DESIGN OF SAFE MEDICAL DEVICE PROTOTYPES - Design and Development of Medical Electronic Instrumentation - page 111

Figure 3.7 This implementation of the general-purpose biopotential ampli fi er of Figure 3.6 requires a single supply of 15 V at 80 mA,

thanks to the AD210's three-port feature.

Figure 3.9 shows a circuit that implements an interesting way of somewhat linearizing
the response of an optoisolator while simplifying the circuitry needed to introduce of ff set

to pass bipolar signals. This isolated EEG ampli er is an adaptation of a circuit by Porr

[2000]. Here, an Analog Devices AMP01 instrumentation ampli

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fi er is used as the high-

input-impedance front-end ampli er for the biopotentials collected from EEG scalp-sur-

face electrodes. The gain of this stage is 20. IC1's output is high-pass

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fi filtered by C5, C6,

and R3 to introduce a frequency of 0.32 Hz. A selectable-gain stage is imple-

mented around op-amp IC2 to boost IC1's output signal approximately 100, 200, 500, or
1000 times. A Sallen-Key second-order low-pass

3-dB cuto ff

fi filter built around IC3A is then used to

limit the bandpass of the EEG ampli er to approximately 34 Hz.

IC3B drives the LEDs of optoisolators IC4 and IC5. The phototransistor in IC4 is used
to set the inverting input of IC3B such that the LED is driven to a point that balances the
signal at IC3's noninverting input. When the phototransistor in IC5 is not illuminated, its
collector is pulled up to the nonisolated positive supply rail by R21. However, as signals
cause IC3B to drive the LED, the phototransistor pulls the collector toward the nonisolated
negative supply rail. The isolated signal is high-pass

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fi filtered by C18 and R22 and bu ff ered

via IC8 before being presented to the output.

LINEAR ANALOG ISOLATION USING OPTOISOLATORS

A way of using an optoisolator for analog signals while maintaining good linearity is rst

to convert the analog signal into a pulse train of variable frequency (or pulse width), which
is then used to drive the optoisolator. At the other side of the optoisolator, the pulse train is
demodulated to render the original signal. Another possibility is to place the optoisolator
within a servo loop that makes use of the loop's error to convey a high-linearity analog
signal. Yet another solution is to convey true-digital data through the optoisolator.
The analog isolator circuit of Figure 3.10 works by pulse-width modulating a pulse train
in proportion to the input voltage. As shown in Figure 3.11, an input signal is presented to

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