DESIGN AND DEVELOPMENT OF MEDICAL ELECTRONIC ...

52 BANDPASS SELECTION FOR BIOPOTENTIAL AMPLIFIERSAs such, this capacitor is practically a short circuit for RF currents, while it leaves lowfrequencysignals pass unimpeded.R5, R6, R9, R10, and C22 implement a low-pass filter. Because of the virtual-groundproperty governing the inputs of an op-amp, we can assume that the low-pass characteristicsof this filter are given by a 94-kΩ resistor (either R5 and R6 in series, or R9 andR10 in series) and the 470-pF capacitor. The 3-dB cutoff frequency for an RC low-passfilter isf 3dB 2π1RC which provides the input network with a first-order low-pass cutoff of approximately3.6 kHz.The biopotential amplifier’s main low-pass filters are implemented by two cascaded RCpassive filters. An op-amp unity-gain follower (IC4) buffers the signals between the cascadedsections. Both RC sections are identical, therefore setting a pole at the same frequency.However, the effect of the second RC can be suppressed by disconnecting itscapacitor through switch SW2. When SW2 is open, signals at the output of IC4 are fed tounity-gain buffer IC6 through R11. Since the input impedance of IC6 is practically infinite,R11 has no effect on the signal. However, when SW2 is closed, R11 and the capacitorselected by SW1B form a low-pass filter. The nominal cutoff frequencies that can beselected for the second-order filter were selected to be close to 1, 2, 5, 10, 20, 50, 100, 200,and 500 Hz. The exact 3-dB cutoff frequencies are shown in Table 2.3.The high-pass filters are implemented in essentially the same way as the low-pass sections.In Figure 2.8, however, the RC elements are reversed. Each high-pass section has acapacitor (C50 and C53) which opposes current flow with an impedance that variesinversely with frequency, and a resistor of selectable value that shunts the load. Both RCsections are identical, therefore setting a pole at the same frequency. However, the effectof the second RC can be suppressed by shorting C53 through SW5. Op-amp IC13 buffersthe signal between the stages. The nominal cutoff frequencies that can be selected for thesecond-order filter were selected to be close to 1, 2, 5, 10, 20, 50, 100, 200, and 500 Hz.The exact 3-dB cutoff frequencies are shown in Table 2.4.The first follower (IC13) in the high-pass filter is implemented using a LTC1152 insteadof a UPC4250 op-amp as in the case of the other followers because higher current outputis required to drive the lowest resistor values associated with the highest 3-dB cutoffTABLE 2.3 Low-Pass 3-dB Cutoff Frequencies for the BiopotentialAmplifier of Figure 2.7 Selected through SW1 a3-dB Cutoff Frequency for 3-dB Cutoff Frequency forSecond-Order Low-Pass Filter First-Order Low-Pass FilterSW1 Position (Hz) (SW2 Closed) (Hz) (SW2 Open)1 1.02 1.592 1.83 2.843 4.65 7.234 10.22 15.925 21.75 33.866 46.47 72.347 102.2 159.28 217.5 338.69 464.7 723.4aSW2 selects between first- or second-order response.