DESIGN AND DEVELOPMENT OF MEDICAL ELECTRONIC ...

PASSIVE FILTERS 49be powered from batteries or through a properly rated isolation power supply. The sameisolation requirements apply to the output of the amplifier.BOOTSTRAPPED AC-COUPLED BIOPOTENTIAL AMPLIFIERDirect ac coupling of the instrumentation amplifier’s inputs by way of RC high-pass filtersacross the inputs degrades the performance of the amplifier. This practice loads the inputof the amplifier, which substantially lowers input impedance and degrades the CMRR ofthe differential amplifier. Although unity-gain input buffers can be used to present a highinputimpedance to the biopotential source, any impedance mismatch in the ac coupling ofthese to an instrumentation amplifier stage degrades the CMR performance of the biopotentialamplifier.Suesserman has proposed an interesting modification of the standard biopotential instrumentationamplifier to yield an ac-coupled differential amplifier that retains all of the superiorperformance inherent in dc-coupled instrumentation amplifier designs. The circuit ofFigure 2.6 is described by Suesserman [1994] in U.S. patent 5,300,896. If capacitors C3 andC4 were not present, the circuit of Figure 2.6 would be very similar to that of the ac-coupledinstrumentation amplifier described earlier in the chapter. ICIA IC1 without C3 and C4would be ac-coupled to the biopotential signal via capacitors C1 and C2. Just as in the earlierac-coupled biopotential amplifier, resistors R1, R2, R3, and R4 are needed to provide a dcpath to ground for the amplifier’s input bias currents. In this circuit, these resistors wouldlimit the ac impedance of each input to 2 MΩ (R1 R2 and R3 R4) referred to ground.With C3 and C4 as part of the circuit, however, ac voltages from the outputs of theICIA’s differential input stage are fed to the inverting inputs of their respective amplifiers.This causes the ac voltage drop across R1 and R4 to be virtually zero. Ac current flowthrough resistors R1 and R4 is practically zero, while dc bias currents can flow freely toground. This technique is known as bootstrapping, referring allegorically to the way inwhich the amplifier nulls its own ac input currents, as when one pulls his or her own bootstrapsto put boots on.Since bootstrapping capacitors C3 and C4 almost completely eliminate ac current flowthrough R1 and R4, the input current through ac-coupling capacitors C1 and C2 would alsodrop close to zero, which by Ohm’s law translates into an almost infinite input impedance(since R V/i; R tends to ∞ as i approaches 0). Suesserman described this biopotentialamplifier as having an impressive 120-dB CMRR (at 100 Hz) with an input impedance ofmore than 75 MΩ.PASSIVE FILTERSThe simplest filters are those that comprise only passive components. These filters containsome combination of resistive (R), capacitive (C), and inductive (L) elements. The inductiveand/or capacitive components are required because these elements present varying impedanceto ac currents at different frequencies. As a refresher, you may remember that inductivereactance increases with frequency, whereas capacitive reactance decreases with frequency.Most passive filters used in the processing of biopotential signals are the resistive–capacitiveor RC kind. This is because relatively large and heavy inductors would be required toimplement filters at the low-frequency bands where biopotential signals reside, makinginductive–capacitive (LC) filters impractical.Despite their simplicity, RC filters are very common and effective in processing a widevariety of biopotential signals. Take, for example, the complete biopotential amplifier