**5. Signal filters and operational amplifiers**

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 from 1μV to 100mV, 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 displays, recorders, or A/D converters for computerized equipment. Amplifiers adequately 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.

#### **5.1. Basic signal amplifier**

The basic requirements that a biopotential amplifier has to satisfy are:


A typical configuration for the measurement of biopotentials as illustrated in figure 29. Three electrodes, two of them are used to pick up the biological signal and the third providing the reference potential, connect the subject to amplifier. The input signal to the amplifier consists of five components:(1) the desired biopotential, (2)undesired biopotential, (3) a power line interference signal of 60Hz (50Hz in some countries) and its harmonics, (4)interference signal 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 designing deferential amplifier is to reject the line frequency interference that is electrostatically or magnetically coupled into subject. The desired biopotential appears as a voltage between two input terminals of differential amplifier and is referred to as the differential signal. The line frequency reference signal shows only small differences in amplitude and phase between the two measuring electrodes, causing approximately the same potential at both inputs, and thus

**Figure 29.** Typical configuration for the measurement of biopotentials

The initial current in above equation, *i*(*t*0), is usually defined with the same polarity as *i*, which means *i*(*t*0) is a positive quantity. If the polarity of the initial current *i*(*t*0) is in the opposite

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 from 1μV to 100mV, 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 displays, recorders, or A/D converters for computerized equipment. Amplifiers adequately 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

**1.** The physiological process to be monitored should not be influenced in any way by the

**3.** The amplifier has to provide protection of patient from any hazard of electrical shock.

**4.** 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 instru‐

A typical configuration for the measurement of biopotentials as illustrated in figure 29. Three electrodes, two of them are used to pick up the biological signal and the third providing the reference potential, connect the subject to amplifier. The input signal to the amplifier consists of five components:(1) the desired biopotential, (2)undesired biopotential, (3) a power line interference signal of 60Hz (50Hz in some countries) and its harmonics, (4)interference signal 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 designing deferential amplifier is to reject the line frequency interference that is electrostatically or magnetically coupled into subject. The desired biopotential appears as a voltage between two input terminals of differential amplifier and is referred to as the differential signal. The line frequency reference signal shows only small differences in amplitude and phase between the two measuring electrodes, causing approximately the same potential at both inputs, and thus

direction, then *i*(*t*0) is negative.

212 Advances in Bioengineering

biopotential amplifiers.

amplifier.

mentation.

**5.1. Basic signal amplifier**

**5. Signal filters and operational amplifiers**

The basic requirements that a biopotential amplifier has to satisfy are:

**2.** The measurement signals should not be distorted.

appears only between the inputs and ground and is called common mode signal. Strong rejection of the common mode signal is one of the most important characteristics of a good biopotential amplifier.

In order to provide optimum signal quality and adequate voltage level for further signal processing, amplifier has to provide a suitable gain range and needs to maintain a possible signal-to-noise ratio. The presence of the high level interference signals not only deteriorates the quality of the physiological signals, but also restricts the design of the biopotential amplifier. For example, electrode half-cell biopotentials limit the gain factor of the first amplifier stage since their amplitude can be several orders of magnitude larger than the amplitude of physiological signal. To prevent the amplifier from going to saturation, this component has to be eliminated before the required gain be provided for physiological signal.

**Figure 30.** 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 interference, biological signal is connected to an output lowpass filter through an isolation stage which provides electrical safety to the patient, prevents patient loops, and reduces the influence of interference signals.

A typical design of the various stage of a biopotential amplifier is shown in figure 30. The three electrodes which provide the transition between the ionic flow of currents in biological tissue and electronic flow of currents in amplifier represent a complex electrochemical system. To a large extent, these electrodes determine the composition of the measured signal. The pream‐ plifier represents the most critical part of a amplifier since it sets the stage for the quality of the biosignal. With proper design, the preamplifier can eliminate, or at least minimize, the most signal interfering with the measurement of biopotentials. In addition to electrode biopotentials and electromagnetic interference, noise which is generated by the amplifier and the connection between biological source and amplifier has to be taken into account when designing a preamplifier.

(a) One-order low-pass filter for its circuit and logarithmic amplitude-frequency characteristics

(b) One-order high-pass filter for its circuit and logarithmic amplitude-frequency characteristics

Figure6.30 Four types of filters and its amplitude-frequency characteristics

according to the frequency range of signals there are four classes of filters: low-pass filter (LPF), high-pass filter (HPF), band-pass filter (BPF) and band elimination filter (BEF). These

After biosignals is preamplified, some unuseful signal has to be eliminated or filtered to highlight the useful biosignal. Such function can be realized by all kinds of filters. In circuit, **Figure 31.** Four types of filters and its amplitude-frequency characteristics

four types of filters are shown in figure6.30.

After biosignals are preamplified, some unuseful signal have to be eliminated or filtered to highlight the useful biosignal. Such function can be realized by all kinds of filters. In circuit, according to the frequency range of signals there are four classes of filters: low-pass filter (LPF), high-pass filter (HPF), band-pass filter (BPF) and band elimination filter (BEF). These four types of filters are shown in figure 31.

## **5.2. Operational amplifiers**

and electronic flow of currents in amplifier represent a complex electrochemical system. To a large extent, these electrodes determine the composition of the measured signal. The pream‐ plifier represents the most critical part of a amplifier since it sets the stage for the quality of the biosignal. With proper design, the preamplifier can eliminate, or at least minimize, the most signal interfering with the measurement of biopotentials. In addition to electrode biopotentials and electromagnetic interference, noise which is generated by the amplifier and the connection between biological source and amplifier has to be taken into account when

(a) One-order low-pass filter for its circuit and logarithmic amplitude-frequency characteristics

(b) One-order high-pass filter for its circuit and logarithmic amplitude-frequency characteristics

(c) Band-pass filter principle diagram (d)Band elimination filter priciple diagram Figure6.30 Four types of filters and its amplitude-frequency characteristics After biosignals is preamplified, some unuseful signal has to be eliminated or filtered to highlight the useful biosignal. Such function can be realized by all kinds of filters. In circuit, according to the frequency range of signals there are four classes of filters: low-pass filter (LPF), high-pass filter (HPF), band-pass filter (BPF) and band elimination filter (BEF). These

four types of filters are shown in figure6.30.

**Figure 31.** Four types of filters and its amplitude-frequency characteristics

designing a preamplifier.

214 Advances in Bioengineering

Operational amplifiers play an important role that they amplify a weak signal and adjust voltage or current in detecting circuit. An operation amplifier is an electronic device that consists of plenty of transistors, resistors, and capacitors. Fully understanding its operation requires that people have the knowledge of diodes and transistors. Circuit involving opera‐ tional amplifier forms the cornerstone of any bioinstrumentation, from amplifiers to filters. Amplifiers used in biomedical applications have very high-input impedance to keep the current down from the system being measured. Most body signals have small magnitudes. For example, ECG has a magnitude in millivolts and the EEG has a magnitude in microvolt. Analog filters are often used to remove noise from a signal, typically through frequency domain analysis to design the filter.


The operational amplifier is an amplifier, but when it is combined with other circuit elements, it may integrate, differentiate, product, divide, sum, and subtract. In circuit, there are three basic types of proportional operation amplifiers: inverse scaling operation circuit, noninverse scaling operation circuit, and differential scaling operation circuit. These three types of operation circuit are compared as illustrated in table 3.

According to the information listed in table 3, we could know that operational amplifier is drawn with the symbol in figure 32. The input terminals are labeled the no inverting input (+) and inverting input (-). The power supply terminals are labeled *V*+ and *V*−, which are frequently omitted, since they do not affect the circuit behavior except in saturation conditions. Most people shorten the name of operational amplifier to the "op amp".

**Figure 32.** Circuit element symbol for the operational amplifier

Figure 33 shows an ideal mode of op amp, focusing on the internal behavior of input and output terminals. The input-output relationship is the following:

**Figure 33.** An internal mode of the op amp

*uo* = *A*(*Vp* −*Vn*)

**Figure 34.** Idealized mode of the op amp

Since the terminal resistance is very large, we may replace it with an open circuit to simplify analysis, leaving us with the op amp model shown in figure 34.

With the replacement of the internal resistance in an open circuit, the input current is zero (*i <sup>p</sup>* =*i <sup>n</sup>* =0*A*). In addition, the output current (*i <sup>A</sup>*) of the op amp is not zero. Because the output current (*i <sup>A</sup>*) is not known, seldom is KCL applied at the output junction. In solving the op amp problems, KCL is always applied at input terminals.

#### **Example problem 1**

Find the output voltage *uo* for the following circuit.

#### **Solution**

The operational amplifier is an amplifier, but when it is combined with other circuit elements, it may integrate, differentiate, product, divide, sum, and subtract. In circuit, there are three basic types of proportional operation amplifiers: inverse scaling operation circuit, noninverse scaling operation circuit, and differential scaling operation circuit. These three types of

According to the information listed in table 3, we could know that operational amplifier is drawn with the symbol in figure 32. The input terminals are labeled the no inverting input (+) and inverting input (-). The power supply terminals are labeled *V*+ and *V*−, which are frequently omitted, since they do not affect the circuit behavior except in saturation conditions. Most

Figure 33 shows an ideal mode of op amp, focusing on the internal behavior of input and

*uo* = *A*(*Vp* −*Vn*)

operation circuit are compared as illustrated in table 3.

216 Advances in Bioengineering

**Figure 32.** Circuit element symbol for the operational amplifier

**Figure 33.** An internal mode of the op amp

output terminals. The input-output relationship is the following:

people shorten the name of operational amplifier to the "op amp".

Using the op amp model shown in figure 33, we may apply KCL at the inverting terminal and gain the following:

$$i\_1 + i\_2 = 0$$

since currents do not flow into the op amp's input terminals. Replacing the current using ohm's law gives:

$$\frac{V\_s - V\_n}{R\_1} + \frac{V\_o - V\_n}{R\_2} = 0$$

Multiplying by R1R2 and collecting like terms, we could gain:

$$R\_2 V\_s = (R\_1 + R\_2) V\_n - R\_1 V\_o$$

Now *Vo* = *A*(*Vp* −*Vn*), and since the no inverting terminal is connected to the ground, *Vp* =0,

*Vo* = − *AVn*

Substituting *Vn* into the KCL inverting input equation gives

$$R\_2 V\_s = \left( R\_1 + R\_2 \right) \left( -\frac{V\_o}{A} \right) - R\_1 V\_o$$

Namely, *Vo* <sup>=</sup> <sup>−</sup> *<sup>R</sup>*2*Vs <sup>R</sup>*<sup>1</sup> <sup>+</sup> *<sup>R</sup>*<sup>1</sup> <sup>+</sup> *<sup>R</sup>*<sup>2</sup> *A*

When a goes to infinity, the above equations could be simplified into:

$$V\_o = -\frac{\mathcal{R}\_2}{\mathcal{R}\_1} V\_s$$

Interestingly, with A going to infinity, output voltage *Vo* remain finite due to the resistor *R*2.This happens because a negative feedback path exists between the output and the inverting input terminal through resistor *R*2. This circuit is called the inverting scaling operation circuit. Of course, similar circuits have differential circuit, integrating circuit, summing circuit, index circuit, logarithmic circuit and dividing circuit. These circuits could be found and read in the analog circuit textbook.

#### **5.3. Bioinformation acquisition**

Biological signals are often very small and typically contain some unwanted noise or interfer‐ ence. Such interference could determine the effect of obscuring relevant information that may be available in the measured signal. Noise can be extraneous in nature, arising from sources outside the body, such as thermal noise in sensors or noise in the electronic components of the acquisition system. Noise can be intrinsic to the biological media, meaning that it can arise from adjacent tissues or organs. ECG measurement from the heart can be affected by bioelectric activity from the adjacent muscles.

In order to extract the meaningful information from biological signals, sophisticated bioinfor‐ mation acquisition techniques and equipment are commonly utilized and explored. Equip‐ ments with high-precision low-noise are very necessary to minimize the effect of unwanted noise. Basic components include sensors, amplifiers, analog signal conditioner, data acquisi‐ tion, data storage and display, digital signal processing circuit. The bioinformation acquisition procedure is shown in figure 35.

**Figure 35.** Procedure of obtaining biological information

*Vo* = − *AVn*

*Vo <sup>A</sup>* ) <sup>−</sup>*R*1*Vo*

*R*2*Vs* =(*R*<sup>1</sup> + *R*2)( −

*Vo* = −

*R*2 *R*1 *Vs*

Interestingly, with A going to infinity, output voltage *Vo* remain finite due to the resistor *R*2.This happens because a negative feedback path exists between the output and the inverting input terminal through resistor *R*2. This circuit is called the inverting scaling operation circuit. Of course, similar circuits have differential circuit, integrating circuit, summing circuit, index circuit, logarithmic circuit and dividing circuit. These circuits could be found and read in the

Biological signals are often very small and typically contain some unwanted noise or interfer‐ ence. Such interference could determine the effect of obscuring relevant information that may

When a goes to infinity, the above equations could be simplified into:

Substituting *Vn* into the KCL inverting input equation gives

Namely, *Vo* <sup>=</sup> <sup>−</sup> *<sup>R</sup>*2*Vs*

218 Advances in Bioengineering

analog circuit textbook.

**5.3. Bioinformation acquisition**

*<sup>R</sup>*<sup>1</sup> <sup>+</sup> *<sup>R</sup>*<sup>1</sup> <sup>+</sup> *<sup>R</sup>*<sup>2</sup> *A*

> In figure 35, sensors feel the biological signal that is being observed into an analog signal conditioner to adapt the requirement of data acquisition system. Here data acquisition system converts the analog signals into a calibrated digital signal that can be stored. Digital signal processing techniques are employed here to reduce the noise and extract additional informa‐ tion that can improve understanding of physiological meaning of original parameter. Throughout the data acquisition shown in figure 35, it's very critical that the information and structure of original biological signal of interests be faithfully preserved. Because these signals are often used to help people diagnose the pathological disorder. The procedure of analog signal conditioner, data acquisition system, analog amplifying and signal filtering, and A/D conversion should not generate misleading or untraceable distortion. Signal distortion would lead to an improper diagnosis on biological body.

> In bioinstrumentation, after biological signal has been detected with an appropriate sensor, it is amplified and filtered. Operational amplifiers are electronic circuits that are used to adjust the amplitude or size of biological signal. Analog filter may be used to remove the noise hiding in biological signal or compensate for distortions caused by sensors. Amplification and

filtering of biological signal may be necessary to meet the requirement of hardware specifica‐ tions of signal conversion procedure. Continuous signal needs to be limited to a certain band of frequencies before signal can be digitized with an analog-to-digital converter, prior to storing in a digital computer.
