DESIGN OF SAFE MEDICAL DEVICE PROTOTYPES - Design and Development of Medical Electronic Instrumentation - page 118

transfer characteristic of the complete circuit is highly linear regardless of LED nonlin-
earity, aging, temperature, or LED drive current dynamics.
In this circuit, the LED driver is prebiased by introducing a negative o set voltage into the

inverting input of op-amp IC1. The feedback photodiode sources current to R7, which is trans-
lated into the control voltage for the servo loop. The op-amp will drive transistor Q1 to supply
LED current to force su

ff

cient photocurrent to keep the voltage at the noninverting input of
IC1 equal to that at its inverting input. The output photodiode is connected to a noninverting
voltage follower ampli

er. The photodiode load resistor R11 performs the current-to-voltage
conversion. IC3 is used to bu

fi

set the output signal's baseline.
Note that none of the optocoupler-based isolators described above generate their own
isolated power. Once an isolation dc/dc converter has to be added, and considering the lim-
ited performance of these circuits, applications that require only one or two isolated signal
channels may be better off

ff er R11 as well as to zero-o ff

using commercial, self-contained analog isolation ICs. Isolators
like the optocoupler circuits shown are most often found in multichannel applications,
where the cost of a large number of self-contained isolation ICs would be prohibitive.

ff

DIGITAL ALTERNATIVE TO SIGNAL ISOLATION

The large majority of modern medical electronic instruments make use of either an embed-
ded microcomputer or an external PC for control, data processing, and display. This implies
that in most cases, an analog-to-digital converter is used at some point within the instrument
to support data acquisition functions. The circuit of Figure 3.14 places the A/D converter in
direct connection with the applied part of the medical instrument, and relays digital rather
than analog signals across the isolation barrier. This alternative over analog signal isolation
has the advantage that the additional noise, nonlinearity, and complexity of the latter can be
avoided by translating signals to digital format early in the process. Furthermore, optoiso-
lators for high-speed digital signals are inexpensive and widely available. In addition, serial
data formats can be used to minimize the number of digital signals that must be communi-
cated concurrently through the isolation barrier.
Many modern high-end medical instruments make extensive use of this philosophy. If
you have the opportunity, examine the circuit schematics of one of today's electrocardio-

Figure 3.14 The majority of modern medical electronic instruments make use of either an embed-
ded microcomputer or an external PC for control, data processing, and display, meaning that an A/D
converter is used at some point within the instrument to support data acquisition functions. This
circuit places the A/D converter in direct connection with the applied part of the medical instrument,
and relays digital rather than analog signals across an optical isolation barrier.