BIOPOTENTIAL AMPLIFIERS - Design and Development of Medical Electronic Instrumentation - page 10

Figure 1.8 Printed circuit board for a high-input-impedance buffer array. The output of each chan-
nel is used to drive guard rings which form low-impedance isopotential barriers that shield all input
paths from leakage currents.

laid out and constructed with care to take advantage of the op-amp's high input impedance.
As shown in the PCB layout of Figure 1.8, the output of each channel is used to drive guard
rings that form low-impedance isopotential barriers that shield all input paths from leak-
age currents.
The selection of op-amps from the TLC27 family has the additional advantage that
electrostatic display (ESD) protection circuits that may degrade high input impedance are
unnecessary because LinCMOS chips have internal safeguards against high-voltage static
charges. Applications requiring ultrahigh input impedances (on the order of 10 10

) neces-
sitate additional precautions to minimize stray leakage. These precautions include main-
taining all surfaces of the printed circuit board (PCB), connectors, and components free of
contaminants, such as smoke particles, dust, and humidity. Residue-free electronic-grade
aerosols can be used e

Ω

particles from surfaces. Humidity must be
leached out from the relatively hygroscopic PCB material by drying the circuit board in a
low-pressure oven at 40

ff ectively to dust off ff

C for 24 hours and storing in sealed containers with dry silica gel.
If even higher input impedances are required, approaching the maximal input impedance
of the TLC24L4, you may consider using Te fl on 2 PCB material instead of the more com-

mon glass-epoxy type.
Typical applications for this circuit include active medallions , which are electrode con-
nector blocks mounted in close proximity to the subject or preparation. The low input
noise (68 nV/ H

) and high bandwidth (dc—10 kHz) make it suitable for a broad range of
applications. For example, 32 standard Ag/AgCl electroencephalography (EEG) electrodes
for a brain activity mapper could be connected to such a medallion placed on a headcap.
Figure 1.9 shows another application for the circuit as an active electrode array in elec-
tromyography (EMG). Here eight arrays were used to pick up muscle signals from 256
points. Connectors J1 in each of the circuits were made of L-shaped gold-plated pins that are
used as electrodes to form an array with a spatial sampling period of 2.54 mm (given by the
pitch of a standard connector with 0.1-in. pin center to center). The outputs of the op-amp
bu

z

ff ers can then carry signals to the main biopotential signal ampli fi ers and signal processors

2 Te

fl on is a trademark of the DuPont Corporation.