DESIGN AND DEVELOPMENT OF MEDICAL ELECTRONIC ...

POWER SUPPLIES 123Figure 3.17 The number of channels that can be acquired through the isolated A/D can beexpanded by using additional multiplexers. This circuit implements a 64-channel analog signal multiplexerwith isolated control.analog signal multiplexer with isolated control. Through its use, up to 64 high-level analogsignals may be connected selectively to an A/D or other acquisition and recordinginstrumentation. Signals may originate from biopotential amplifiers, physiological sensors,or a combination of both.The channel addressing is controlled digitally through a parallel asynchronous input.Optical isolation is implemented onboard to maintain patient-contact-rated isolation betweenthe analog electronics and the digital control. Additional multiplexer circuits may be stackedto increase the channel capability of a system. In PC-based data acquisition systems, thisboard may be used to collect analog signals from up to 64 channels through a single analoginput of the A/D. The digital control is easily obtainable from a parallel output port availablein most every PC data acquisition board.Typical multichannel biopotential signal acquisition applications include topographicbrain mappers, body potential mapping (BPM) ECG, the recording of surface arrayEMG signals, and so on. The signal range for the DG508 multiplexers in the circuit ofFigure 3.18 is 12 V with a bandwidth of at least 50 kHz. The in–out resistance ofa selected channel is less than 600 . Scanning of an array can be done at a maximumfrequency of 1000 channels/s. Channel selection is accomplished via optoisolators,which can be driven directly by TTL logic. The circuit requires 12 V isolated powerto operate.POWER SUPPLIESAs Figure 3.1 showed, having reinforced insulation between the applied part and everyother part of the medical instrument does not mean that similarly strong insulation is notneeded between a mains part and other live or nonlive parts besides the applied part. Thisimplies that although you may be using a component which itself powers the applied partacross an appropriate isolation barrier (e.g., ISO107, 284J), the instrument’s power supplymust still meet the same requirements as a safety isolation transformer.