- •Analysis and Application of Analog Electronic Circuits to Biomedical Instrumentation
- •Dedication
- •Preface
- •Reader Background
- •Rationale
- •Description of the Chapters
- •Features
- •The Author
- •Table of Contents
- •1.1 Introduction
- •1.2 Sources of Endogenous Bioelectric Signals
- •1.3 Nerve Action Potentials
- •1.4 Muscle Action Potentials
- •1.4.1 Introduction
- •1.4.2 The Origin of EMGs
- •1.5 The Electrocardiogram
- •1.5.1 Introduction
- •1.6 Other Biopotentials
- •1.6.1 Introduction
- •1.6.2 EEGs
- •1.6.3 Other Body Surface Potentials
- •1.7 Discussion
- •1.8 Electrical Properties of Bioelectrodes
- •1.9 Exogenous Bioelectric Signals
- •1.10 Chapter Summary
- •2.1 Introduction
- •2.2.1 Introduction
- •2.2.4 Schottky Diodes
- •2.3.1 Introduction
- •2.4.1 Introduction
- •2.5.1 Introduction
- •2.5.5 Broadbanding Strategies
- •2.6 Photons, Photodiodes, Photoconductors, LEDs, and Laser Diodes
- •2.6.1 Introduction
- •2.6.2 PIN Photodiodes
- •2.6.3 Avalanche Photodiodes
- •2.6.4 Signal Conditioning Circuits for Photodiodes
- •2.6.5 Photoconductors
- •2.6.6 LEDs
- •2.6.7 Laser Diodes
- •2.7 Chapter Summary
- •Home Problems
- •3.1 Introduction
- •3.2 DA Circuit Architecture
- •3.4 CM and DM Gain of Simple DA Stages at High Frequencies
- •3.4.1 Introduction
- •3.5 Input Resistance of Simple Transistor DAs
- •3.7 How Op Amps Can Be Used To Make DAs for Medical Applications
- •3.7.1 Introduction
- •3.8 Chapter Summary
- •Home Problems
- •4.1 Introduction
- •4.3 Some Effects of Negative Voltage Feedback
- •4.3.1 Reduction of Output Resistance
- •4.3.2 Reduction of Total Harmonic Distortion
- •4.3.4 Decrease in Gain Sensitivity
- •4.4 Effects of Negative Current Feedback
- •4.5 Positive Voltage Feedback
- •4.5.1 Introduction
- •4.6 Chapter Summary
- •Home Problems
- •5.1 Introduction
- •5.2.1 Introduction
- •5.2.2 Bode Plots
- •5.5.1 Introduction
- •5.5.3 The Wien Bridge Oscillator
- •5.6 Chapter Summary
- •Home Problems
- •6.1 Ideal Op Amps
- •6.1.1 Introduction
- •6.1.2 Properties of Ideal OP Amps
- •6.1.3 Some Examples of OP Amp Circuits Analyzed Using IOAs
- •6.2 Practical Op Amps
- •6.2.1 Introduction
- •6.2.2 Functional Categories of Real Op Amps
- •6.3.1 The GBWP of an Inverting Summer
- •6.4.3 Limitations of CFOAs
- •6.5 Voltage Comparators
- •6.5.1 Introduction
- •6.5.2. Applications of Voltage Comparators
- •6.5.3 Discussion
- •6.6 Some Applications of Op Amps in Biomedicine
- •6.6.1 Introduction
- •6.6.2 Analog Integrators and Differentiators
- •6.7 Chapter Summary
- •Home Problems
- •7.1 Introduction
- •7.2 Types of Analog Active Filters
- •7.2.1 Introduction
- •7.2.3 Biquad Active Filters
- •7.2.4 Generalized Impedance Converter AFs
- •7.3 Electronically Tunable AFs
- •7.3.1 Introduction
- •7.3.3 Use of Digitally Controlled Potentiometers To Tune a Sallen and Key LPF
- •7.5 Chapter Summary
- •7.5.1 Active Filters
- •7.5.2 Choice of AF Components
- •Home Problems
- •8.1 Introduction
- •8.2 Instrumentation Amps
- •8.3 Medical Isolation Amps
- •8.3.1 Introduction
- •8.3.3 A Prototype Magnetic IsoA
- •8.4.1 Introduction
- •8.6 Chapter Summary
- •9.1 Introduction
- •9.2 Descriptors of Random Noise in Biomedical Measurement Systems
- •9.2.1 Introduction
- •9.2.2 The Probability Density Function
- •9.2.3 The Power Density Spectrum
- •9.2.4 Sources of Random Noise in Signal Conditioning Systems
- •9.2.4.1 Noise from Resistors
- •9.2.4.3 Noise in JFETs
- •9.2.4.4 Noise in BJTs
- •9.3 Propagation of Noise through LTI Filters
- •9.4.2 Spot Noise Factor and Figure
- •9.5.1 Introduction
- •9.6.1 Introduction
- •9.7 Effect of Feedback on Noise
- •9.7.1 Introduction
- •9.8.1 Introduction
- •9.8.2 Calculation of the Minimum Resolvable AC Input Voltage to a Noisy Op Amp
- •9.8.5.1 Introduction
- •9.8.5.2 Bridge Sensitivity Calculations
- •9.8.7.1 Introduction
- •9.8.7.2 Analysis of SNR Improvement by Averaging
- •9.8.7.3 Discussion
- •9.10.1 Introduction
- •9.11 Chapter Summary
- •Home Problems
- •10.1 Introduction
- •10.2 Aliasing and the Sampling Theorem
- •10.2.1 Introduction
- •10.2.2 The Sampling Theorem
- •10.3 Digital-to-Analog Converters (DACs)
- •10.3.1 Introduction
- •10.3.2 DAC Designs
- •10.3.3 Static and Dynamic Characteristics of DACs
- •10.4 Hold Circuits
- •10.5 Analog-to-Digital Converters (ADCs)
- •10.5.1 Introduction
- •10.5.2 The Tracking (Servo) ADC
- •10.5.3 The Successive Approximation ADC
- •10.5.4 Integrating Converters
- •10.5.5 Flash Converters
- •10.6 Quantization Noise
- •10.7 Chapter Summary
- •Home Problems
- •11.1 Introduction
- •11.2 Modulation of a Sinusoidal Carrier Viewed in the Frequency Domain
- •11.3 Implementation of AM
- •11.3.1 Introduction
- •11.3.2 Some Amplitude Modulation Circuits
- •11.4 Generation of Phase and Frequency Modulation
- •11.4.1 Introduction
- •11.4.3 Integral Pulse Frequency Modulation as a Means of Frequency Modulation
- •11.5 Demodulation of Modulated Sinusoidal Carriers
- •11.5.1 Introduction
- •11.5.2 Detection of AM
- •11.5.3 Detection of FM Signals
- •11.5.4 Demodulation of DSBSCM Signals
- •11.6 Modulation and Demodulation of Digital Carriers
- •11.6.1 Introduction
- •11.6.2 Delta Modulation
- •11.7 Chapter Summary
- •Home Problems
- •12.1 Introduction
- •12.2.1 Introduction
- •12.2.2 The Analog Multiplier/LPF PSR
- •12.2.3 The Switched Op Amp PSR
- •12.2.4 The Chopper PSR
- •12.2.5 The Balanced Diode Bridge PSR
- •12.3 Phase Detectors
- •12.3.1 Introduction
- •12.3.2 The Analog Multiplier Phase Detector
- •12.3.3 Digital Phase Detectors
- •12.4 Voltage and Current-Controlled Oscillators
- •12.4.1 Introduction
- •12.4.2 An Analog VCO
- •12.4.3 Switched Integrating Capacitor VCOs
- •12.4.6 Summary
- •12.5 Phase-Locked Loops
- •12.5.1 Introduction
- •12.5.2 PLL Components
- •12.5.3 PLL Applications in Biomedicine
- •12.5.4 Discussion
- •12.6 True RMS Converters
- •12.6.1 Introduction
- •12.6.2 True RMS Circuits
- •12.7 IC Thermometers
- •12.7.1 Introduction
- •12.7.2 IC Temperature Transducers
- •12.8 Instrumentation Systems
- •12.8.1 Introduction
- •12.8.5 Respiratory Acoustic Impedance Measurement System
- •12.9 Chapter Summary
- •References
8
Instrumentation and Medical
Isolation Amplifiers
8.1Introduction
Instrumentation amplifiers (IAs) are basically differential amplifiers characterized by very high input impedance, very high common-mode rejection ratio (CMRR), and differential gain set by a single resistor, generally in the range from ∞1 to ∞1000. In addition, IAs also have low noise, offset voltage, bias current, and offset current.
Medical isolation amplifiers (IsoAs) provide an ultra-low conductive pathway between the input (patient) terminals and the output terminals and ground. This pathway provides what is called ohmic or galvanic isolation for a patient. In medical applications, this isolation is required for reasons of patient safety. The dc resistance between input and output terminals is typically on the order of gigaohms (thousands of megohms); at ac, capacitance between input and output terminals is on the order of single picofarad. There are five established isolation architectures
1.Transformer isolation in which power is coupled to the (isolated) input stage by high-frequency current and signal is coupled to the output stage also by transformer, by modulating the power supply oscillator frequency
2.Photo-optic coupling in which the isolated conditioned input signal is coupled to the output by means of photo-optic couplers (using an LED and a photodiode or photoresistor). Power is still supplied through high-frequency isolation transformer.
3.Capacitive coupling of a signal-modulated, high-frequency digital carrier from the isolated input stage through a pair of 1-pF capacitors to a demodulator in the output stage
4.Magnetic coupling using giant magnetoresistive resistors (GMRs) in a Wheatstone bridge. A GMR’s resistance is altered by its local magnetic field. The isolated input signal is converted to a current
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© 2004 by CRC Press LLC
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Analysis and Application of Analog Electronic Circuits |
that is passed through coils in close proximity to two GMRs in a bridge. The Rs unbalance the bridge, which is on the output side of the IA. The unbalance is detected by a DA, which generates a current used to re-null the bridge. The renulling current is sensed and is proportional to the input voltage. Isolation is maintained by the ohmic isolation between the input coils and the GMR bridge resistors.
5.The flying capacitor chopper circuit uses a small capacitor charged up by the signal voltage and then switched by a high-speed DPDT relay to an output amplifier that reads the voltage across the capacitor. Such switched-capacitor IAs have ohmic isolation set by the relay structure and are useful only for dc or very low-frequency signals.
8.2Instrumentation Amps
IAs, per se, are not suitable for medical applications because medical amplifiers require severe ohmic (galvanic) isolation. Medical amplifiers can use IAs in their front ends, however. Several IC manufacturers make IAs, e.g., Burr–Brown and Analog Devices.
This section first examines some of the properties of a low-cost, low-power IA, the AD620. This device can have its differential gain set from 1 to 103 by a single resistor. The −3-dB bandwidth (BW) varies with gain in accordance with the gain ∞ bandwidth constancy relation. At Av = 103, the BW is approximately 10 kHz; at Av = 100, the BW is approximately 120 kHz; at Av = 10, the BW is approximately 400 kHz; and at Av = 1, the BW is approximately
1MHz.
The equivalent short-circuit input voltage noise root power spectrum
depends on Av. For Av = 100, 103, ena = 9 nV/ Hz at 1 kHz. The input noise increases as the gain Av decreases. Input current noise ina = 100 fA RMS/ Hz,
regardless of gain. The AD620’s CMRR varies with gain, ranging from 90 dB at Av = 1 to 130 dB at Av = 103. This IA’s input bias current, IB, = 0.5 nA; the input offset voltage, Vos, is 15 μV. The input impedance is 10 GΩ˙2 pF for common-mode and difference mode inputs (see Chapter 3).
A low-noise precision IA, the AMP01 by Analog Devices, offers slightly different specifications. The gain is set between 1 and 103 by a single resistor. The −3-dB bandwidths are approximately 26, 82, 100, and 570 kHz for gains
of 103, 102, 10, and 1, respectively. The short-circuit input voltage noise root spectrum is 5 nV/ Hz at Av = 103, and 1 kHz, and the current noise spectrum is 0.15 pA/ Hz at Av = 103, and 1 kHz. The AMP01’s input resistance is 1 GΩ
for difference-mode signals at Av = 103 and 20 GΩ for common-mode inputs at Av = 103. The input bias current is typically 1 nA and the input offset
© 2004 by CRC Press LLC
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FIGURE 8.1
Simplified schematic of an Analog Devices’ AD620 instrumentation amplifier.
voltage is typically ±20 μV. The AMP01’s CMRR is 130, 130, 120, and 100 dB for gains of 103, 102, 10, and 1, respectively.
The advantage of using a “boughten” IA instead of a “homebrew” IA made from three op amps (see Section 3.7.2 in Chapter 3) is that, in order to meet the out-of-the-box CMRR specifications of the commercial IA, the designer needs resistors matched to at least 0.02% and the input op amps also need to be matched. Components matched to 0.02% are expensive in terms of time or of money. Figure 8.1 illustrates a simplified schematic of the AD620 from its data sheet supplied by Analog Devices. Super-beta transistors Q1 and Q2 connected as a differential pair are the input elements of this IA. The 400-Ω resistors and diodes at each transistor base serve to protect the IA from transient overvoltages. According to Analog Devices (1994):
[Negative] [Feedback through the Q1–A1–R1 loop and the Q2–A2–R2 loop maintains constant collector current of the input devices Q1, Q2 thereby impressing the input voltage across the external gain setting resistor Rgain. This creates a differential gain from the inputs to the A1/A2 outputs given by G = (R1 + R2)/RG + 1. The unity gain subtractor [DA] A3 removes any common-mode signal, yielding a single-ended output referred to as the REF [VREF] pin potential.
The value of RG also determines the transconductance of the preamp stage. As RG is reduced for larger gains, the transconductance increases
© 2004 by CRC Press LLC