Mindray Passport 5-Lead, Service Manual

Page: 218 / 544

Block Diagrams Theory of Operation
2 - 16 0070-00-0420 Passport 5-Lead, 5L, LT, XG Service Manual
The patient’s resistance variation due to respiration (.05 ohm to 5 ohms, out of 50 ohms to
2K unmodulated impedance) reflects back through the transformer and results in a
voltage variation at Patient Drive Amplifier U11 pin 7 of approximately 69 microvolts to 6.9
millivolts. This modulated voltage sits on a carrier of approximately 3 to 8 volts. The above
modulation and carrier values assume zero cable capacitance shunting the patient
impedance. For typical cable capacitances (325 pf for 10’ patient cable) and reasonable
patient impedances of less than 1 kilohm, this attenuation is not appreciable, nor is any noise
created by moving the patient leads. Even for the smallest of modulated signals (about 70
microvolts), the carrier amplitude at the output of U11 is sufficient (about 3 volts p-p, centered
around 0 volts) to forward bias rectifier CR35. After halfwave rectification, the signal
amplitude is roughly 700 microvolts per ohm. The carrier portion of the signal ranges from
approx. +0.9 to +3.4 volts (which is half the voltage less a .6 volt diode drop). Differenced
with +2.5 volts and amplified by DC Amplifier U13, this yields a carrier range of -2.2 to
+4.2 volts at U13 pin 1. Increased cable capacitance would serve to make these values
more negative. With as much as a 500 pf cable capacitance (325pf +/-25% is more
realistic), the largest patient impedance of 2000 ohms will receive the greatest attenuation
resulting in a carrier at U13 pin 1 of -1.5 volts. At the other extreme, a patient impedance of
200 ohms, the same cable yields approximately -2.2 volts.
The signal from U13 pin 1 is AC coupled at approximately 0.1 Hz by C5 and R176, then
amplified with a gain of 580 by AC Amplifier U44 for a net gain of 1 volt per ohm (at zero
cable capacitance). This amplifier rolls off at about 4 Hz. Combined with the high frequency
cutoff of the demodulator and the AC Amplifier, this results in an overall low pass corner
frequency cutoff of about 3Hz.
Comparator U13 compares the voltage at U13 pin 1 with a 2.3 volt reference in order to
generate a “ Hi Impedance ” or “lead off” indicator signal, causing a Capacitor Reset to be
initiated by an analog switch in U18 and R103. This speeds up the signal’s return to within
the active range of the opamps. After being clipped to 5 volt level by CR41, the indicator
signal is delayed by resistor R184 and C11 just long enough for C5 to sample and capture
the proper voltage following restoration of an open lead. The lead off threshold is set just
beyond the maximum patient impedance, or approximately 2200 ohms at the leads
(assuming zero cable capacitance; greater with shunting capacitance).
The impedances of capacitors on the Interconnect Board connected to the RRA and RLA
wires, along with the clamping of CR32, CR38, and shield in T2, protect U11 from
defibrillator discharges.
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