DESIGN AND DEVELOPMENT OF MEDICAL ELECTRONIC ...

322 STIMULATION OF EXCITABLE TISSUES(a)+V(b)-V327+-6IoutRin234-+6IoutVin4Vin7-V+VLOADLOADRfIout =______ VinRSenseRSenseIout =-Vin Rf_________RSense RinRSense(c)RinRf(d)RinRfRin23-V4-+6RSenseIoutVinRin23-V4-+6RSenseIoutVin7+VRfLOAD7+VRfLOADVin RfIout = -_________Vin RfRSense Rin___________Iout =RSense RinFigure 7.11 Symmetrical biphasic current waveforms are often generated through voltage-to-current converters: (a) floating-load noninvertingconfiguration; (b) floating-load inverting configuration; (c) grounded-load noninverting improved Howland current pump; (d) invertingconfiguration of the Howland pump.As an example, Figure 7.12 presents the output stage of a backpack stimulator based onthe one developed by Livnat et al. [1981] for animal experiments involving chronic stimulationof the central nervous system with implanted electrodes. Pulses are generated byIC1 at a frequency set by R3 and R4 (e.g., 160 Hz). IC2A, IC3A, IC4A, IC4B, and IC5reshape the output of timer IC1 into a symmetric bipolar square-pulse train. The voltagewaveform at the output of IC5 is converted into a constant-current waveform by the invertingHowland pump built around IC6. The stimulation current can be selected by the settingof R1. In Livnat’s stimulator, power for the output stage was turned on and off througha set of CMOS switches driven by a low-frequency sequencer that selected the burst durationand frequency at which bursts were delivered to the animal. The stimulator output wasdesigned to deliver up to 1 mA into electrodes with impedances of up to 10 kΩ.Another way of producing biphasic current pulses is to use a unipolar current source thatis switched through an H-bridge so that the polarity presented to the tissue can be reversed.Take, for example, the optically isolated bipolar current source of Figure 7.13. This veryneat circuit configuration was proposed by Woodward [1998] as a laboratory stimulator forneurophysiology research. In this circuit, the four optocoupler transistors of the H-bridge(IC2) are used not only to switch the polarity of the current source, they are themselves the