Elektronika BIOMEDIC

Nagel, J. H. “Biopotential Amplifiers.”
The Biomedical Engineering Handbook: Second Edition.
Ed. Joseph D. Bronzino
Boca Raton: CRC Press LLC, 2000

Biopotential Amplifiers

70.1 Basic Amplifier Requirements
Interferences
70.2 Special Circuits
Instrumentation Amplifier • Isolation Amplifier and Patient
Safety • Surge Protection • Input Guarding • Dynamic
Range and Recovery • Passive Isolation Amplifiers • Digital
Electronics
Biosignals are recorded as potentials, voltages, and electrical field strengths generated by nerves and
muscles. The measurements involve voltages at very low levels, typically ranging between 1 µV and
100 mV, with high source impedances and superimposed high level interference signals and noise. The
signals need to be amplified to make them compatible with devices such as displays, recorders, or A/D
converters for computerized equipment. Amplifiers adequate to measure these signals have to satisfy very
specific requirements. They have to provide amplification selective to the physiological signal, reject
superimposed noise and interference signals, and guarantee protection from damages through voltage
and current surges for both patient and electronic equipment. Amplifiers featuring these specifications
are known as
biopotential amplifiers
. Basic requirements and features, as well as some specialized systems,
will be presented.
70.1 Basic Amplifier Requirements
The basic requirements that a biopotential amplifier has to satisfy are:
• the physiological process to be monitored should not be influenced in any way by the amplifier
• the measured signal should not be distorted
• the amplifier should provide the best possible separation of signal and interferences
• the amplifier has to offer protection of the patient from any hazard of electrical shock
• the amplifier itself has to be protected against damages that might result from high input voltages
as they occur during the application of defibrillators or electrosurgical instrumentation
A typical configuration for the measurement of biopotentials is shown in Fig. 70.1. Three electrodes,
two of them picking up the biological signal and the third providing the reference potential, connect the
subject to the amplifier. The input signal to the amplifier consists of five components: (1) the desired
biopotential, (2) undesired biopotentials, (3) a power line interference signal of 60 Hz (50 Hz in some
countries) and its harmonics, (4) interference signals generated by the tissue/electrode interface, and
(5) noise. Proper design of the amplifier provides rejection of a large portion of the signal interferences.
The main task of the differential amplifier as shown in Fig. 70.1 is to reject the line frequency interference
that is electrostatically or magnetically coupled into the subject. The desired biopotential appears as a
voltage between the two input terminals of the differential amplifier and is referred to as the
differential
Joachim H. Nagel
University of Stuttgart






FIGURE 70.1
Typical configuration for the measurement of biopotentials. The biological signal V biol appears between the two measuring electrodes at the right and left arm of the patient, and is fed to the inverting and the non-inverting inputs of the differential amplifier. The right leg electrode provides the reference potential for the amplifier with a common mode voltage Vc
as indicated.

signal
. The line frequency interference signal shows only very small differences in amplitude and phase
between the two measuring electrodes, causing approximately the same potential at both inputs, and
thus appears only between the inputs and ground and is called the common mode signal . Strong rejection
of the common mode signal is one of the most important characteristics of a good biopotential amplifier.
The common mode rejection ratio (or CMRR) of an amplifier is defined as the ratio of the differential
mode gain over the common mode gain. As seen in Fig. 70.1, the rejection of the common mode signal
in a biopotential amplifier is both a function of the amplifier CMRR and the source impedances Z 1 and
Z 2 . For the ideal biopotential amplifier with Z 1 = Z 2 and infinite CMRR of the differential amplifier, the
output voltage is the pure biological signal amplified by G D , the differential mode gain: V out = G D ·V biol. With finite CMRR, the common mode signal is not completely rejected, adding the interference term
G D ·V c /CMRR to the output signal. Even in the case of an ideal differential amplifier with infinite CMRR,
the common mode signal will not completely disappear unless the source impedances are equal. The
common mode signal Vc causes currents to flow through Z1 and Z2
. The related voltage drops show a difference if the source impedances are unequal, thus generating a differential signal at the amplifier input which, of course, is not rejected by the differential amplifier. With amplifier gain GD and input impedance Zin , the output voltage of the amplifier is:

The output of a real biopotential amplifier will always consist of the desired output component due to a differential biosignal, an undesired component due to incomplete rejection of common mode interference signals as a function of CMRR, and an undesired component due to source impedance unbalance allowing a small proportion of a common mode signal to appear as a differential signal to the amplifier. Since source impedance unbalances of 5,000 to 10,000 Ω
, mainly caused by electrodes, are not uncommon, and sufficient rejection of line frequency interferences requires a minimum CMRR of 100 dB, the input impedance of the amplifier should be at least 109 Ω
at 60 Hz to prevent source impedance
unbalances from deteriorating the overall CMRR of the amplifier. State-of-the-art biopotential amplifiers provide a CMRR of 120 to 140 dB.
In order to provide optimum signal quality and adequate voltage level for further signal processing, the amplifier has to provide a gain of 100 to 50,000 and needs to maintain the best possible signal-tonoise ratio. The presence of high level interference signals not only deteriorates the quality of the physiological signals, but also restricts the design of the biopotential amplifier. Electrode half-cell potentials, for example, limit the gain factor of the first amplifier stage since their amplitude can be several orders of magnitude larger than the amplitude of the physiological signal. To prevent the amplifier from

 

FIGURE 70.2 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.

going into saturation, this component has to be eliminated before the required gain can be provided for the physiological signal.

A typical design of the various stages of a biopotential amplifier is shown in Fig. 70.2. The electrodes which provide the transition between the ionic flow of currents in biological tissue and the electronic flow of current in the amplifier, represent a complex electrochemical system that is described elsewhere in this handbook. The electrodes determine to a large extent the composition of the measured signal. The preamplifier represents the most critical part of the amplifier itself since it sets the stage for the quality of the biosignal. With proper design, the preamplifier can eliminate, or at least minimize, most of the signals interfering with the measurement of biopotentials.

In addition to electrode potentials and electromagnetic interferences, noise—generated by the amplifier and the connection between biological source and amplifier—has to be taken into account when designing the preamplifier. The total source resistance R s, including the resistance of the biological source and all transition resistances between signal source and amplifier input, causes thermal voltage noise with a root mean square (rms) value of:

 

where k = Boltzmann constant, T = absolute temperature, Rs = resistance in Ω, and B = bandwidth in Hz.

Additionally, there is the inherent amplifier noise. It consists of two frequency-dependent components,
the internal voltage noise source en and the voltage drop across the source resistance Rs caused by an internal current noise generator in. The total input noise for the amplifier with a bandwidth of B = f2 – f1
is calculated as the sum of its three independent components:

High signal-to-noise ratios thus require the use of very low noise amplifiers and the limitation of bandwidth. Current technology offers differential amplifiers with voltage noise of less than 10 nV and current noise less than 1 pA . Both parameters are frequency dependent and decrease approximately with the square root of frequency. The exact relationship depends on the technology of the amplifier input stage. Field effect transistor (FET) preamplifiers exhibit about five times the voltage noise density compared to bipolar transistors but a current noise density that is about 100 times smaller.

The purpose of the high pass and low pass filters in Fig. 70.2 is to eliminate interference signals like electrode half-cell potentials and preamplifier offset potentials and to reduce the noise amplitude by the limitation of the amplifier bandwidth. Since the biosignal should not be distorted or attenuated, higher order sharp-cutting linear phase filters have to be used. Active Bessel filters are preferred filter types due to their smooth transfer function. Separation of biosignal and interference is in most cases incomplete due to the overlap of their spectra.

The isolation stage serves the galvanic decoupling of the patient from the measuring equipment and provides safety from electrical hazards. This stage also prevents galvanic currents from deteriorating the signal-to-noise ratio especially by preventing ground loops. Various principles can be used to realize the isolation stage. Analog isolation amplifiers use either transformer, optical, or capacitive couplers to transmit the signal through the isolation barrier. Digital isolation amplifiers use a voltage/frequency converter to digitize the signal before it is transmitted easily by optical or inductive couplers to the output frequency/voltage converter. The most important characteristics of an isolation amplifier are low leakage current, isolation impedance, isolation voltage (or mode) rejection (IMR), and maximum safe isolation voltage.

Interferences

The most critical point in the measurement of biopotentials is the contact between electrodes and biological tissue. Both the electrode offset potential and the electrode/tissue impedance are subject to changes due to relative movements of electrode and tissue. Thus, two interference signals are generated as motion artifacts: the changes of the electrode potential and motion-induced changes of the voltage drop caused by the input current of the preamplifier. These motion artifacts can be minimized by providing high input impedances for the preamplifier, usage of non-polarized electrodes with low halfcell potentials such as Ag/AgCl electrodes, and by reducing the source impedance by use of electrode gel. Motion artifacts, interferences from external electromagnetic fields, and noise can also be generated in the wires connecting electrodes and amplifier. Reduction of these interferences is achieved by using twisted pair cables, shielded wires, and input guarding.

Recording of biopotentials is often done in an environment that is equipped with many electrical systems which produce strong electrical and magnetic fields. In addition to 60 Hz power line frequency and some strong harmonics, high frequency electromagnetic fields are encountered. At power line frequency, the electric and magnetic components of the interfering fields can be considered separately. Electrical fields are caused by all conductors that are connected to power, even with no flow of current. A current is capacitively coupled into the body where it flows to the ground electrode. If an isolation amplifier is used without patient ground, the current is capacitively coupled to ground. In this case, the body potential floats with a voltage of up to 100 V towards ground. Minimizing interferences requires increasing the distance between power lines and the body, use of isolation amplifiers, separate grounding of the body at a location as far away from the measuring electrodes as possible, and use of shielded electrode cables.

The magnetic field components produce eddy currents in the body. The amplifier, the electrode cable, and the body form an induction loop that is subject to the generation of an interference signal. Minimizing this interference signal requires increasing the distance between the interference source and patient, twisting the connecting cables, shielding of the magnetic fields, and relocating the patient to a place and orientation that offers minimum interference signals. In many cases, an additional narrow band-rejection filter (notch filter) is implemented as an additional stage in the biopotential amplifier to provide sufficient suppression of line frequency interferences.

In order to achieve optimum signal quality, the biopotential amplifier has to be adapted to the specific application. Based on the signal parameters, both appropriate bandwidth and gain factor are chosen. Figure 70.3 shows an overview of the most commonly measured biopotentials and specifies the normal ranges for amplitude and bandwidth.

 

FIGURE 70.3 Amplitudes and spectral ranges of some important biosignals. The various biopotentials completely cover the area from 10 – 6 V to almost 1 V and from dc to 10 kHz.

A final requirement for biopotential amplifiers is the need for calibration. Since the amplitude of the biopotential often has to be determined very accurately, there must be provisions to easily determine the gain or the amplitude range referenced to the input of the amplifier. For this purpose, the gain of the amplifier must be well calibrated. In order to prevent difficulties with calibrations, some amplifiers that need to have adjustable gain use a number of fixed gain settings rather than providing a continuous gain control. Some amplifiers have a standard signal source of known amplitude built in that can be momentarily connected to the input by the push of a button to check the calibration at the output of the biopotential amplifier.

70.2 Special Circuits

Instrumentation Amplifier

An important stage of all biopotential amplifiers is the input preamplifier which substantially contributes to the overall quality of the system. The main tasks of the preamplifier are to sense the voltage between two measuring electrodes while rejecting the common mode signal, and minimizing the effect of electrode polarization overpotentials. Crucial to the performance of the preamplifier is the input impedance which should be as high as possible. Such a differential amplifier cannot be realized using a standard single operational amplifier (op-amp) design since this does not provide the necessary high input impedance.
The general solution to the problem involves voltage followers, or noninverting amplifiers, to attain high input impedances. A possible realization is shown in Fig. 70.4a. The main disadvantage of this circuit is that it requires high CMRR both in the followers and in the final op-amp. With the input buffers working at unity gain, all the common-mode rejection must be accomplished in the output amplifier, requiring very precise resistor matching. Additionally, the noise of the final op-amp is added at a low signal level,

 

FIGURE 70.4 Circuit drawings for three different realizations of instrumentation amplifiers for biomedical applications. Voltage follower input stage (a), improved, amplifying input stage (b), and 2-op-amp version (c).

decreasing the signal-to-noise ratio unnecessarily. The circuit in Fig. 70.4b eliminates this disadvantage. It represents the standard instrumentation amplifier configuration. The two input op-amps provide high differential gain and unity common-mode gain without the requirement of close resistor matching. The differential output from the first stage represents a signal with substantial relative reduction of the common-mode signal and is used to drive a standard differential amplifier which further reduces the common-mode signal. CMRR of the output op-amp as well as resistor matching in its circuit are less critical than in the follower type instrumentation amplifier. Offset trimming for the whole circuit can be done at one of the input op-amps. Complete instrumentation amplifier integrated circuits based on this standard instrumentation amplifier configuration are available from several manufacturers. All components except R1, which determines the gain of the amplifier, and the potentiometer for offset trimming are contained on the integrated circuit chip. Figure 70.4c shows another configuration that offers high input impedance with only two op-amps. For good CMRR, however, it requires precise resistor matching.

In applications where DC and very low frequency biopotentials are not to be measured, it would be desirable to block those signal components at the preamplifier inputs by simply adding a capacitor working as a passive high-pass filter. This would eliminate the electrode offset potentials and permit ahigher gain factor for the preamplifier and thus a higher CMRR. A capacitor between electrodes and amplifier input would, however, result in charging effects from the input bias current. Due to the difficulty of precisely matching capacitors for the two input leads, they would also contribute to an increased source impedance unbalance and thus reduce CMRR. Avoiding the problem of charging effects by adding aresistor between the preamplifier inputs and ground as shown in Fig. 70.5a also results in a decrease of CMRR due to the diminished and mismatched input impedance. A 1% mismatch for two 1-MΩ resistors

 

FIGURE 70.5 AC coupled instrumentation amplifier designs. The classical design using an RC high-pass filter at the inputs (a), and a high CMRR “quasi-high-pass” amplifier as proposed by Lu (b).

can already create a –60 dB loss in CMRR. The loss in CMRR is much greater if the capacitors are mismatched, which cannot be prevented in real systems. Nevertheless, such realizations are used where the
specific situation allows. In some applications, a further reduction of the amplifier to a two-electrode amplifier configuration would be convenient, even at the expense of some loss in the CMRR. Figure 70.6 shows a preamplifier design working with two electrodes and providing AC coupling as proposed by Pallás-Areny [1990].

 A third alternative of eliminating DC and low frequencies in the first amplifier stage is a directly coupled quasi-high-pass amplifier design, which maintains the high CMRR of DC coupled high input impedance instrumentation amplifiers [Song et al., 1998]. In this design, the gain determining resistor R1 (Fig. 70.5a) is replaced by a first order high-pass filter consisting of R1 and a series capacitor Cf. The signal gain of the amplifier is

 

FIGURE 70.6 Composite instrumentation amplifier based on an ac-coupled first stage. The second stage is
based on a one op-amp differential amplifier which can be replaced by an instrumentation amplifier.

Thus, DC gain is 1, while the high frequency gain remains at G = 1 + 2R2/R1 . A realization using an offthe-
shelf instrumentation amplifier (Burr-Brown INA 118) operates at low power (0,35 mA) with low offset voltage (11 µV typical) and low input bias current (1 nA typical), and offers a high CMRR of 118 dB at a gain of G = 50. The very high input impedance (10 G Ω) of the instrumentation amplifier renders it insensitive to fluctuations of the electrode impedance. Therefore, it is suitable for bioelectric measurements using pasteless electrodes applied to unprepared, i.e., high impedance skin.

The preamplifier, often implemented as a separate device which is placed close to the electrodes or even directly attached to the electrodes, also acts as an impedance converter which allows the transmission of even weak signals to the remote monitoring unit. Due to the low output impedance of the preamplifier, the input impedance of the following amplifier stage can be low, and still the influence of interference signals coupled into the transmission lines is reduced.

Isolation Amplifier and Patient Safety

Isolation amplifiers can be used to break ground loops, eliminate source ground connections, and provide isolation protection to patient and electronic equipment. In a biopotential amplifier, the main purpose of the isolation amplifier is the protection of the patient by eliminating the hazard of electric shock resulting from the interaction among patient, amplifier, and other electric devices in the patient’s environment, specifically defibrillators and electrosurgical equipment. It also adds to the prevention of line frequency interferences.

Isolation amplifiers are realized in three different technologies: transformer isolation, capacitor isolation, and opto-isolation. An isolation barrier provides a complete galvanic separation of the input side, i.e., patient and preamplifier, from all equipment on the output side. Ideally, there will be no flow of electric current across the barrier. The isolation-mode voltage is the voltage which appears across the isolation barrier, i.e., between the input common and the output common (Fig. 70.7). The amplifier has to withstand the largest expected isolation voltages without damage. Two isolation voltages are specified

 

FIGURE 70.7 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.

for commercial isolation amplifiers: (1) the continuous rating and (2) the test voltage. To eliminate the need for longtime testing, the device is tested at about two times the rated continuous voltage. Thus, for a continuous rating of 2000 V, the device has to be tested at 4000 to 5000 V for a reasonable period of time.
Since there is always some leakage across the isolation barrier, the isolation mode rejection ratio (IMRR) is not infinite. For a circuit as shown in Fig. 70.7, the output voltage is:

 

where G is the amplifier gain, V D, VCM, and V ISO are differential, common mode, and isolation voltages,
respectively, and CMRR is the common mode rejection ratio for the amplifier [Burr-Brown, 1994].
Typical values of IMRR for a gain of 10 are 140 dB at DC, and 120 dB at 60 Hz with a source unbalance
of 5000Ω. The isolation impedance is approximately 1.8 pF||1012Ω

Transformer coupled isolation amplifiers perform on the basis of inductive transmission of a carrier signal that is amplitude modulated by the biosignal. A synchronous demodulator on the output port reconstructs the signal before it is fed through a Bessel response low-pass filter to an output buffer. A power transformer, generally driven by a 400 to 900 kHz square wave, supplies isolated power to the amplifier.

Optically coupled isolation amplifiers can principally be realized using only a single LED and photodiode combination. While useful for a wide range of digital applications, this design has fundamental limitations as to its linearity and stability as a function of time and temperature. A matched photodiode design, as used in the Burr-Brown 3650/3652 isolation amplifier, overcomes these difficulties [Burr-Brown, 1994]. Operation of the amplifier requires an isolated power supply to drive the input stages.
Transformer coupled low leakage current isolated DC/DC converters are commonly used for this purpose. In some particular applications, especially in cases where the signal is transmitted over a longer distance by fiber optics, e.g., ECG amplifiers used for gated magnetic resonance imaging, batteries are used to power the amplifier. Fiber optic coupling in isolation amplifiers is another option that offers the advantage of higher flexibility in the placement of parts on the amplifier board..

Biopotential amplifiers have to provide sufficient protection from electrical shock to both user and patient. Electrical-safety codes and standards specify the minimum safety requirements for the equipment, especially the maximum leakage currents for chassis and patient leads, and the power distribution system [Webster, 1992; AAMI, 1993].

Special attention to patient safety is required in situations where biopotential amplifiers are connected to personal computers which are more and more often used to process and store physiological signals 

and data. Due to the design of the power supplies used in standard PCs permitting high leakage currents—
an inadequate situation for a medical environment—there is a potential risk involved even when the patient is isolated from the PC through an isolation amplifier stage or optical signal transmission from the amplifier to the computer. This holds especially in those cases where, due to the proximity of the PC to the patient, an operator might touch patient and computer at the same time, or the patient might touch the computer. It is required that a special power supply with sufficient limitation of leakage currents is used in the computer, or that an additional, medical grade isolation transformer is used to provide the necessary isolation between power outlet and PC.

Surge Protection

The isolation amplifiers described in the preceding paragraph are primarily used for the protection of the patient from electric shock. Voltage surges between electrodes as they occur during the application of a defibrillator or electrosurgical instrumentation also present a risk to the biopotential amplifier. Biopotential amplifiers should be protected against serious damage to the electronic circuits. This is also part of the patient safety since defective input stages could otherwise apply dangerous current levels to the patient. To achieve this protection, voltage limiting devices are connected between each measuring electrode and electric ground. Ideally, these devices do not represent a shunt impedance and thus do not lower the input impedance of the preamplifier as long as the input voltage remains in a range considered safe for the equipment. They appear as an open circuit. As soon as the voltage drop across the device reaches a critical value Vb, the impedance of the device changes sharply and current passes through it to such an extent that the voltage cannot exceed Vb due to the voltage drop across the series resistor R as indicated in Fig. 70.8.

Devices used for amplifier protection are diodes, Zener diodes, and gas-discharge tubes. Parallel silicon diodes limit the voltage to approximately 600 mV. The transition from nonconducting to conducting state is not very sharp, and signal distortion begins at about 300 mV which can be within the range of input voltages depending on the electrodes used. The breakdown voltage can be increased by connecting several diodes in series. Higher breakdown voltages are achieved by Zener diodes connected back to back. One of the diodes will be biased in the forward direction and the other in the reverse direction. The breakdown voltage in the forward direction is approximately 600 mV, but the breakdown voltage in the reverse direction is higher, generally in the range of 3 to 20 V, with a sharper voltage-current characteristic than the diode circuit.

 

FIGURE 70.8 Protection of the amplifier input against high-voltage transients. The connection diagram for voltagelimiting elements is shown in panel (a) with two optional resistors R′at the input. A typical current-voltage characteristic is shown in panel (b). Voltage-limiting elements shown are the anti-parallel connection of diodes (c), anti-parallel connection of Zener diodes (d), and gas-discharge tubes (e).

A preferred voltage-limiting device for biopotential amplifiers is the gas-discharge tube. Due to its extremely high impedance in the nonconducting state, this device appears as an open circuit until it reaches its breakdown voltage. At the breakdown voltage which is in the range of 50 to 90 V, the tube switches to the
conducting state and maintains a voltage that is usually several volts less than the breakdown voltage. Though the voltage maintained by the gas-discharge tube is still too high for some amplifiers, it is low enough to allow the input current to be easily limited to a safe value by simple circuit elements such as resistors like the resistors R′ indicated in Fig. 70.8a. Preferred gas discharge tubes for biomedical applications are miniature neon lamps which are very inexpensive and have a symmetric characteristic.

 

FIGURE 70.9 Instrumentation amplifier providing input guarding.

Input Guarding

The common mode input impedance and thus the CMRR of an amplifier can be greatly increased by guarding the input circuit. The common mode signal can be obtained by two averaging resistors connected
between the outputs of the two input op-amps of an instrumentation amplifier as shown in Fig. 70.9. The buffered common-mode signal at the output of op-amp 4 can be used as guard voltage to reduce the effects of cable capacitance and leakage.

In many modern biopotential amplifiers, the reference electrode is not grounded. Instead, it is connected to the output of an amplifier for the common mode voltage, op-amp 3 in Fig. 70.10, which works as an inverting amplifier. The inverted common mode voltage is fed back to the reference electrode. This negative feedback reduces the common-mode voltage to a low value [Webster, 1992]. Electrocardiographs based on this principle are called driven-right-leg systems replacing the right leg ground electrode of ordinary electrocardiographs by an actively driven electrode.

 

FIGURE 70.10Driven-right-leg circuit reducing common-mode interference.

Dynamic Range and Recovery

With an increase of either the common mode or differential input voltage there will be a point where the amplifier will overload and the output voltage will no longer be representative for the input voltage. Similarly, with a decrease of the input voltage, there will be a point where the noise components of the output voltage cover the output signal to a degree that a measurement of the desired biopotential is no longer possible. The dynamic range of the amplifier, i.e., the range between the smallest and largest possible input signal to be measured, has to cover the whole amplitude range of the physiological signal of interest. The required dynamic range of biopotential amplifiers can be quite large. In an application like fetal monitoring for example, two signals are recorded simultaneously from the electrodes which are quite different in their amplitudes: the fetal and the maternal ECG. While the maternal ECG shows an amplitude of up to 10 mV, the fetal ECG often does not reach more than 1 µV. Assuming that the fetal ECG is separated from the composite signal and fed to an analog/digital converter for digital signal processing with a resolution of 10 bit (signed integer), the smallest voltage to be safely measured with the biopotential amplifier is 1/512 µV or about 2 nV vs. 10 mV for the largest signal, or even up to 300 mV in the presence of an electrode offset potential. This translates to a dynamic range of 134 dB for the signals alone and 164 dB if the electrode potential is included in the consideration. Though most applications are less demanding, even such extreme requirements can be realized through careful design of the biopotential amplifer and the use of adequate components. The penalty for using less expensive amplifiers with diminished performance would be a potentially severe loss of information.

Transients appearing at the input of the biopotential amplifier, like voltage peaks from a cardiac pacemaker or a defibrillator, can drive the amplifier into saturation. An important characteristic for the amplifier is the time it takes to recover from such overloads. The recovery time depends on the characteristics of the transient, like amplitude and duration, the specific design of the amplifier, like bandwidth, and the components used. Typical biopotential amplifiers may take several seconds to recover from severe overload. The recovery time can be reduced by disconnecting the amplifier inputs at the discovery of a transient using an electronic switch.

Passive Isolation Amplifiers

Increasingly, biopotentials have to be measured within implanted devices and need to be transmitted to an external monitor or controller. Such applications include cardiac pacemakers transmitting the intracardiac ECG and functional electrical stimulation where, e.g., action potentials measured at one eyelid serve to stimulate the other lid to restore the physiological function of a damaged lid at least to some degree. In these applications, the power consumption of the implanted biopotential amplifier limits the lifespan of the implanted device. The usual solution to this problem is an inductive transmission of power into the implanted device that serves to recharge an implanted battery. In applications where the size of the implant is of concern, it is desirable to eliminate the need for the battery and the related circuitry by using a quasi passive biopotential amplifer, i.e., an amplifier that does not need a power supply.

The function of passive telemetric amplifiers for biopotentials is based on the ability of the biological source to drive a low power device such as a FET and the sensing of the biopotentials through inductive or acoustic coupling of the implanted and external devices [Nagel, 1982]. In an inductive system, a FET serves as a load to an implanted secondary LC-circuit which is stimulated inductively by an extracorporal oscillator (Fig. 70.11). Depending on the special realization of the system, the biopotential is available in the external circuit from either an amplitude or frequency-modulated carrier-signal. The input impedance of the inductive transmitter as a function of the secondary load impedance Z 2 is given by:

 

FIGURE 70.11 The passive isolation amplifier can be operated without the need for an isolated power supply. The biological source provides the power to modulate the load impedance of an inductive transformer. As an easy realization shown in panel (b), a FET can be directly connected to two electrodes. The source-drain resistance changes as a linear function of the biopotential which is then reflected by the input impedance of the transformer.

In an amplitude-modulated system, the resistive part of the input-impedance Z 1 must change as alinear function of the biopotential. The signal is obtained as the envelope of the carrier signal, measured across a resistor Rm
. A frequency-modulated system is realized when the frequency of the signal generator is determined at least in part by the impedance Z1 of the inductive transmitter. In both cases, the signaldependent changes of the secondary impedance Z2 can be achieved by a junction-FET. Using the field effect transistor as a variable load resistance changing its resistance in proportion to the source-gate voltage which is determined by the electrodes of this two-electrode amplifier, the power supplied by the biological source is sufficient to drive the amplifier. The input impedance can be in the range of 1010Ω
.
Optimal transmission characteristics are achieved with AM systems. Different combinations of external and implanted resonance circuits are possible to realize in an AM system, but primary parallel withsecondary serial resonance yields the best characteristics. In this case, the input impedance is given by:

 

The transmission factor (L1/M)2 is optimal since the secondary inductivity, i.e., the implanted inductivity, can be small, only the external inductivity determines the transmission factor and the mutual inductivity should be small, a fact that favors the loose coupling that is inherent to two coils separated by skin and tissue. There are, of course, limits to M which cannot be seen from Eq. (70.7). In a similar fashion, two piezoelectric crystals can be employed to provide the coupling between input and output. This 2-lead isolation amplifier design is not limited to telemetric applications. It can also be used in all other applications where its main advantage lies in its simplicity and the resulting substantial cost savings as compared to other isolation amplifiers which require additional amplifier stages and an additional isolated power supply. FIGURE 70.11
The passive isolation amplifier can be operated without the need for an isolated power supply. The biological source provides the power to modulate the load impedance of an inductive transformer. As an easy realization shown in panel (b), a FET can be directly connected to two electrodes. The source-drain resistance changes as a linear function of the biopotential which is then reflected by the input impedance of the transformer.

Digital Electronics

The ever increasing density of integrated digital circuits together with their extremely low power consumption
permits digitizing and preprocessing of signals already on the isolated patient-side of the amplifiers, thus improving signal quality and eliminating the problems normally related to the isolation barrier, especially those concerning isolation voltage interferences and long-term stability of the isolation amplifiers. Digital signal transmission to a remote monitoring unit, a computer system, or computer network can be achieved without any risk of picking up transmission line interferences, especially when implemented with fiberoptical cables.
Digital techniques also offer an easy means of controlling the front-end of the amplifier. Gain factors can be easily adapted, and changes of the electrode potential resulting from electrode polarization or from interferences which might drive the differential amplifier into saturation can easily be detected and compensated.

Summary

Biopotential amplifiers are a crucial component in many medical and biological measurements, and largely determine the quality and information content of the measured signals. The extremely wide range of necessary specifications with regard to bandwidth, sensitivity, dynamic range, gain, CMRR, and patient safety leaves only little room for the application of general purpose biopotential amplifiers, and mostly requires the use of special purpose amplifiers.

Defining Terms

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 = G D/G CM. Expressed in decibels, the common mode rejection is 20 log10 CMRR. The common mode rejection is a function of frequency and source-impedance unbalance.

Isolation Mode Rejection Ratio (IMRR):
The ratio between the isolation voltage, V ISO , 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 = V ISO /V OUT
.
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 openloop 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 [Horowitz and Hill, 1980].