DESIGN AND DEVELOPMENT OF MEDICAL ELECTRONIC ...

DIRECT STIMULATION OF NERVE AND MUSCLE 325current-regulating elements of the current source. In a nutshell, the circuit works as follows:IC1B and Q2 will drive current through either the LEDs of IC2A and IC2D or those ofIC2B and IC2C, depending on the polarity of the input signal. This will cause current fromthe battery to be driven into the load connected to the electrodes by the H-bridge transistorsof multichannel optocoupler IC2. The current delivered to the bridge is sensed by optocouplerIC3B. The current-control loop is closed via IC1A, which receives setpoint and feedbackcurrents through IC3A and IC3B, respectively. Q1 and LED D1 turn on when IC1Adrives excessive current through the bridge, providing an indication that the voltage complianceof the source has been reached.Op-Amp Bridge StimulatorsAnother stimulator output stage configuration that is worth exploring is the op-amp bridge.The bridge connection of two op-amps provides output voltage swings twice that of one opamp.This makes it possible to reach higher stimulation or compliance voltages in applicationswith low supply voltages or applications that operate amplifiers near their maximumvoltage ratings in which a single amplifier could not provide sufficient drive. 2 In addition,the bridge configuration is the only way to obtain bipolar dc-coupled drive in single-supplyapplications.Figure 7.14a shows a generic voltage-output bridge connection of two op-amps. AmplifierA1 is commonly referred to as the master and A2 as the slave. The master amplifieraccepts the input signal and provides the gain necessary to develop full output swing fromthe input signal. The total gain across the load is thus twice the gain of the master amplifier.The master amplifier can be set up as any op-amp circuit: inverting or noninverting, differentialamplifier, or as a current source such as a Howland current pump. In the latter, onlythe master amplifier is configured as a current source. As shown in Figure 7.14b, the slaveremains as an inverting voltage amplifier.This is, however, not the only current-source circuit that can be built in bridgeconfiguration. In fact, U.S. patent 4,856,525 to van den Honert [1989] shows how to usetwo current sources per channel of a multichannel stimulator to minimize the crosscouplingof stimulating currents between channels. As shown in Figure 7.15a, eachchannel of a multichannel stimulator may have a current source that drives currentbetween one electrode and a common ground electrode. However, a single currentsource for each channel requires only that the current in the loop containing the electrodeand electrical ground be equal to the value of the current source and not necessarilythat the current passing to ground go through any particular one of electrodescoupled to electrical ground. If the electrical power supplies of the channels of the electricalstimulator are not isolated electrically, a cross current from one electrode pair toanother electrode pair could occur while maintaining the loop current requirements ofeach channel’s single current source. Van den Honert invented a multichannel currentsource configuration with improved channel isolation that does not require electricallyisolated power supplies.As shown in Figure 7.15b, each channel of the electrical stimulator has a pair of currentsources that work in concert. One current source is coupled to each electrode of eachchannel’s electrode pair. The current sources operate harmoniously. When one currentsource is sourcing (or sinking) a certain current, its complementary current source is sinking(or sourcing) an almost identical current. As such, the current passing between the2Op-amp bridge current sources are not only limited to low-voltage applications in direct stimulation of tissues.There are a number of op-amps that will operate with very high supply voltages, making it possible to use the sametechniques in the design of high-voltage external stimulators. For example, the PA89 by Apex Microtechnology israted for a total supply voltage of 1200 V.