.
Digital Section
For the purpose of counting charge, a special digital
section is designed to achieve this. The output of the input
section is fed to a very low noise JFET input Op Amp
used as a buffer and subsequently to a low cost Voltage to
Frequency Converter. The chosen VFC has a linearity
error of 0.03% for a 250 KHz F.S, low drift of 4µV/0C
and overall temperature coefficient of ± 50 ppm/0C.
Figure 3: Current Integrator being tested with Keithley Pico Ampere Source
and it converts the voltage at its input to its actual
corresponding frequency. At full scale of the instrument,
output frequency of 164 kHz is obtained, which is further
fed to a 14 stage binary counter to obtain an output
frequency of 10Hz F.S. This pulse is then fed to a low
power precision monostable which generates a pulse
having a width of 20 milli second that is required to drive
the six digit mechanical counter used in the circuit. The
output of the monoshot can also be obtained at TTL
PULSE OUT, which is an output port in the front panel,
through a BNC connector.
Power Supply Section:
A simple yet effective power supply has been
developed for this unit. The input Op Amp requires a
dedicated power supply of ± 5VDC and all the other
blocks of the circuit requires ± 15VDC besides the 6 digit
mechanical counter which requires + 24VDC. Hence,
easily available 3 terminal Voltage Regulators has been
used for the purpose with standard protection. A LED is
also provided in the front panel which indicates that
power supply is ON.
TEST RESULTS
Multi level testing has been performed in developing
this unit. The printed circuit board has been tested
thoroughly for track short, track open or any cracks. The
input Op Amp and the feedback Op Amp has been tested
for a long time using offset adjustments with current input
fed before final optimization. The accuracy and stability
has been found to be very much satisfactory. Similarly,
the feedback resistors have been tested to have very low
tolerance and their value was exact to that of the true
value. Finally the meter has been calibrated to read the
exact value of the current input. The unit has been
thoroughly tested using Keithley Current Source, model
6221 as shown in Fig.3. Counts observed in the
mechanical counter were recorded simultaneously in an
external NIM Scalar using the pulse output to check for
any disparity.. Each input range has been tested for long
period of time. After careful study for more than a month,
the accuracy and long terrn drift were found to be not
more than 1% of F.S. The input and the feedback op amp
have been selected to have the minimal offset voltage and
minimum offset voltage drift as described previously.
ACKNOWLEDGMENT
The authors gratefully acknowledge the contributions
of R. N. Singaraju, Shuaib. A. Khan, J. Saini and G.C.
Santra for their valuable suggestions and extended help
during the period. They would also like to express their
gratitude to S. Chatterjee and P.S. Chakraborty, Cyclotron
Operation, VECC for their valuable feedback time to time
besides all other staff members of Nuclear Electronics
Section who have been a constant source of support and
encouragement.
REFERENCES
[1] Low Current Measurement Handbook, 6th Edition, Keithley
Inc.
[2] Techniques for Grounding Printed Circuit Boards, Texas
Instruments.
[3] Applications of Precision Op Amps, AN6, Linear
Technology.
[4] Linear Technology Product Catalogue.
.