PIERS Proceedings, Marrakesh, MOROCCO, March 20–23, 2011
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The Design of High-impedance and High-voltage Input Amplifier for
Measurement of Electropotentials on Solid-liquid Phase Boundary
Z. Roubal1 , Z. Szabó1 , M. Steinbauer1 , D. Heger2 , and R. Kubásek1
1
Department of Theoretical and Experimental Electrical Engineering, Brno University of Technology
Kolejnı́ 2906/4, Brno 612 00, Czech Republic
2
Department of Chemistry, Faculty of Science, Masaryk University
Kamenice 5/A8, Brno 625 00, Czech Republic
Abstract— On the interface of solid-liquid phases of water solutions, certain electric potential
occurs during the freezing process. This is caused uneqal distribution of ions between the solid
and liquid phases. As freezing is often used for the preservation of biological samples, the influence
of the electric field induced by this process upon biological samples is the subject of investigation.
For this purpose, we constructed specialized measuring devices to facilitate the measurement of
this potential.
In this paper, a design will be analyzed of an electrometric amplifier for the measurement of
voltage in the order of hundreds volts. Because the measured source shows a very high inner
resistance and low capacity, the amplifier input resistance must be greater than 1014 Ω with a
negligible parallel capacity. For the same reason, using an input voltage divider is a problematic
step. Such a high input impedance can be achieved only when applying a special input amplifier.
A standard operational amplifier shows a measurement range of about ±10 V, for the expansion
of this range excluding the input voltage divider, it is necessary to use voltage shifting for the
operational amplifier power supply. A circuit with floating supplies is susceptible to oscillation
unless supported by the right frequency corrections. The proposed electrometric amplifier with
a high voltage input range will be simulated using Pspice.
1. INTRODUCTION
In the field of research into biochemical substances there has emerged the need to measure the
electric potential of phase changes upon solidification of aqueous solutions. This potential is referred
to as freezing potential or Workam-Reynolds phenomenon [1–5]. At this point, it is necessary to
note that the results obtained by the related researchers differed in respect to the applied measuring
method, measurement system configuration, and methodology. When performing a comparison of
these results, readers of the herein mentioned reports can identify within the measured potentials
a scatter that ranges between 100 mV and hundreds of volts for a chemically identical sample. The
principle of freezing potential consists in the generation of an electric charge on the interface of
fluid and solid [2]; the described process is typical of water or liquid solutions! A separation of
charge occurs between ice and a sulution. This separation results from differences in the partition
coefficients of anions and cations and it generates an electric potential, which is known as freezing
potential. The mobility of ions is changed upon freezing. A model of freezing potential was
first created by Lefebre [2]; the proposed approach includes charge generation, redistribution and
neutralization. This model was later perfected by Bronshteyn and Chernov, who included a charge
redistribution in ice due to ionic diffusion and H+ ion flow driven by the electrical field in the
crystal.
2. THE MEASURING APPARATUS
The basic research was materialized in laboratory conditions showing a lower degree of repeatability.
For this reason, we designed and built a measuring apparatus (Fig. 1); the illustration of its primary
internal structure is provided in Fig. 2. In the described measurement device concept, cooling has
been preset in the direction from the bottom to the upper sections of the apparatus. Thus, we
can attain repeated generation and measurement of an electric potential on the interface of the
sample solid phase. At the moment when ice reaches the inner electrode, discharge occurs and the
measured electric voltage will drop to zero.
The entire measuring device is positioned in a thermally insulated vessel where liquid nitrogen
will be produced (nitrogen boiling temperature equals to −195, 80◦ C). The lower section of the
vessel shows a shape and configuration enabling high-quality accumulation and transfer of heat
(with cooling realized by means of liquid nitrogen); simultaneously, however, the vessel facilitates
Progress In Electromagnetics Research Symposium Proceedings, Marrakesh, Morocco, Mar. 20–23, 2011 1163
Figure 1: The assembled apparatus.
Figure 2: Internal structure of the measuring apparatus.
Figure 3: The measuring apparatus overall diagram.
the elimination of problems resulting from the change in linear expansion. In the shielded vessel
having a hot and a cold section there exists free, gas-filled space that prevents the occurrence of
air humidity freezing. At the very initial stage of measurement, the head housing a capillary as
well as the sample to be tested is inserted in an overcooled duralumin monobloc; thus, a repeatable
refrigeration process starts. At the moment of the sample insertion in the overcooled space, the
tested sample phase begins to change and the fluid-solid phase interface progressively moves upwards; now, freezing potential is measured. Following the phase change reach of the other electrode,
freezing potential will dischare itself.
3. THE PROPOSED MEASURING STRING
An electric potential on the interface between two phases of the sample behaves as a source of
potential with a high differential resistance. The measurement must be realized using a system
with an electrometric amplifier at the input. The duralumin monobloc temperature is measured
by the help of a PT100-type metal resistive sensor.
The supply of the sensor materializes through the source of constant current 1mA; in addition,
voltage scanning is realized on the sensing device (element). The digitization of the related two
voltages takes place through an Agilent U2352A data acquisition measurement module. Further,
the data measured are processed by a PC using the Agilent VEE environment. The temperature provided by the PT100-type platinum sensor is evaluated by means of solving the quadratic
equation. The measurement result consists in the time behaviour of freezing potential in the time
domain.
4. THE ELECTROMETRIC AMPLIFIER
In the process of designing an electrometric amplifier there may occur a certain technical problem
concerning high input voltage. As a consequence, the measured voltage value can range within
several hundreds of volts. A standard solution consists in applying a resistor divider at the input,
Fig. 4. In electrobiology, however, we can not use this type of solution as the signal source contains
capacity in orders of pF; even when special high-ohm 100 GΩ resistors are used, the discharge time
constant of the circuit ranges within orders of tenths of seconds. it is therefore obvious from the
description that the discussed solution does not help us to meet the desired target.
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PIERS Proceedings, Marrakesh, MOROCCO, March 20–23, 2011
One of the proposed methods of solution to the problem lies in the application of an electrometric
amplifier not equipped with any input divider; in this type of amplifier, then, we assume floating
power supply in relation to the input voltage. Fig. 8 shows the diagram of such an amplifier. Here,
the input voltage is amplified by an electrometric amplifier supplied from a floating source. The
related output is connected to a high-voltage amplifier supplied by a raising voltage changer. This
output manages a high-voltage straight line source, which shifts voltage levels of the electrometric
amplifier. The input voltage is read at the high-voltage source output. The internal resistance of
this configuration is defined only by the electrometric amplifier volume resistivity and may reach up
to 1014 Ω. The discharge time constant is, with inner capacity of the signal source, approximately
1000 s, which will not affect the measured values of freezing potential.
Figure 4: Resistor divider at the input.
Figure 5: The block diagram of an electrometric amplifier with floating power supply.
Figure 6: A floating supply electrometric amplifier:
an instance of oscillation.
Figure 7: A frequency compensated amplifier.
A diagram of this type of electrometric amplifier has been designed and simulated using Spice. In
the design, an OZ LMC6041 was utilized as an electrometric amplifier; typically, its input current
is 2 fA. Fig. 6 presents the overall diagram. Operational amplifiers having the input voltage of
300 V are generally not available, therefore we built a high-voltage amplifier based on discrete hv
transistors. Owing to the connection sensitivity to oscillation, it is necessary to use correct values
of capacitor C1 . With respect to the maximum input resistance, the amplifier does not have input
protection. This problem is solved through the application of RC filter(s) R10 and C2 . The filter
restrains voltage spikes at the input and the amplifier is capable of monitoring the changes occurring
at its own input.
Figure 7 provides an example of possible oscillations: the input voltage is shown as the violet
course, while the output voltage pertains to the dark green course. Fig. 8 illustrates the situation following compensation. At the beginning of the measurement, the amplifier input must
be short-circuited in order to facilitate stabilization of the initial conditions. Simulation in the
Pspice environment indicates input resistance at 1014 ohm. The amplifier input current is markedly
represented by the charging of capacitor C2 during the input voltage changes.
Progress In Electromagnetics Research Symposium Proceedings, Marrakesh, Morocco, Mar. 20–23, 2011 1165
Figure 8: Concrete diagram of a floating supply electrometric amplifier (after compensation).
14
0
12
10
U [V]
U [V]
-2
-4
-6
8
6
4
-8
2
0
-10
0
100
200
300
0
50
Figure 9: Only the charging of spurious capacities.
150
Figure 10: The freezing potential with a parasitic
exponential.
2
6
4
0
2
-2
-4
0
U [V]
U [V]
100
t [s]
t [s]
-2
-6
-8
-4
-10
-6
-12
-8
-14
0
20
40
t [s ]
60
80
0
20
40
60
80
t [s ]
Figure 11: Freezing potential with the elimination of spurious capacities charging for two instances of
measurement (the second measurement of refrigeration commencement at sec. 30).
5. THE MEASURED DATA
Using a special electrometric amplifier as well as measurement apparatuses, we measured the potentials of chemical solutions. At the initial stage of the experiments, the measurement was degraded
by an electric charge in certain parts of the measuring apparatus. The effects on the concerned
parts manifested themselves adversely during the experiment evaluation. Voltage surge caused by
the freezing of the solution occurred non-repestedly and its amplitude showed different character-
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PIERS Proceedings, Marrakesh, MOROCCO, March 20–23, 2011
istics. The situation is described in Fig. 9, which shows the charging of spurious capacities. Fig. 10
shows the freezing potential added to an erroneous signal from the electric charge of structural
parts of the apparatus.
6. CONCLUSION
A special electrometric amplifier has been designed and frequency-compensated. The amplifier is
capable of performing measurement in the order of hundreds of volts with a large input resistivity.
By the help of the measuring apparatus, we measured potential at the interface of the sample
phase change. The measured data show that this potential can be repeatedly measured using a
structurally modified apparatus.
ACKNOWLEDGMENT
The work described in the paper was financially supported by the research project GA102/09/0314,
GACR 203/09/P445, research plan MSM 0021630513, and project of the BUT Grant Agency
FEKT-S-10-13.
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