DESIGN AND DEVELOPMENT OF MEDICAL ELECTRONIC ...

38 BIOPOTENTIAL AMPLIFIERSUsing off-the-shelf components, however, the construction of such a circuit would introduceparasitic capacitances in the same order of magnitude as the sample-and-hold capacitances,resulting in errors that the internal charge-balancing circuitry within the integratedcircuit cannot cancel. For this reason, this design includes two noninverting amplifiers IC1Band IC1D which present the switched-capacitor block with a signal level that is compatiblewith a larger-valued sampling capacitor, effectively eliminating the problems related to parasiticcapacitances. The output of each of these amplifiers is given byandV A 1 R 2R1V 1 (R 2)V 2R1V B 1 R 3R1V 2 (R 3)V 1R1Thus, if R2 R3, the theoretical differential voltage presented to the sampling capacitor isV A V B (V 2 V 1 ) 1 2R 2 R1CMOS op-amps IC1A and IC1C are configured as unity-gain buffers and serve as ultrahighimpedance to low-impedance transformers so that the biopotential signal may be carriedwith negligible loss and contamination to the instrumentation stage. In criticalapplications, these could be mounted in close proximity to the electrodes used to detect thebiopotentials. In addition, if the biopotential amplifier can be mounted close enough to thesubject, IC1A and IC1C may be omitted.In order not to reduce the high common-mode rejection that may be achieved throughuse of a switched-capacitor instrumentation block, the use of high-precision componentsis mandatory, so that the gain of the chain formed by IC1A and IC1B will closely matchthat of IC1C and IC1D. In addition, an adequate layout of the printed circuit board orbreadboard, using guard rings and shielding the sampling capacitor from external parasiticcapacitances, is necessary to preserve the common-mode rejection from being degraded.This also helps maintain the inherent ultrahigh impedance of the CMOS input buffers.An additional high-performance CMOS operational amplifier IC2, configured as a noninvertingfollower, amplifies the single-ended output of the instrumentation stage. The ultrahighinput impedance of this amplifier ensures that the performance of the switched-capacitor stageis not affected by the output load. The dc gain of the noninverting follower is given byG IC2 1 R 5R4which is multiplied by its own transfer function, the dc gain of the input amplifiers andbuffers, their transfer function, and the transfer function of the switched-capacitor instrumentationblock to yield the frequency-dependent gain of the complete system. However, theflat-response bandwidth of any modern operational amplifier is by far wider than that ofbiopotential signals, and by selecting a very high sampling frequency and the correct capacitanceratio, a virtually flat frequency response within the bandwidth of interest is achievable.Figure 1.32 shows an array of these switched-capacitor instrumentation amplifiers used todetect myoelectric signals from muscle fibers stimulated by an electrical current. Artifactsinduced by the high-voltage surface neuromuscular stimulation can be rejected by extendingthe even-phase switching interval during stimulation. To do so, an external clock drives theswitched-capacitor timing logic. Just prior to stimulation, the clock is isolated from theamplifiers by a logic AND gate, and all switched-capacitor blocks are set unconditionally toeven-phase mode. Shortly after stimulation ceases, switching at clock speed is restored.