Mindray Passport 5-Lead, Руководство по обслуживанию

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Passport 5-Lead, 5L, LT, XG Service Manual 0070-00-0420 2 - 11
Theory of Operation Block Diagrams
Daughter Board
The daughter board connects to the front end board at the existing J41 header, and also at a
new connector installed in the location formerly occupied by multiplexer U3. The J41 header
was the original attachment for the interconnect board in the 3-lead version. The respiration,
pressure and temperature signals simply pass through the daughter board from the
interconnect board to their original pins on J41. The ECG signals, however, are intercepted
to provide the 5-lead functions. Certain signals not available on J41, such as power and the
lead selection lines, are obtained from the connector installed in place of U3.
Each of the leadwires RA, LA, LL, RL and C, is brought from the interconnect board through
J2. The active leads (RA, LA, LL and C) are passed through an ESIS filter, consisting of two
cascaded R-C stages. For RA, this consists of R2-C1 and R19-C5, while identical circuits are
provided in the remaining leadwires. Each signal is then buffered by one section of quad
opamp U1, which is connected as a voltage follower. The buffer amplifier is used to provide
a bootstrapped lead bias for lead fault detection. Again using RA as a example, the series
connected diode pair CR1 and resistor R26 produce a level shifted version of the buffered
RA signal. This level shifted signal will always track two diode drops, or about 1.2 volts,
more negative than the RA signal. Bias resistor R1 is connected from this level shifted signal
to the RA lead input. Therefore, regardless of the voltage at the RA input (within range) a bias
current equal to (1.2 volts)/R1, or about 55 nA, flows as the lead bias. This bias current is
negative, meaning that the buffered lead signal will assume a strong negative voltage (near
the negative supply rail) when the leadwire is disconnected. Each of the active leads has an
identical circuit.
The RL lead has a different circuit, because it is used only as a drive lead, and never as an
active (input) lead. When displaying any lead except, I, II or III, a 5-wire cable is needed,
and RL is the drive lead. However, to allow compatibility with 3-wire cables when displaying
leads I, II or III, the LL, LA and RA wires (respectively) are used as the drive. The selection of
the drive lead is performed by multiplexer IC U4. Resistor R32 is used to attenuate the drive
signal (supplied from the front end board) so that it cannot exceed the +5 volt supply rail of
U4. Were this to occur, U4 would malfunction, and lead fault would not be reliably detected.