BIOPOTENTIAL AMPLIFIERS - Design and Development of Medical Electronic Instrumentation - page 14

contamination. IC1B, also a unity-gain bu er, is fed by the input signal, and its output

drives a shield that protects the input from leaks and noise. Resistors R3 and R2 reduce the
gain of the shield driver to just under unity in order to improve the stability of the guard-
ing circuit. Capacitor C1 limits the bandwidth of input signals bu

ff

ff ered by IC1A. The cir-

cuit is powered by a single supply of 4 V dc. Miniature power supply decoupling

capacitors are mounted in close proximity to the op-amp.
IC1A and IC1B are each one-half of a TLC277 precision dual op-amp's IC. Here again,
the selection of op-amps from the TLC27 family has the additional advantage that ESD
protection circuits which may degrade high input impedance are unnecessary because
LinCMOS chips have internal safeguards against high-voltage static charges. Note that this
circuit shows no obvious path for op-amp dc bias current. This is true if we assume that all
elements are ideal or close to ideal. However, the imperfections in the electrode anodiza-
tion, as well as in the dielectric separations and circuit board, provide su

cient paths for

the very weak dc bias required by the TL082 op-amp.
The circuit is constructed on a miniature PCB in which ground planes, driven shield
planes, and rings have been etched. The circuit is placed on top of a 1-cm 2 plate of thin
aluminum coated with hard anodization Super used as the bioelectrode. A grounded con-
ductive

flexible printed circuit ribbon
cable, which carries power for both the circuit and the signal output.
Figure 1.12 presents a prototype bioelectrode array designed to record frontal EEG sig-
nals measured di

fi film layer shields the encapsulated bioelectrode and fl

erentially (between positions Fp1 and Fp2 of the International 10-20
System), as required for an experimental GLOC detection system. One of the bioelec-
trodes contains the same circuitry as that described above. The second, in addition to the
bu

ff

er and shield drive circuits, also contains a high-accuracy monolithic instrumentation
ampli

ff

filtered signals which may
be carried to remotely placed processing stages with minimal signal contamination from
noisy electronics in the helmet and elsewhere in the cockpit.
A miniaturized version of the circuit may be assembled on a single

fi er and fi filters. Such a con fi guration provides high-level fi

fl flexible printed cir-

cuit. Driven and ground shields, as well as the flat cables used to interconnect the elec-

trodes and carry power and output lines, may be etched on the same printed circuit. As
shown in Figure 1.13, the thin assembly may then be encapsulated and embedded at the
appropriate position within the inner padding of a

fl

fl flight helmet. Nonactive reference for

the instrumentation ampli er may be established by using conductive foam lining the

headphone cavities (approximating positions A1 and A2 of the International 10-20
System) or as cushioning for the chin strap.

fi

Figure 1.12 Block diagram of a capacitive bioelectrode array with integrated amplification and filter circuits designed to record frontal EEG
signals. One of the bioelectrodes contains the same circuitry as Figure 1.11. The second also contains a high-accuracy monolithic instrumen-
tation amplifier and filters. (Reprinted from Prutchi and Sagi-Dolev [1993], with permission from the Aerospace Medical Association.)