Mindray Passport 5-Lead, Руководство по обслуживанию

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Passport 5-Lead, 5L, LT, XG Service Manual 0070-00-0420 2 - 15
Theory of Operation Block Diagrams
Respiration
In Fig. 2-8, the connection to the patient is made through two leads marked RRA and RLA,
which ultimately become RA and LA on the body. A 114 KHz, 500 microamp RMS sinewave
is applied to the patient through the electrodes (which are positioned more to either side of
the chest than usual while doing a 3 lead ECG). The signal is applied to the electrodes
through a small isolation Transformer (T2) and the respiration signal is reflected back through
this same transformer. The purpose of the isolation is not for patient safety (that barrier is
elsewhere) but to eliminate other stray return paths for the signal and thus an error in the
signal.
FIGURE 2-8
Respiration Preamp Block Diagram
Generation of the patient drive starts at U18, which chops between a 5 volt reference and
ground to produce a 114 kHz square wave of a precision amplitude with respect to the
analog to digital converter (i.e., the same 5 volt reference is used for the A/D reference).
The respiration drive can be disabled by a logic high on the RESPEN* signal, which inhibits
U18. A second order Bandpass Filter, U11 and associated parts, transforms this square
wave into an approximation of a sinewave, approximately 4 volts peak to peak, centered on
0 volts. This sinewave drives the inverting input of U11, the Patient Drive Amplifier, through
R91. Thus, the current in Transformer T2’s primary (being in the feedback loop of U11) is
constrained to be the sinewave voltage divided by the value of R75, resulting in
approximately 500 microamperes. The secondary signal, which is the same as the primary,
because the transformer has a 1:1 winding ratio, is applied to the electrodes via resistors
and capacitors on the Interconnect board. The capacitors behave as a short to the 114 kHz
sinewave carrier, but as a very high (greater than 5 megohms at 10 Hz) shunting impedance
to the ECG electrodes so as not to attenuate the ECG signal.
Transformer
Rectifier
Amplifier
Bandpass Filter
Squarewave
to
Sinewave
Generator
DC Amplifier AC Amplifier
Hi-Impedance
Indicator
Comparator
Capacitor
Reset
U18 Resp
Reset
RRA
RLA
Resp
RESPEN*
HI Z
114 kHz
Respiration
Pre Amp
Patient Drive
T2
U18,U11
U13
U14
U13
U11