BIOPOTENTIAL AMPLIFIERS - Design and Development of Medical Electronic Instrumentation - page 16

Figure 1.14 Single-lead ECG recordings: (a) using an Ag/AgCl standard bioelectrode; (b) using
the capacitive active bioelectrode. (Reprinted from Prutchi and Sagi-Dolev [1993], with permission
from the Aerospace Medical Association.)

is built around one-half of two TL064 quad op-amps. Eight copies of this circuit
constitute the 16 identical biopotential ampli cation channels. Operation of a single

channel is described in the following discussion.
A biopotential signal detected by a bioelectrode is coupled to the noninverting inputs of

fi

the er. The input impedance is given

mostly by the input impedance of the front-stage op-amps, yielding

fi first-stage ampli fi er and the shield driver ampli fi

100 M Ω paralleled

with 100 pF. R1 limits the current that can flow through the input lead, while diodes D1

and D2 shunt to ground any signal that exceeds their zener voltage. This arrangement pro-
tects the inputs of the ampli

fl

fi ers from ESD and from the high voltages present during car-

diac de brillation. Furthermore, it protects the subject from currents that may leak back

from the ampli

fi

ers or associated circuitry.
The shield driver is con

fi

er. The actual drive, however, deter-
mined by R2 and R3, is set to 99% of the signal magnitude at the inner wire to stabilize

fi gured as a unity-gain bu ff