BIOPOTENTIAL AMPLIFIERS - Design and Development of Medical Electronic Instrumentation - page 22

skin- electrode impedance and is given by

V test ( Z i R2)

1

V i G i

R

where G i is the gain of each ampli er in the array.

For simplicity and convenience, the test signal can be generated by a computer
and phased demodulation can be implemented in software. Impedance tests can be
performed just prior to data collection as well as at selected times throughout an
experiment, making it easy to locate faulty electrode-skin connections even in large
ampli

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er arrays. Further theoretical and practical considerations regarding the construc-
tion of large single-ended biopotential ampli

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fi er arrays may be found in a paper by Van

Rijn et al. [1990].
To use the BPD circuit in conjunction with biopotential ampli ers, connect the BPD

reference terminal (J1-1) to the reference electrode ( subject ground ) of the biopotential
ampli

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er system. Place the sense electrode (e.g., a standard Ag/AgCl ECG electrode) in
contact with the body in the proximity of the biopotential ampli

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er's active electrode(s)
and connect it to J1-2 of the BPD circuit using shielded cable (with the shield connected
to J1-1). A similar electrode placed at a distant point on the body should be connected to
the “drive'' output (J1-3) of the BPD. Upon hooking up a 9-V alkaline battery to the appro-
priate power inputs (

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terminal to J1-6), common-mode signals
should be neutralized. The moving dot on the display shows the relative maximum ampli-
tude of the BPD voltage. This can be used to assess the conditions of the recording envi-
ronment.
In general, use of a separate sense electrode is not be recommended for any newly
designed equipment. Whenever active common-mode suppression is required, the instru-
ment should be designed such that the common-mode potential used for BPD is obtained
from the outputs of the biopotential ampli

terminal to J1-5 and

er's front end. However, a stand-alone BPD
such as the one shown in Figure 1.19 can be used to boost the performance of older

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Figure 1.19 A body potential driver can be constructed as a stand-alone unit powered by a 9-V bat-
tery. This circuit can be used in conjunction with existing biopotential ampli ers to boost the com-

mon-mode rejection of older equipment. The LED display shows the relative maximum amplitude
of the BPD voltage to assess the conditions of the recording environment.

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