BIOPOTENTIAL AMPLIFIERS - Design and Development of Medical Electronic Instrumentation - page 15

Figure 1.13 A miniaturized version of the capacitive bioelectrode array may be assembled on a
single flexible printed circuit. This assembly can be encapsulated and embedded at the appropriate
position within the inner padding of a flight helmet for differential measurement of the EEG between
positions Fp1 and Fp2 of the International 10-20 System. Conductive foam is used to establish non-
active reference either at positions A1 and A2 or at the chin of the subject. (Reprinted from Prutchi
and Sagi-Dolev [1993], with permission from the Aerospace Medical Association.)

EEG and ECG signals recorded using the new pasteless bioelectrodes compare very well
to recordings obtained through standard Ag/AgCl electrodes. Figure 1.14 presents a
digitized tracing of a single-lead ECG signal detected with a capacitive pasteless bioelec-
trode as well as with a standard Ag/AgCl electrode. Figure 1.15 shows digitized EEG sig-
nals recorded from a frontal di erential pair with a reference at A2 using a pasteless

biopotential electrode array and with standard Ag/AgCl electrodes.

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SINGLE-ENDED BIOPOTENTIAL AMPLIFIER ARRAYS

Single-ended op-amp ampli fi ers were in the past used as front-end stages for biopotential

ampli fi ers. As we will see later, the advent of low-cost integrated instrumentation

ampli fi ers has virtually eliminated the need to design single-ended biopotential ampli fi ers,

and as such, the use of single-ended biopotential ampli ers is not recommended. Despite

this, this section has strong educational value because it demonstrates the design principles
of using single-ended ampli

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fi ers, which are common in the stages that follow the

bioampli fi er's front end. Figure 1.16 shows an array of 16 single-ended biopotential

ampli ers. A number of these circuits may be stacked up to form very large arrays, which

made them common for applications such as body potential mapping electrocardiography
in the days when single op-amps were expensive.
Each biopotential ampli

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cation channel features high-impedance ESD-protected
inputs, current limiting, and de

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brillation protection. Individual shield drives are used to
protect each input lead from external noise. Each channel provides a

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fi fixed gain of 1000

within a fi xed ( 3-dB) bandpass of 0.2 to 100 Hz. The chief advantage of the single-

ended con guration is its simplicity, but this comes at the cost of lacking high immunity

to common-mode signals. Because of this, single-ended biopotential ampli

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ers are
usually found in equipment that incorporates other ways of suppressing common-mode
signals. In this circuit, an onboard adjustable 50/60-Hz notch

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filter is connected at the
output of each channel. The schematic diagram of Figure 1.17 shows how each channel

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