Mindray Passport 5-Lead, Service Manual

Page: 230 / 544

Block Diagrams Theory of Operation
2 - 28 0070-00-0420 Passport 5-Lead, 5L, LT, XG Service Manual
Analog Section
The operation of the analog section is controlled by the digital part of the board by:
• Supplying
CLKSEQ
clock signal to the sequencer.
• Latching signals D0..7, into control register with strobe signals
APOUT0
,
APOUT1
, and
APOUT3 .
• Monitoring COMPOUT* and INTERF* signals.
Sequencer controls the front end of analog section. It provides RED DRV and IR DRV
signals for led drive circuit which in turn alternately drive red and infrared LED emitters in the
probe. A single photo-detector on the opposite side of the finger produces current pulses
proportional to the amount of light received. Sequencer also sets the gain of the current to
voltage pre-amp and controls the demultiplexer. Gain values for current to voltage pre-amp
are derived from control register signals
IR0 , IR1 , RED0 , and RED1 . The fact that the
sequencer has synchronous control of led drive, gain of current to voltage pre-amp, and
demultiplexer makes it possible to set different current to voltage gain values for infrared and
red signals.
Control register provides data for the DAC and sets gain values of dc gain ( DCG0 , DCG1 ) ,
ac gain ( ACG0 , ACG1 ), and current to voltage pre-amp ( IR0 , IR1 , RED0 , RED1 ) stages.
Wide gain range gives the board enough flexibility to acquire signals from fingers spanning
a wide range of thicknesses or alternatively from other sites of the body such as ears, noses
or toes. It also controls the remultiplexer (
CHNLMUX ) and provides calibration signal
RUN/CAL , which is used by the sequencer to determine the operation mode, and test signal
DIAG , which supplies a fixed voltage source at demultiplexer input for circuit diagnostics.
Again because control register can synchronously control the remultiplexer and gain values
for ac and dc gain stages, different gain settings can be selected for red and infrared
signals.
Interference detector monitors the output of current to voltage preamp for voltages less than -
7.2 V in amplitude. This information is sent to the digital section through
INTERF* signal.
The probe patient signal enters the board as current pulses in the range of .48 - 64 A. After
signal goes through current to voltage preamp stage it is separated by the demultiplexer
circuit, which steers each voltage pulse to one of two signals A and B. In addition, the circuit
sends a negatively amplified version of the signal level between LED pulses to both channels.
This residual signal is caused by ambient light on the photo-detector and offset voltages from
the preceding circuitry. The negative amplification sets-up cancellation of the extraneous
effect of the residual signal by the filter circuits that follow.
Signals A and B are then filtered identically by two parallel and matched filters. Filters also
reduce the effect of any noise source, which might interfere with the measurement, such as an
electro-surgical unit.
After going through filter blocks red and infrared signals and are alternately selected by the
remultiplexer for further processing. Next signal is amplified by the dc gain stage. Having
the ability to apply a different gain to the two components, this block functions as a coarse
equalization of the multiplexed signal.