GE Corometrics 126, Service Manual

Page: 120 / 440

4-62 120 Series Maternal/Fetal Monitor Revision B
2015590-001
Theory of Operation: MECG Board
Single-Wire ECG Amplifier with Right Leg Drive
The ECG amplifier itself consists of an instrumentation amplifier, an amplifier/
limiter stage, a low-pass filter/output buffer stage, an integrator stage for the right
leg drive, and an integrator stage for baseline restoration. Before connecting to any
active circuitry, the patient signals from MUX U14 are first low-pass filtered by a
differential filter consisting of resistors R19 and R22, and capacitors C15, C16, and
C18. This filter prevents high-frequency energy from affecting the normal operation
of the ECG amplifier. Resistors R23 and R24 are used to limit input current to the
amplifier due to fault conditions. After filtering and passing through the protection
circuitry, the patient signals connect to the inputs of instrumentation amplifier U7,
which provides a gain of 10. The single-ended output of U7 connects to the input of
an amplifier/limiter stage consisting of resistors R36–R39, op-amp U6, capacitor
C33, and diodes D13–D18. R36, R37, D13, and D14 form an input limiter to this
stage that prevents the input voltage to U6 from exceeding ±0.6 V. Since the
maximum output of U7 under normal signal conditions is ±0.05 V, this limiter will
only have an effect for overload conditions such as a pacemaker pulse input. U6 is
configured as a non-inverting amplifier with a gain of 100. Even with the input
limited to ±0.6 V, U6 would saturate under an overload condition. To prevent this,
diodes D15–D18 in the feedback of op-amp U6 will cause the amplifier to revert to
unity gain for output levels that exceed ±10.5 V. Zener diodes D17 and D18 have a
breakdown voltage of 8.7 V. Depending on the polarity of voltage across them, one
of the two zeners will be forward biased providing a 0.6 V drop. This makes the
combine breakdown voltage of the pair 9.3 V. Back-to-back diodes, D15 and D16,
add an additional 0.6 V to the total. With the input of U6 (pin 12) limited to ±0.6 V,
the feedback point at pin 13 must also be limited to this same potential. By adding
up the combination of breakdown voltages of the feedback diodes (9.9 V) and the
maximum voltage at pin 13, the output must reach ±10.5 V before the limiting
diodes conduct and short out R38, making U6 a unity gain amplifier. Both the input
limiter, and the feedback limiter are used to reduce the overload recovery time of
amplifier U6 from pacemaker spikes or other large signals. With the pacemaker
rejection turned off, the output of U6 connects to a single-pole low-pass filter
composed of R67 and C37 through FET switch U2. This filter has a 3 dB point of
59 Hz. The filter output is buffered by another op-amp of U6 which is configured as
a unity gain non-inverting amplifier. The output of this buffer represents the final 60
dB output stage of the ECG front end circuitry. Since the entire amplifier chain is
DC coupled, some form of high-pass function or DC correction is necessary to
eliminate baseline offsets and to maintain the ECG signals within the dynamic range
of the system. This is accomplished with an inverting integration stage composed of
resistors R31–R35, capacitors C31 and C31, diodes D11 and D12, and op-amp U10.
The final output of the ECG amplifier at U6 pin 8 is attenuated by resistor divider
R31 and R34. This attenuation is necessary to adjust for the gain difference (x100)
between the output of the limiter/amplifier stage at U6 and the offset feedback point
which is the reference input to instrumentation amplifier U7 (pin 10). The output of
the attenuator connects to an integrator stage which produces an output voltage that
is proportional to the inverse of its input voltage and the amount of time the voltage
is present. When the DC level at the output increases or decreases from 0 V, the
integrator over time will produce a feedback voltage that restores the output to 0 V.
Since the integration process is a function of time, the lower the frequency
component applied to the input, the larger the voltage swing will be at the output.
The effect of this is a cancellation of low-frequency signals at the output of the ECG
amplifier chain. This makes the ECG amplifier appear to have a high-pass filter
installed somewhere in the chain. The attenuator and time constants used in the
integrator were selected to provide a –3 dB high-pass point of 0.6 Hz. Diodes D11
and D12 along with resistor R32 speed up the response of the integrator by