ECG Acquisition in a Computer - ECG Acquisition and Automated Remote Processing - page 57

body) up to 100 lA at not less than 0.1 Hz. Normally, patient safety is achieved
in two ways, viz., providing complete galvanic isolation between the patient
and acquisition circuit (or biomedical equipment) and using surge protection
arising from defibrillator or electrosurgical equipment [ 21 ]. Galvanic isolation
is achieved by electrically separating the input stage of the isolation amplifier
(IsoA) from the output stage. That is, the input stage has a separate floating
power supply and a 'ground' that are connected to the output side of the IsoA
by a high resistance (around 1,000 MX) and a low parallel capacitance (around
picofarad range). The signal terminals of the input stage are isolated from the
IsoA's output by similar high impedance. An IsoA is realized by any of the
following techniques, viz. (1) isolation transformer through high-frequency
magnetic coupling, (2) optical isolation using LED-phototransistor combina-
tion, and (3) capacitive coupling using a signal modulated high-frequency
digital carrier from the input stage (isolated) through a pair of capacitors to a
demodulator at the output stage.

In transformer isolation, transformer with a toroidal core is used to couple high-
frequency (around 500 kHz) AC power from output to isolated input stages where
it is rectified and filtered. The output from the isolated phase modulates an AC
carrier magnetically coupled to a demodulator at the output stage [ 22 ]. Using
magnetic isolation, a breakdown voltage of 7 kV can be achieved. Optical isola-
tion uses either linear analog photo-optic coupling or digital modulation of the
amplified signal. However, in optical coupling, separate isolated power supply is
required to the input stages through high-frequency isolation amplifiers. Linear
analog photo-optic coupling can be implemented one infrared LED, optically
coupled with two identical phototransistors, either photoconductive or photovol-
taic mode of operation. Figure 3.4 shows schematic circuit of LOC 110 linear
optocouplers from Clare Inc. [ 23 ], operated in photoconductive mode. One pho-
totransistor (T 1 ) provides a servo feedback to stabilize the LED (D) driving circuit
current, and the other (T 2 ) one provides the galvanic isolation between input and
output. The basic operation of the circuit is given as follows:
As the input V IN increases, I F increases and D starts to conduct. The incident
optical flux on T 1 causes a current I 1 to flow, which in turn increases the voltage at
inverting terminal of OPAMP U1 by a drop I 1 R 1 . The voltage V A at non-inverting
terminal gradually becomes equal to V IN , and then, no further increase in I F occurs.
Thus, for a particular I F , the circuit generated a fixed current I 2 (the output of T 2 ),
which is proportional to LED flux and hence equal to I 1 . Hence, the output of the
amplifier is given as

V OUT ¼ I 2 R 2 ¼ V IN
R 1

ð 3 : 5 Þ

R 2

Using optical coupling, a breakdown voltage up to 4-7 kV can be achieved. In
capacitive IsoA, an internal oscillator is used to modulate the analog differential
signal to a digital pulse train, which is transmitted across the isolation barrier, built
around a matched pair of capacitors (1-3 pfd range). At the output stage, the