Mindray Passport 5-Lead, Service Manual

Page: 219 / 544

Passport 5-Lead, 5L, LT, XG Service Manual 0070-00-0420 2 - 17
Theory of Operation Block Diagrams
Pressure
The Pressure Preamplifier (Figure 2-9 on page 2-17) is comprised of two channels of
identical Differential Amplifiers. The P1 channel amp is comprised of U20 and associated
components. The amp has a gain of 195 at DC, a bandwidth of approximately 15 Hz (the -
3 dB point), an output biased of 1.8 volts. This biasing is necessary to allow for transducers
that have positive or negative offsets as large as 120 millimeters of mercury (mmHg). Full
scale output is limited to 300 mmHg positive pressure, relative to atmosphere, and is scaled
at 4.88 millivolts (one A/D lsb) per mmHg, assuming a transducer scale factor of 5
microvolts per volt excitation per mmHg.
FIGURE 2-9
Pressure Preamp Block Diagram
This amplifier will accept, as an input, a standard invasive pressure transducer which
appears as a resistive bridge, with each leg being approximately 350 ohms, excited using
+5 volts and ground. This excitation voltage is provided via the +IN and -IN wires, while the
transducer output is input to the amplifier via the +OUTP1 and -OUTP1 wires at
approximately a +2.5 volt common mode voltage. Sensor disconnect indication, a
permanent full scale output, is created by the input resistors R13 and R21 when no
transducer is attached.
The operation of the second channel, P2, is identical to the above description of P1 (with
different input and output labels), using U20 and associated components. The P1 and P2
signals are measured by the analog to digital converter, U21.
Bandpass Filter
Pulse Detect
+ OUT P1
- OUT P1
+ OUT P2
- OUT P2
+ IN
- IN
Pressure
Pre Amp
+5 T
Differential Amplifier
Differential Amplifier
U20
U20
U21
U21
P Trigger
P1
P2