BIOMEDICAL IMSTRUMENTION UNIT III SIGNAL CONDITIONING CIRCUITS
- 1. KONGUNADU COLLEGE OF ENGINEERING AND TECHNOLOGY (AUTONOMOUS) NAMAKKAL- TRICHY MAIN ROAD, THOTTIAM DEPARTMENT OF BIOMEDICAL ENGINEERING Ms. M. Thendral, Assistant Professor / BME KNCET 20BM502 – BIOMEDICAL INSTRUMENTATION (REGULATION-KNCET - UGR2020) UNIT III – SIGNAL CONDITIONING CIRCUITS
- 2. UNIT III SIGNAL CONDITIONING CIRCUITS •Need for bio-amplifier - single ended bio- amplifier, differential bio-amplifier, Impedance matching circuit, isolation amplifiers - transformer and optical isolation - isolated DC amplifier and AC carrier amplifier., Power line interference, Right leg driven ECG amplifier, Band pass filtering.
- 3. NEED FOR BIO-AMPLIFIER Bio amplifiers • Generally, Bio signals are having low amplitude and low frequency. Amplifiers are needed to boost the amplitude level of the bio signals. The output of this amplifier is displayed as EEG or ECG waveform. These amplifiers are known as bio amplifiers or bio medical amplifiers. Need of Bio Amplifier: • The biological amplifier should have a high input impedance value. The range of value lies between 2 MΩ and 10 MΩ depending on the applications. Higher impedance value reduces distortion of the signal. • When electrodes pick up biopotentials from the human body, the input circuit should be protected. Every bio-amplifier should consist of isolation and protection circuits, to prevent the patients from electrical shocks.
- 4. NEED FOR BIO-AMPLIFIER • Since the output of a bioelectric signal is in millivolts or microvolt range, the voltage gain value of the amplifier should be higher than 100dB. • Throughout the entire bandwidth range, a constant gain should be maintained. • A bio-amplifier should have a small output impedance. • A good bio-amplifier should be free from drift and noise. • Common Mode Rejection Ratio (CMRR) value of amplifier should be greater than 80dB to reduce the interference from common mode signal. • The gain of the bio-amplifier should be calibrated for each measurement.
- 5. SINGLE ENDED BIO-AMPLIFIER • A single-ended amplifier is an amplifier circuit configuration where there is only one active device (commonly a vacuum tube or transistor) in the signal path, and all other circuit elements serve to provide power to this device. • The output is taken directly from the active device, hence the name ‘single-ended’. The single-ended amplifier is the simplest of all amplifier circuit configurations.
- 6. Inverting amplifier: • An inverting amplifier (also known as an inverting operational amplifier or an inverting op-amp) is a type of operational amplifier circuit which produces an output which is out of phase with respect to its input by 180o . • The voltage gain of the inverting operational amplifier or inverting op amp is,
- 7. Non Inverting amplifier: • A non-inverting amplifier is an OPAMP circuit configuration whose output is in phase with the input signal at the non- inverting input. The input signal is applied at the non-inverting input of the opamp. A non- inverting amplifier also acts as a voltage follower circuit. • The non-inverting amplifiers also have negative feedback which is used to control the gain of the amplifier. Feedback contains a voltage divider circuit that provides a part of the output to the input terminal. This OPAMP has a high input impedance and a low output impedance.
- 8. DIFFERENTIAL BIO-AMPLIFIER • A differential amplifier is a type of electronic amplifier that amplifies the difference between two input voltages and rejects the average or common mode value of the two voltages. • V0 is the output voltage • V1 and V2 are the input voltages • Ad is the gain of the amplifier (i.e. the differential amplifier gain) When V1 = V2, the output voltage V0 is equal to zero, and hence the output voltage is suppressed. But any difference between inputs V1 and V2 is amplified by the differential amplifier gain Ad.
- 9. DIFFERENTIAL BIO-AMPLIFIER • Differential amplifier is also known as a difference amplifier because the difference between the input voltages is amplified. • Hence its output voltage will be equal to the sum of the output voltages produced by the Op-Amp circuit operating as an inverting amplifier and the Op-Amp circuit operating as a non-inverting amplifier. Thus, one gets:
- 10. DIFFERENTIAL BIO-AMPLIFIER Differential gain: • Differential-mode input voltage is the voltage difference between V1 and V2. • Differential mode can readily act as a subtractor amplifier as it results in an output voltage given by: • Where V1 and V2 represent the voltages applied at its inverting and non-inverting input terminals and Ad refers to its differential gain. Common mode gain(Ac): • Common-mode input voltage is the average value of V1 and V2. Vc = • The output of an ideal differential amplifier is given by
- 11. IMPEDANCE MATCHING CIRCUIT •Impedance matching is defined as the process of designing the input impedance and output impedance of an electrical load to minimize the signal reflection or maximize the power transfer of the load. This source impedance is equivalent to resistance in series with reactance. • According to the maximum power transfer theorem, when the load resistance is equal to the source resistance and load reactance is equal to negative of the source reactance, the maximum power is transferred from source and load. It means that the maximum power can be transfer if the load impedance is equal to the complex conjugate of the source impedance. • Consider that, the Resistor (R) and Inductor (L) are in series. And this combination is in parallel with the Capacitor (C). Hence, the Impedance is,
- 12. Block diagram of Impedance matching • Source: This represents the signal source. Source" represents the device that generates the signal. • Impedance Matching Circuit. It typically consists of passive components like resistors, capacitors, and inductors arranged in a specific configuration. Its purpose is to adjust the impedance of the source or load to match the other side. • Load: "Load" is the device that receives the signal.
- 13. Condition of impedance matching • ZS is donated as source impedance. ZL is donated as load impedance. 1. ZS =ZL • If the source impedance is equal to the load impedance, the maximum power will transfer from source to load. 1. ZS ≠ZL • If the source impedance is not equal to the load impedance, the maximum power will not transfer from source to load. The signal reflection will occur.
- 14. ISOLATION AMPLIFIERS • Isolation amplifiers are known as Pre- amplifier isolation circuits. An isolation amplifier increases the input impedance of a patient monitoring system. It also helps to isolate the patient from the device. Using the isolation amplifier prevents accidental internal cardiac shock. • The electrical signals are obtained with electrodes. The signals received goes to the amplifier block, where signals amplification occurs. After amplification, the signal enters the modulation block. When either it goes to the isolation barrier, optical cable or transformer can be used. If in case of optical cable, modulator output travels to LED.
- 15. ISOLATION AMPLIFIERS • The LED converts electrical signals into light energy. If the transformer acts an isolation barrier, modulator output connects the primary winding of the transformer. Energy from primary transfers to the secondary winding based on the mutual induction principle. At the next stage, secondary output enters the demodulation block. Finally, the amplified demodulated signal is obtained.
- 16. TRANSFORMER ISOLATION AMPLIFIERS • A transformer -isolated amplifier relies on transformer coupling of a high-frequency carrier signal between input and output. Some models also include a transformer-isolated power supply, that may also be used to power external signal processing devices on the isolated side of the system. • An isolation transformer is a transformer used to transfer electrical power from a source of alternating current (AC) power to device while isolating the powered device from the power source. • In theory, the definition of 'isolation transformer' applies to any transformer where there is no direct connection between the primary and the secondary windings. The windings are connected only by the magnetic flux in the core. • The electrical signals are obtained with electrodes. The signals received goes to the amplifier block, where signals amplification occurs. After amplification, the signal enters the modulation block.
- 17. TRANSFORMER ISOLATION AMPLIFIERS • When either it goes to the isolation barrier transformer can be used. If the transformer acts an isolation barrier, modulator output connects the primary winding of the transformer. • Energy from primary transfers to the secondary winding based on the mutual induction principle. At the next stage, secondary output enters the demodulation block. Finally, the amplified demodulated signal is obtained.
- 18. OPTICAL ISOLATION AMPLIFIERS • An optically isolated amplifier modulates current through an LED optocoupler. The linearity is improved by using a second optocoupler within a feedback loop. Some devices provide up to 60 kHz bandwidth. • In isolation amplifier the coupling method between input and output is optical. The most often isolation amplifier contains an input amplifier of LED and a silicon photodiode an output amplifier. The following figure shows such configuration of isolation amplifier.
- 19. ISOLATED DC AMPLIFIER • An isolated DC amplifier is a device used to amplify a direct current (DC) signal while providing electrical isolation between the input and output. This isolation helps prevent noise, ground loop issues, and other interference from affecting the signal. The primary goal is to maintain the integrity of the DC signal and prevent interference or ground loop issues.
- 20. ISOLATED DC AMPLIFIER Input Circuit: • Input circuit is designed to take a DC signal as an input. Isolation Barrier: • Use an optocoupler or a transformer to provide electrical isolation between the input and output circuits. Amplification Circuit: • The amplification circuit is used to amplify the isolated DC signal. Output Circuit: • The output circuit provide an isolated amplified DC signal as output.
- 21. Carrier Amplifier • A direct-current amplifier • The dc input signal is filtered by a low pass filter, then used to modulate a carrier so it can be amplified conventionally as an alternating-current signal; • The amplified dc output is obtained by rectifying and filtering the rectified carrier signal.
- 22. Carrier Amplifier 1. Carrier Oscillator: • used to energize the transducer with an alternating carrier voltage. 2. Strain gauge transducer: • The information signal from the body electrodes reaches the transducer where it is amplitude modulated using carrier signal from the carrier oscillator. The transducer changes the amplitude of carrier signal with respect to the physiological variable being measured. The output of transducer is amplitude modulated signal. 3. Amplifier: Amplifier used is Multistage Capacitance coupled Amplifier. The modulated signal from the transducer is given to this amplifier. The first stage produces amplification of AM signal. Second stage responds to signal frequency components of carrier signal only. Further amplified in the third stage.
- 23. Carrier Amplifier 4. Rectifier Output from the amplifier is converted into unidirectional signal using a rectifier. 5. Phase sensitive Detector The signal is demodulated and extracts the amplified information signal. 6. Direct Writing Recorder The voltage produced by the detector stage is then fed to the driver stage of the recording system.
- 24. POWER LINE INTERFERENCE •Power line interference, also referred to as electrical interference or electrical noise, is a phenomenon where unwanted electrical signals disrupt the normal functioning of electronic devices and communication systems. •These unwanted signals can originate from a variety of sources, such as power lines, electronic equipment, radio frequency (RF) radiation, and other electromagnetic sources.
- 25. POWER LINE INTERFERENCE • The powerline interference represents a common noise source in the ECG and other physiologic signals recorded from the body surface. • Depending on the country region, such noise is characterized by a 50 or 60 Hz sinusoidal interference, possibly accompanied by harmonics. • Powerline interference (50 or 60 Hz noise from mains supply) can be removed by using a notch filter of 50 or 60 Hz cut-off frequency.
- 27. There are several types of power line interference: • Electromagnetic Interference (EMI): This type of interference occurs when electromagnetic fields generated by power lines or electronic devices induce unwanted currents or voltages in nearby conductors. These induced currents can disrupt the operation of sensitive equipment. • Radio Frequency Interference (RFI): RFI is caused by radio frequency signals emitted by various sources, including wireless communication devices, broadcast stations, and radar systems. These signals can interfere with electronic devices that are not properly shielded against such frequencies. • Harmonic Distortion: Power lines can sometimes carry harmonics, which are multiples of the fundamental frequency (usually 50 or 60 Hz). These harmonics can lead to voltage and current distortions that affect the performance of connected devices.
- 28. RIGHT LEG DRIVEN ECG AMPLIFIER •A Driven Right Leg circuit or DRL circuit, also known as Right Leg Driving technique, is an electric circuit that is often added to biological signal amplifiers to reduce common-mode interference. Biological signal amplifiers such as ECG (electrocardiogram) EEG (electroencephalogram) or EMG circuits measure very small electrical signals emitted by the body, often as small as several micro-volts (millionths of a volt). •However, the patient's body can also act as an antenna which picks up electromagnetic interference, especially 50/60 Hz noise from electrical power lines. This interference can obscure the biological signals, making them very hard to measure. Right leg driver circuitry is used to eliminate interference noise by actively cancelling the interference. • In most modern electrocardiographic systems, the patient is not grounded at all. Instead, the right-leg electrode is connected to the output of an auxiliary op amp. The common-mode voltage on the body is sensed by the two averaging resistors Ra, inverted, amplified, and fed back to the right leg. This negative feedback drives the common-mode voltage to low value. The body's displacement current flows not to ground but rather to the op-amp output circuit. This reduces the interference as far as the ECG amplifier is concerned and effectively grounds the patient.
- 29. RIGHT LEG DRIVEN ECG AMPLIFIER
- 30. Band pass filter • A band pass filter (also known as a BPF or pass band filter) is defined as a device that allows frequencies within a specific frequency range and rejects (attenuates) frequencies outside that range. • The low pass filter is used to isolate the signals which have frequencies higher than the cutoff frequency. Similarly, the high pass filter is used to isolate the signals which have frequencies lower than the cutoff frequency. • By the cascade connection of high pass and low pass filter makes another filter, which allows the signal with specific frequency range or band and attenuate the signals which frequencies are outside of this band. This type of filter is known as Band Pass Filter.
- 31. Active Band Pass Filter • The circuit diagram of Active Band Pass Filter is divided into three parts. The first part is for a high pass filter. Then the op-amp is used for the amplification. The last part of the circuit is the low pass filter.
- 32. Active Band Pass Filter
- 33. Passive Band Pass Filter • The passive filter used only passive components like resistors, capacitors, and inductors. Therefore, the passive band pass filter is also used passive components and it does not use the op- amp for amplification. So, like an active band pass filter, the amplification part is not present in a passive band pass filter. The passive band pass filter is a combination of passive high pass and passive low pass filters.
- 34. Wide band pass filter • The circuit diagram of this filter is as shown in the below figure where the first half is for active high pass filter and the second half is for active low pass filter. It is easy to design the circuit for a wide range of bandwidth.
- 35. Narrow band pass filter The bandwidth of this filter is narrow. Therefore, it allows the signal with a small range of frequencies. It has multiple feedback. This band pass filter uses only one op-amp. In this band pass filter, the op-amp is used in non-inverting mode.
- 37. Thank you