DESIGN AND DEVELOPMENT OF MEDICAL ELECTRONIC ...

36 BIOPOTENTIAL AMPLIFIERSresults in an appropriate frequency response may be selected. Assuming that the switchingfrequency allows for a large oversampling of the desired passband, z can be approximatedby the continuous Laplace term (1 sT ); thenH Φinodd;Φoutodd z1sT(z) ⎯⎯→ H(s) sT(1 C1H/C S ) 1Comparing this to the continuous frequency-domain transfer function of a simple RC lowpassfilter yields1H(s) (s/ω 1)1where ω 1 1/RC. Then a cutoff frequency of f 3dB 1/2πRC may be obtained through acapacitance ratio of C H 1C 2πfSThe analogy to an RC low-pass filter is not a mere coincidence, because the DPDTswitches together with C S constitute the parallel switched-capacitor resistor realization ofFigure 1.30f, whose value is given by3dBTR equivalent TC SThe capacitance of C S must be computed to comply with a desired input impedance, whichis dependent on the sampling frequency. It must be noted that because of the fact that common-modesignals are not sampled, their spectral content may well exceed the samplingfrequency without encountering aliasing.It may be seen from the analysis presented above that after charge reorganization hasbeen achieved during an even-phase interval, the output voltage is held as long as the samplingcapacitor does not bring a new sample in contact with the hold capacitor. This propertymay be used to reject stimulation artifacts (i.e., high-amplitude spikes caused bycurrents caused by a pulse generator intended to cause tissue stimulation) by extending theeven-phase interval, making it slightly longer than the stimulation pulse to be rejected. Thistechnique effectively isolates the stimulation artifact from the high-gain amplification andprocessing circuitry, following the instrumentation stage, allowing for the immediately consecutivedetection of biopotentials.Although switched-capacitor instrumentation stages are not very common in patientmonitors, they are often used as the core of biopotential amplifiers in implantable devices(e.g., pacemakers). In addition, many modern analog signal processing applications relyon switched-capacitor sampled-data processing techniques implemented through the useof CMOS charge manipulation circuits. In these applications, CMOS application-specificICs (ASICs) contain switches and capacitors that are used as an economical means of massproducing sophisticated signal processing functions, such as amplification, analog arithmetic,nonlinear functions, and filtering [Allen and Sanchez-Sinencio, 1984].In the circuit of Figure 1.31, IC3, a monolithic charge-balanced dual switched-capacitorinstrumentation building block (Linear Technology’s LTC1043), implements all of therequired charge manipulation functions. Within this integrated circuit, a nonoverlappingclock controls two DPDT CMOS switch sections. If the switched-capacitor stage IC3would be connected directly to a differential biopotential source detected by small-areasurface electrodes, and the sampling frequency is chosen to be 100 times that of the highestspectral component of the signal, the optimal value of the sampling capacitor wouldresult in the picofarad range to present an input impedance in the gigaohm range.