CAPACITIVELY COUPLED BIOSIGNAL INSTRUMENTATION SYSTEM FOR POWER AND SIGNAL ISOLATION
SEMINAR REPORT
Submitted by
Archana Nair . I
Seventh Semester B.Tech
Applied Electronics and Instrumentation
DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING
COLLEGE OF ENGINEERING
TRIVANDRUM
2010
ABSTRACT
Requirements for patient safety and a high interference rejection ratio in medical equipment create a demand for effective isolation devices. A system scale approach that uses capacitive coupling for power and signal isolation is presented. In addition, the development of an instrumentation system prototype that applies microwaves for power exchange and bidirectional data transfer across the isolation barrier is described. The system consists of an isolated transducer unit, a central unit, and a single coaxial cable between the units. The isolation capacitance is as low as 1.6 pF, inclusive of the digital data transfer and power exchange up to 600 mW of isolated direct current (dc) power. The system is suitable for line-powered biopotential measurements and it is shown that reducing the isolation capacitance from 180 to 1.6 pF improves the power line rejection by 30 dB in a typical electrocardiogram (ECG) measurement setup.
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CHAPTER 1
INTRODUCTION
BIOPOTENTIAL signals such as electrocardiogram (ECG), electromyography (EMG), and electroencephalography (EEG) are often recorded in the presence of electromagnetic interference. The input-referred interfering voltage due to common mode voltage (VCM) is mainly determined by the potential divider effect caused by the essential differences in impedance between individual electrodes. This is the main reason to reduce the VCM in respect to ground-referred body voltage(VB) and it is done in two ways (Fig.1) as follows. 1) A neutral electrode is connected between the body and the amplifier to provide a low-impedance path between the patient and the isolated ground. A driven-right-leg (DRL) circuit is typically applied to further lower the effective impedance of the neutral electrode . 2) Isolation devices are used to provide data and power transfer and the galvanic insulation between the amplifier and the grounded electronics. This is mainly for safety reasons , but the isolation capacitance also determines the induced VCM.
Fig 1.1. Typical biopotential measurement setup with the neutral electrode and isolation
device.
The commercially available isolation device lineup includes analog isolation amplifiers, isolated digital couplers, optocouplers, and isolated direct current (dc/dc) converters. Some of the analog isolation amplifiers and digital couplers also include small levels ( 50 mW) of isolated power. Inside the isolation device, a number of barrier arrangements and signal modulation schemes are available. The three techniques in common use for on-chip signal isolation are optical isolation, transformer isolation and capacitive isolation. In addition, optic fibers and radio transceivers are applied for signal transfer in battery powered systems. Power isolation is typically accomplished by a transformer coupling inside a dc/dc converter, but also an optical power supply and on-chip capacitive coupling power exchange can be used. Regardless of the coupling technology, there is a finite amount of capacitance between the input and output of all isolation devices. The CISO is the total capacitance of all the parallel isolation devices and it determines the alternating current (ac) characteristics of the isolation barrier. For example, the input–output capacitance of a commercially available isolated dc/dc converter is typically 15–150pF.
Fig 1.2. Diagram of (a) a typical bedside patient monitor with passive lead trunk and (b) the new solution using the transducer unit with isolation.
Fig. 1.2(a) presents a conventional way to set up a patient monitoring system. Electrode leads are collected to a trunk cable, which connects to the amplifier input with the patient cable. The amplifier modules with isolation are placed in a chassis that provides a connection to the display and user interface via common data bus. In this topic a new technology that allows bidirectional data transfer and power exchange across an isolation barrier using capacitive coupling is described. Further, the development of instrumentation for measuring biopotentials using the new technology is described. Also the amount of isolated dc power is measured, evaluate the system’s suitability for biopotential measurements and its capability to reduce common mode interference in a typical ECG measurement setup.
CHAPTER 2
BIOPOTENTIALS
2.1 Action Potential
In physiology an action potential is a short-lasting event in which the electrical membrane potential of a cell rapidly rises and falls, following a stereotyped trajectory. Action potentials occur in several types of animal cells, called excitable cells, which include neurons, muscle cells, and endocrine cells. In neurons, they play a central role in cell-to-cell communication. In other types of cells, their main function is to activate intracellular processes. In muscle cells, for example, an action potential is the first step in the chain of events leading to contraction. In beta cells of the pancreas, they provoke release of insulin.
Fig 2.1 Biopotentials
Action potentials are generated by special types of voltage-gated ion channels embedded in a cell's plasma membrane. These channels are shut when the membrane potential is near the resting potential of the cell, but they rapidly begin to open if the membrane potential increases to a precisely defined threshold value. When the channels open, they allow an inward flow of sodium ions, which changes the electrochemical gradient, which in turn produces a further rise in the membrane potential. This then causes more channels to open, producing a greater electric current, and so on. The process proceeds explosively until all of the available ion channels are open, resulting in a large upswing in the membrane potential. The rapid influx of sodium ions causes the polarity of the plasma membrane to reverse, and the ion channels then rapidly inactivate. As the sodium channels close, sodium ions can no longer enter the neuron, and they are actively transported out the plasma membrane. Potassium channels are then activated, and there is an outward current of potassium ions, returning the electrochemical gradient to the resting state. After an action potential has occurred, there is a transient negative shift, called the afterhyperpolarization or refractory period, due to additional potassium currents. This is the mechanism which prevents an action potential traveling back the way it just came.
Fig2.2 Propagation of action potential
Ions will flow across a membrane from the higher concentration to the lower concentration (down a concentration gradient), causing a current. However, this creates a voltage across the membrane that opposes the ions motion. When this voltage reaches the equilibrium value, the two balance and the flow of ions stops.
Fig 2.3 Polarization and depolarization.
Electrical signals within biological organisms are, in general, driven by ions. The most important cations for the action potential are sodium (Na+) and potassium (K+). Both of these are monovalent cations that carry a single positive charge. Action potentials can also involve calcium (Ca2+), which is a divalent cation that carries a double positive charge. The chloride anion (Cl−) plays a major role in the action potentials of some algae, but plays a negligible role in the action potentials of most animals.
2.2 Cell membrane
Ions cross the cell membrane under two influences: diffusion and electric fields. Each neuron is encased in a cell membrane, made of a phospholipid bilayer. This membrane is nearly impermeable to ions. To transfer ions into and out of the neuron, the membrane provides two structures.
1. Ion channels
2. Ion pumps
2.2.1 Ion channels
Ion channels are integral membrane proteins with a pore through which ions can travel between extracellular space and cell interior. Most channels are specific (selective) for one ion. The channel pore is typically so small that ions must pass through it in single-file order. Channel pore can be either open or closed for ion passage, although a number of channels demonstrate various sub-conductance levels. When a channel is open, ions permeate through the channel pore down the transmembrane concentration gradient for that particular ion. Rate of ionic flow through the channel, is determined by the maximum channel conductance and electrochemical driving force for that ion. The action potential is a manifestation of different ion channels opening and closing at different times. A channel may have several different states (corresponding to different conformations of the protein), but each such state is either open or closed. In general, closed states correspond either to a contraction of the pore—making it impassable to the ion—or to a separate part of the protein, stoppering the pore.
Fig 2.3 Distribution of ions
2.2.2 Ion pumps
The ionic currents of the action potential flow in response to concentration differences of the ions across the cell membrane. These concentration differences are established by ion pumps, which are integral membrane proteins that carry out active transport, i.e., use cellular energy (ATP) to "pump" the ions against their concentration gradient. Such ion pumps take in ions from one side of the membrane (decreasing its concentration there) and release them on the other side (increasing its concentration there). The ion pump most relevant to the action potential is the sodium–potassium pump, which transports three sodium ions out of the cell and two potassium ions in. As a consequence, the concentration of potassium ions K+ inside the neuron is roughly 20-fold larger than the outside concentration, whereas the sodium concentration outside is roughly ninefold larger than inside. Ion pumps influence the action potential only by establishing the relative ratio of intracellular and extracellular ion concentrations. The action potential involves mainly the opening and closing of ion channels, not ion pumps. If the ion pumps are turned off by removing their energy source, or by adding an inhibitor such as ouabain, the axon can still fire hundreds of thousands of action potentials before their amplitudes begin to decay significantly. In particular, ion pumps play no significant role in the repolarization of the membrane after an action potential.
Fig 2.4 Action potential
CHAPTER 3
BIOPOTENTIAL AMPLIFIER
Amplifiers are an important part of modern instrumentation systems for measuring biopotentials. Such measurements involve voltages that often are at low levels, have high source impedances, or both. Amplifiers are required to increase signal strength while maintaining high fidelity. Amplifiers that have been designed specifically for this type of processing of biopotentials are known as biopotential amplifiers.
Fig 3.1 Schematic design of the main stages of a biopotential amplifier. Three electrodes connect the patient to a preamplifier stage. After removing dc and low-frequency interferences, the signal is connected to an output lowpass filter through an isolation stage which provides electrical safety to the patient, prevents ground loops, and reduces the influence of interference signals.
3.1 Functions
The essential function of a biopotential amplifier is to take a weak electric signal of biological origin and increase its amplitude so that it can be further processed, recorded, or displayed. Usually such amplifiers are in the form of voltage amplifiers, because they are capable of increasing the voltage level of a signal. Nonetheless, voltage amplifiers also serve to increase power levels, so they can be considered power amplifiers as well. In some cases, biopotential amplifiers are used to isolate the load from the source. In this situation, the amplifiers provide only current gain, leaving the voltage levels essentially
unchanged.
3.2 Requirements
To be useful biologically, all biopotential amplifiers must meet certain basic requirements. They must have high input impedance, so that they provide minimal loading of the signal being measured. The characteristics of biopotential electrodes can be affected by the electric load they see, which, combined with excessive loading, can result in distortion of the signal. Loading effects are minimized by making the amplifier input impedance as high as possible, thereby reducing this distortion. Modern biopotential amplifiers have input impedances of at least 10 MΩ. The input circuit of a biopotential amplifier must also provide protection to the organism being studied. Any current or potential appearing across the amplifier input terminals that is produced by the amplifier is capable of affecting the biological potential being measured. In clinical systems, electric currents from the input terminals of a biopotential amplifier can result in microshocks or macroshocks in the patient being studied—a situation that can have grave consequences. To avoid these problems, the amplifier should have isolation and protection circuitry, so that the current through the electrode circuit can be kept at safe levels and any artifact generated by such current can be minimized. The output circuit of a biopotential amplifier does not present so many critical problems as the input circuit. Its principal function is to drive the amplifier load, usually an indicating or recording device, in such a way as to maintain maximal fidelity and range in this readout. Therefore, the output impedance of the amplifier must be low with respect to the load impedance, and the amplifier must be capable of supplying the power required by the load. Biopotential amplifiers must operate in that portion of the frequency spectrum in which the biopotentials that they amplify exist. Because of the low level of such signals, it is important to limit the bandwidth of the amplifier so that it is just great enough to process the signal adequately. In this way, we can obtain optimal signal-to-noise ratios (SNRs). Biopotential signals usually have amplitudes of the order of a few millivolts or less. Such signals must be amplified to levels compatible with recording and display devices. This means that most biopotential amplifiers must have high gains—of the order of 1000 or
greater. Very frequently biopotential signals are obtained from bipolar electrodes. These electrodes are often symmetrically located, electrically, with respect to ground. Under such circumstances, the most appropriate biopotential amplifier is a differential one. Because such bipolar electrodes frequently have a common-mode voltage with respect to ground that is much larger than the signal amplitude, and because the symmetry with respect to ground can be distorted, such biopotential differential amplifiers must have high commonmode- rejection ratios to minimize interference due to the common-mode
signal.
A final requirement for biopotential amplifiers that are used both in medical applications and in the laboratory is that they make quick calibration possible. In recording biopotentials, the scientist and clinician need to know not only the waveforms of these signals but also their amplitudes. To provide this information, the gain of the amplifier must be well calibrated. Frequently biopotential amplifiers have a standard signal source that can be momentarily connected to the input, automatically at the start of a measurement or manually at the push of a button, to check the calibration. Biopotential
amplifiers that need to have adjustable gains usually have a switch by which different, carefully calibrated fixed gains can be selected, rather than having a continuous control (such as the volume control of an audio amplifier) for adjusting the gain. Thus the gain is always known, and there is no chance of it being accidentally varied by someone bumping the gain control.
3.3 Defining Terms
i. Common Mode Rejection Ratio (CMRR):
The ratio between the amplitude of a common mode signal and the amplitude of a differential signal that would produce the same output amplitude or as the ratio of the differential gain over the common-mode gain: CMRR = GD/GCM. Expressed in decibels,
the common mode rejection is 20 log10CMRR. The common mode rejection is a function of frequency and source-impedance unbalance.
ii. Isolation Mode Rejection Ratio (IMRR):
The ratio between the isolation voltage, VISO, and the amplitude of the isolation signal appearing at the output of the isolation amplifier, or as isolation voltage divided by output voltage V OUT in the absence of differential and common mode signal: IMRR =VISO/VOUT
.
iii. Operational Amplifier (op-amp):
A very high gain dc-coupled differential amplifier with single-ended output, high voltage gain, high input impedance, and low output impedance. Due to its high open loop gain, the characteristics of an op-amp circuit only depend on its feedback network. Therefore, the integrated circuit op-amp is an extremely convenient tool for the realization of linear amplifier circuits.
CHAPTER 4
ISOLATION BARRIERS
4.1 Types of Isolation Barriers
Three types of isolation barriers commonly used in the conventional biopotential amplifier set up are
Transformer coupling
Optical coupling
Capacitive coupling
Fig 4.1 Equivalent circuit of an isolation amplifier. The differential amplifier on the left transmits the signal through the isolation barrier by a transformer, capacitor, or an opto-coupler.
a) Transformer coupling
Transformers are inherently high frequency AC devices. If transformer coupling is used, then there is a need for modulation and demodulation. Also the system becomes large in size and bulky. The use of transformers as isolation barriers also affects the portability of the system.
b) Optical coupling
In optical coupling optical signal is modulated in proportion to the electric signal and transmitted to the detector. The signal is typically pulse code modulated to circumvent the inherent nonlinearity of the LED-phototransistor combination. In this case the size of the system is decreased. But there arises a problem that, the system may easily crept into an error due to the effect of ambient light. Also the current transfer ratio of optical coupling is very low.
c) Capacitive coupling
Using capacitor as the isolation barrier has increased advantages over the above mentioned methods. The first one is that if the size of the capacitor is reduced then the size of the system can be reduced. As the size of the capacitor reduces the induced common mode voltage also reduces which is an added advantage. The possibility of error in the measurement is very low in the case of capacitive coupling technique.
Fig 4.2 Different coupling methods
CHAPTER 5
NEED FOR ISOLATION
5.1 Macro and Micro Shocks
Due to the exposure of patients to different types of biomedical instrumentation system, the chance that a patient can get a fatal shock is very high. Here comes the need of an isolation barrier. An isolation barrier is a very important part of any biomedical instrumentation system which prevents any kind of shocks affecting the patient’s body. The shocks can be divided into two:
1. MACRO SHOCK
2. MICRO SHOCK
5.1.1 Macro Shock
It is the most common type of shock received and occurs when the human body becomes a conductor of electric current passing by means other than directly through the heart. This effect can readly occur with the use of medical electrical equipment as the natural resistance of the skin to current flow is often reduced or bypassed by electrodes and electorde paste or by invasion into mucous membrane.
Fig 5.1 (a) Macro Shock (b) Micro Shock
Macro Shock can occur because of ground vaults and faulty connections to the housing through an operator touching the equipment. The macro shock occurs above {75mA, 200mA} level while the current is below 1-5A. Macro shock depends on body weight, shock duration (>1 sec, T wave affected) and wetness of skin (R = 15kΩ/cm2 – 1MΩ/cm2).
5.1.2 Micro Shock
Certain medical electrical procedures (cardiac catheterization, cardiac output measurement, intracardiac ECG, and cardiac pacing) involve the introduction of an electrical conductor in direct contact with a ventricular heart muscle. These are the sources of micro shocks. Micro shock may not be obvious as no outward and physically visible stimurus or contraction occurs.It is a risk in patients with intracardiac conductors, such as external pacemaker electrodes or saline filled catheters, within the heart. A current as low as 100μAmps directly through the heart, may send a patient directly into ventricular fibrillation.
Micro shocks can occur during invasive procedures in the emergency room, operating room, intensive care unit and catheterization lab. The following devices might directly connect current to the heart: external pace makers, intra cardiac ECG, HIS bundle recordings, catheters for blood pressure, blood samples, blood flow, injection of drugs.
CHAPTER 6
SYSTEM DESCRIPTION AND DEVELOPMENT
6.1 Architecture
A top-level diagram of the new instrumentation system including the patient is shown in Fig.1.2(b). The system consists of two main parts: the central unit and the remote biomedical transducer unit with isolation. The patient is connected to the isolated transducer unit with electrode leads. In the transducer unit, the measured biopotential signal is conditioned, sampled, and transmitted to the central unit through a coaxial cable using a radio transceiver. The transducer unit has also a control circuit for adjusting the signal conditioning parameters. In the central unit, the radio signal is received with another radio transceiver and further transmitted to the display and user interface device, such as a personal computer (PC). The operating power for the transducer unit is generated in the central unit and transmitted to the transducer unit through the same coaxial cable using microwaves. Because microwaves are used for both data and power transfer, capacitive isolation barrier with very low amount of coupling capacitance can be used without significant attenuation of the signal. Fig.6.1. shows the block diagram of the system.
Fig 6.1 Block diagram of the new instrumentation system
6.2 Datatransfer
Bidirectional data transfer across the isolation barrier is arranged for biosignal transfer and simultaneous control of signal conditioning parameters. The solution described here involves Bluetooth as the communication technology, because Bluetooth is already at an advanced level of development. A Bluetooth module, including radio and baseband hardware and the protocol stack is used in this set up. Also, Bluetooth serial port profile (SPP) with maximum data rate of 230 kb/s is implemented inside the module. Any other suitable technology could be used as well.
The system shown in Fig. 6.1 uses a point-to-point Bluetooth page link for bidirectional communications between the units. Even though technology that is meant for wireless devices (Bluetooth) is applied to communications, we want to emphasize that the system is not wireless. The coaxial cable provides a very low loss radio path for Bluetooth radio page link and power exchange and connects the central unit and the transducer unit together.
6.3 Power Exchange
In addition to data transfer, operating power for isolated electronics must be provided. Microwaves are used also for power transfer to minimize the insertion loss of the barrier coupling capacitors, and consequently, to further improve the efficiency of the power feed. A continuous wave personal communication services (PCS) band (1.98 GHz) carrier is generated by a voltage controlled oscillator (VCO).
PCS Band.
Personal Communications Service or PCS is the name for the 1900 MHz radio band used for digital mobile phone services in Canada, Mexico and the United States. Code Division Multiple Access (CDMA), GSM, and D-AMPS systems can be used on PCS frequencies. The FCC, as well as Industry Canada, set aside the frequency band of 1850-1990 MHz for mobile phone use in 1994, as the original cellular phone band at 800-894 MHz was becoming overcrowded. Dual-band GSM phones are capable of working in both the 850 and 1900 MHz bands, although they are incompatible with 900 and 1800 MHz European and Asian systems. However, GSM "world phones" (some of which are known as tri-band or quad-band phones, because they operate in three or four different frequency bands, respectively) offered by North American carriers support both European and domestic frequencies. Outside the USA, PCS is used to refer to GSM-1900.Sprint was the first company to set up a PCS network. Nowadays, the PCS frequencies have been adopted for usage in most parts of the Americas.
The VCO is followed by a custom power amplifier (PA) in the central unit (Fig. 6.1) resulting in a medium power (33 dBm) carrier at the central unit output. The power amplifier consists of a voltage variable attenuator, a driver amplifier, and a power amplifier . The carrier is fed to the transducer unit through the same coaxial cable and the coupling capacitors that are used in data transfer. Any other frequency bands could be used as well. The PCS band is chosen because of good availability and low cost of the commercially available components.
6.4 Barrier Arrangement
The isolation barrier is arranged by cutting both the signal and the return conductors by commercially available, lumped 0.8-pF capacitors with voltage rating of 1000 V. The coupling capacitors are placed in parallel in the transducer unit and the CISO is the sum of these capacitors (1.6 pF). Since the isolation barrier forms the physical interface between the 50Ω coaxial cable and the associated 50Ω microstrip circuitry in the transducer unit it should be matched to 50Ω as well. The input impedance of the coupling capacitors is compensated with ceramic radio-frequency (RF) chip inductors, that were placed in series with the capacitors (Fig. 6.2). This results as a series resonance circuit that was designed and simulated with a circuit simulator (APLAC). The series resonance frequency was tuned as close to the power feeding frequency (1.98 GHz) as possible to minimize the barrier attenuation.
6.5 Diplexers
A diplexer is a passive device that implements frequency domain multiplexing. Two ports (e.g., L and H) are multiplexed onto a third port (e.g., S). The signals on ports L and H occupy disjoint frequency bands. Consequently, the signals on L and H can coexist on port S without interfering with each other.
The chief advantage of a diplexer is that it allows two different devices to share a common communications channel. Typically the shared channel is a long piece of coaxial cable. Rather than run two separate cables, a single cable with diplexers at each end is used. The plan is economical if the diplexers cost less than running the second cable.
Because a single coaxial cable is used for data transfer and power feed, the power signal (1.98-GHz PCS band) and the Bluetooth signal [2.45-GHz industrial, scientific, and medical (ISM) band] can be separated by difference in frequency with specially designed diplexers (Fig. 6.2). To achieve simple, but effective implementation, the generic diplexer circuit was reduced, and a basic t-junction with parallel resonance circuit at the branching line (Fig. 6.2) was designed and simulated with a circuit simulator (APLAC). The parallel resonance frequency was also tuned as close to the power feeding frequency (1.98 GHz) as possible.
Fig 6.2. Circuit diagram of the transducer RF circuitry including the isolation barrier, diplexer and the RF/dc converter.
6.6 RF/dc converter
The microwave power is converted to dc power with a specially designed RF/dc converter. The RF/dc converter consists of an input-matching circuit, a zero-bias diode rectifier in a voltage doubler configuration, and an output bypass filter (Fig.6.2). Commercially available Schottky diode pair was chosen as the rectifier because of its low series resistance (0.65Ω ) and low junction capacitance (6.7 pF) .
The input-matching circuit and the output bypass filter were implemented with commercially available lumped capacitors and RF chip inductors. The RF/dc converter was designed and simulated with circuit simulator (APLAC) using nonlinear harmonic balance (HB) analysis method and the design was verified by RF/dc converter partial prototype. The scalar reflection coefficient and the output dc power were measured as a function of dc load (RL) with 1-W incident 1.98-GHz microwave power with a custom measurement setup (Fig. 6.4). The simulated and measured results are
shown in Fig 6.3
Fig 6.3 Return loss and output dc power of the RF/dc converter partial prototypes as functions of dc load
CHAPTER 7
SYSTEM PERFORMANCE
7.1 Isolated DC power
First, the return loss of the transducer unit and the coaxial cable [device under test (DUT) in Fig.7.1] was measured as a function of frequency (Fig. 7.2).
Fig 7.1 Test set up for the power feed evaluation
Fig 7.2 . Converted dc power in the transducer unit as a function of the central unit RF generator output power.
Second, the amount of isolated dc power available was measured. The output of the RF/dc converter in the transducer unit was connected to resistive load (56Ω) and the output dc power was measured as a function of output RF power of the central unit at 1.98-GHz frequency. As the power curve in Fig. 12 shows, up to 900 mW of dc power can be momentarily transferred across the 1.6-pF isolation barrier to 56 load in the transducer unit. The conversion efficiency of the RF/dc converter is
From Fig. 6.3, we get return loss of 9.5 dB at 56- load, which correspond to 111-mW reflected RF power when the incident RF power is 1 W. The corresponding output dc power is 630 mW. Now, substituting these values in the equation of efficiency, we get 71% conversion efficiency for the rectifier at 56- load and 30-dBm, 1.98-GHz incident RF power. In long term use, the amount of dc power is limited by the specification of diode rectifier absolute maximum power dissipation (250 mW, ). Comparing that to conversion efficiency, we get the maximum output power of 612mWfor the RF/dc converter. This corresponds to little more than 32 dBm of central unit output RF power .
7.2 Induced Common Mode Voltage
The induced VCM can be calculated using a circuit model to analyze capacitive coupling to the patient in which case the induced common mode voltage becomes VCM=VB(ZEO/ZISO)=VB(REO/(1/jωCISO)) when CB>>CISO is assumed and is the ground- referred voltage induced to the patient.
Fig 7.3 (a) Common mode coupling circuit.
(b) Electrode placement for the real ECG measurement.
The induced VCM was measured by connecting the common mode generator [Fig. 7.3(a)] to the ECG cables of the transducer unit. Additional capacitors were placed in parallel to the actual coupling capacitors to vary the CISO and the measurement was performed as a function of frequency. As the results in Fig. 7.6 show, the amount isolation capacitance affects greatly the amount of induced VCM in respect to ground-referred body voltage . In theory, using 1.6-pF isolation capacitance instead of 180 pF improves the common mode rejection by about 40 dB over the whole frequency band. The experimental result was about 30 dB improved (Fig. 7.4), which is nearly equal to the effect of a classic DRL-circuit .
Fig 7.4. Effect of total isolation capacitance on induced VCM
It is assumed that there is a finite amount of stray capacitance between the isolated transducer unit and the earth ground in the test setup. This stray capacitance would be in parallel to CISO, degrading the common mode performance by about 10 dB in comparison to theoretical calculations in the case of small CISO. This is an interesting result, because the same effect is likely to be generally present in all instrumentation, but rarely discussed in the literature.
Fig 7.5. Matching of the transducer unit and the cable as a function of frequency around the PCS band.
7.3Recorded ECG Signal
Fig.7.6 shows the ECG signal measured between the electrode locations V2 and V4 [Fig. 7.3(b)] with the prototype system. The neutral electrode was connected above the right hip. The measurement was performed with unshielded 0.5-m electrode cables. The offset level and the gain of the amplifier were adjusted to get a proper signal level. The bandwidth of the measurement was 0.05–150 Hz with total gain of 600. The signal was sampled with 16-b resolution, 2.56-V ADC span, and a 1-kHz sampling rate. The power line interference is not seen in the signal, but the random noise is observed during the diastole phase of the cardiac cycle.
Fig 7.6. ECG signal recorded with the new instrumentation system between the electrode locations V2 and V4.
CHAPTER 7
CONCLUSION
The concept of the new solution that provides an interface for isolated data and power transfer using capacitive coupling is proven with a fully functional prototype. Isolation capacitance of 1.6 pF inclusive of the bidirectional data transfer and power exchange was achieved. Advantages of the new instrumentation solution are improved patient safety and common mode rejection due to very low barrier capacitance and still reasonable amount (600 mW) of isolated dc power. It is suggested that the system has plenty of potential targets for applications in the field of bio instrumentation. Further development will focus on increasing the voltage rating of the barrier to achieve medical grade isolation and optimization of the microwave power supply to reduce the internal electromagnetic interference (EMI) and to increase the amount of available isolated dc power and power transfer efficiency. Also, the size of the isolated transducer unit will be reduced to enable seamless integration to the patient cable.
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