DESIGN OF SAFE MEDICAL DEVICE PROTOTYPES - Design and Development of Medical Electronic Instrumentation - page 123

Figure 3.17 The number of channels that can be acquired through the isolated A/D can be
expanded by using additional multiplexers. This circuit implements a 64-channel analog signal mul-
tiplexer with isolated control.

analog signal multiplexer with isolated control. Through its use, up to 64 high-level ana-
log signals may be connected selectively to an A/D or other acquisition and recording
instrumentation. Signals may originate from biopotential ampli

fi ers, physiological sen-

sors, 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 between
the analog electronics and the digital control. Additional multiplexer circuits may be stacked
to increase the channel capability of a system. In PC-based data acquisition systems, this
board may be used to collect analog signals from up to 64 channels through a single analog
input of the A/D. The digital control is easily obtainable from a parallel output port available
in most every PC data acquisition board.
Typical multichannel biopotential signal acquisition applications include topographic
brain mappers, body potential mapping (BPM) ECG, the recording of surface array
EMG signals, and so on. The signal range for the DG508 multiplexers in the circuit of
Figure 3.18 is 12 V with a bandwidth of at least 50 kHz. The in-out resistance of

a selected channel is less than 600 . Scanning of an array can be done at a maximum

frequency of 1000 channels/s. Channel selection is accomplished via optoisolators,
which can be driven directly by TTL logic. The circuit requires

12 V isolated power

to operate.

POWER SUPPLIES

As Figure 3.1 showed, having reinforced insulation between the applied part and every
other part of the medical instrument does not mean that similarly strong insulation is not
needed between a mains part and other live or nonlive parts besides the applied part. This
implies that although you may be using a component which itself powers the applied part
across an appropriate isolation barrier (e.g., ISO107, 284J), the instrument's power supply
must still meet the same requirements as a safety isolation transformer.