DESIGN OF SAFE MEDICAL DEVICE PROTOTYPES - Design and Development of Medical Electronic Instrumentation - page 109

component has a true three-port design structure, which permits it to be applied as an input
or output isolator, in single- or multichannel applications. In the AD210, each port (input,
output, and power) remains independent, with 2500 V RMS (continuous) and 3500 V p-p

isolation between any two ports.
In the circuit of Figure 3.6, an AD210BN is used to power and isolate a general-purpose
instrumentation biopotential ampli er front end. Biopotential signals detected by electrodes

connected to input connector J1 are dc-coupled to the inputs of an AD620 instrumentation
ampli

fi

er via input resistors R5 and R8. Low-leakage diodes D1-D4 are used to protect the
AD620 from high-voltage transients. Resistors R3 and R9 provide a dc path for bias currents
whenever the ampli

fi

fi er is used to detect signals from capacitive sensors.

IC2 is used to bu ff er the o ff set voltage set by trimmer potentiometer R10. The o ff set level

is fed directly to the reference pin of the instrumentation ampli fi er. Gains of 1, 2, 10,

and 100 are selected through jumpers JP1-JP3. The output of the instrumentation ampli-

fi fier is fed to the input of the AD210 isolation ampli fi fier The output of the isolation ampli fi er

is 15 V to power IC1 and

IC2 is generated by the AD210. The AD210 also produces a separate, isolated

fi filtered by the active low-pass fi filter formed around IC4. Isolated

15 V which
is used to power IC4. A single 15 V at 80 mA supply is all that is needed to power the com-
plete circuit, thanks to the AD210's three-port feature. Figure 3.7 shows an implementation
of this circuit where the various insulation barriers can be seen clearly.

ANALOG SIGNAL ISOLATION USING OPTICAL ISOLATION BARRIERS

High performance usually comes at a high price, and the ISO107, 284J, and AD210BN are
no exceptions. The unit price for each of these is over $100, making their use prohibitive in
many low-cost designs as well as in instruments that involve a large number of analog
signals crossing the isolation barrier. In these cases, analog isolators can be built using low-
cost optoisolators as isolation channels. Optoisolators or optocouplers operate by emitting
and detecting modulated light. An input current drives a light-emitting device internal to the
optocoupler, and an internal photodetector drives the output circuitry. Optoisolators usually
consist of an LED and a phototransistor which are galvanically isolated from each other and
are located opposite each other in a lighttight package.
The simplest form of optical isolation for an analog signal is implemented by the cir-
cuit of Figure 3.8. In this circuit, an input voltage is converted by IC1 and transistor Q1 to
a proportional current to drive a LED. The light output of the LED is proportional to the
drive current between 500 A and 40 A. However, since the best linear behavior for opti-

cal output set is introduced in

the voltage-to-current conversion. A 1.2-V reference voltage is generated across reference
diode D1. Resistor R5 and potentiometer R6 select the fraction of the reference voltage
that should be used as an o

fl flux versus input current occurs in the range 5 to 20 A, o ff

set. The voltage divider formed by resistors R1 and R2 and
potentiometer R3 are used to scale the input signal.
The LED D2 and a photodiode D3 are mounted across from each other inside a piece
of dark PVC pipe. The silicon photodiode, operating as a light-controlled current source,
generates an output current that is proportional to the incident optical

ff

flux supplied by the
LED emitter. Translating the photodiode current back to a voltage is done with series resis-
tor R10. IC2 bu

fl

set
caused by keeping the LED always illuminating the photodiode. Although operation of this
circuit seems straightforward, its performance leaves a lot to be desired, especially as far
as stability is concerned. The problem is that the optical output of a LED as a function of
drive current is very unstable, changing widely with the LED's age, temperature, and the
dynamics of the drive current. For this reason, this circuit is not recommended except for
applications where good linearity and precision are not required.

ff ers the voltage across the current-sensing resistor and cancels the o ff