BIOPOTENTIAL AMPLIFIERS - Design and Development of Medical Electronic Instrumentation - page 20

A way of improving the common-mode rejection problem is to use single-ended
ampli

ers concurrently with body potential driver (BPD) circuits to cancel out common-
mode signals. Power line and other contaminating common-mode signals are capacitively
coupled to the body, causing current to

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fl flow through it and into ground. The body, acting

as a resistor through which a current erence between any two

points on it. The goal of a BPD is to detect and eliminate this voltage, e

fl flows, causes a voltage di ff

ectively reducing
common-mode signals between biopotential detection electrodes in the vicinity of its sense
electrode .
A BPD is implemented by detecting the common-mode potential in the area of interest
and then feeding into the body a 180

ff

version of the same signal. A feedback loop is thus
established which cancels out the common-mode potential. Circuits that have feedback are
inherently unstable, and oscillatory behavior must be prevented to make a BPD useful.
This, however, limits the BPD to a range well under its

first resonance. The performance
of the circuit within this range is dependent on the internal delay of the loop and varies
according to the frequency of common-mode signal components.
The common-mode potential used for a BPD is often acquired from the outputs of the
front stages of di

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ers. In electrocardiography, for example, a
composite signal is often generated by summing the various di

ff erential biopotential ampli fi

erential leads. This signal
is inverted and fed back to the subject's body through the right-leg electrode. This prac-
tice, commonly referred to as right-leg driving , is not optimal, especially at higher fre-
quencies where the additional delay caused by the front stages and summing circuits
degrades BPD performance.
Superior performance can be obtained by implementing a separate BPD circuit which
uses an additional electrode (sense). Any modern operational ampli

ff

er operated in open-
loop mode (with a feedback capacitor in the order of a few picofarads) can be used as the
heart of the BPD [Levkov, 1982, 1988]. In the circuit of Figure 1.18, the common-mode
signal is measured between the sense and common electrodes. This signal is applied
through current-limiting resistor R2 to the inverting input of one-half of op-amp IC1.
Operated in open-loop mode, a 180

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out-of-phase signal is injected into the body through
the drive electrode in order to cancel the common-mode voltage. D3 and D4 clip the BPD
output so as not to exceed a safe current determined by resistor R3. In addition, this meas-
ure protects the circuit from de

brillation pulses. D1 and D2 are used to protect the input
of the BPD from ESD and other transients. The low-pass

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filter formed by R2 and C5, as
well as the presence of feedback capacitor C2, stabilize the circuit and prevent it from
entering into oscillation.
The output of the BPD op-amp is recti

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fi ed by the full-wave bridge formed by D5-D8

and then ampli er built using the other half of IC1. The out-

put of this op-amp is measured and displayed by the bar graph voltmeter formed by IC3 in
conjunction with a 10-element LED display DISP1. The LM3914 bar graph driver IC has
constant-current outputs, and thus series resistors are not required with the LEDs. The cur-
rent is controlled by the value of resistors R8 and R9. Resistor values also set the range
over which the input voltage produces a moving dot on the display. Power for the circuit
is supplied by a single 9-V alkaline battery. The

fi ed by the di ff erential ampli fi

9-V supply required by IC1 is gener-
ated using IC2, an integrated-circuit voltage converter. C3, D9, and C4 are required by IC2
to produce an inverted output of the power fed through pin 8.
An additional advantage of using the BPD is the possibility of monitoring the skin-elec-
trode impedance of every electrode connected to the input of a single-ended biopotential
ampli er system. To do so, a test voltage V test fed into the inverting input of the BPD

through J1-4 induces an additional component on each of the ampli

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ed output signals.
Phased demodulation of one of these signals removes components corresponding to
detected biopotentials, leaving only an ampli

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ed version of the detected test signal V i .
Assuming that an ideal BPD is used, the amplitude of this signal depends on the

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