Mindray Passport 5-Lead, Service Manual

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Block Diagrams Theory of Operation
2 - 32 0070-00-0420 Passport 5-Lead, 5L, LT, XG Service Manual
LED driver switch consists of two similar circuits one for each red and infrared LEDs. TTL
pulses are limited to 3.35V by combination of CR5/R29 (CR6/R30). This is done to assure
undeviating LED current. Signal is then AC coupled by C18 (C19) and is used to switch Q2
(Q1). Transorbs CR10 and CR11 are used to provide ESD protection.
Current to Voltage Preamp
The function of Current to voltage preamp is to convert current pulses from sensor’s
photodetector elements to voltage pulses. It is based on U1A and U2B. Preamp gain value
is selected by 63C09 processor through the control register and sequencer (). Signals S1
and S2 are used to control the switches U3A, U3B, and U3C, which change the U1A
feedback resistor value.
Demultiplexer
Under sequencer’s control, demultiplexer separates the signal into red and infrared
components and subtracts DC offset and low frequency noise from the signal. Switches U5A
and U5D route the direct and inverted output of current to voltage preamplifier into red
channel (A). Switches U5B and U5C perform the same function for infrared channel (B). U6
and U7 are configured as integrators, which average inverted and noninverted signals.
Matched 18 Hz Filters
Two identical low pass filters, based on U8 and U9, provide noise rejection for LED and
demultiplexer switching noise (f1.366 KHz) and ESU noise (f500 KHz). Each filter consists
of two equivalent dual pole low pass filters, which when cascaded, provide 80 db per
decade roll off with f018.9 Hz.
Remultiplexer
Remultiplexer is controlled by 63C09 processor by toggling the CHNLMUX signal (See ).
This signal regulates switches U10A and U10D, which steer either red or infrared signals
into Programmable Gain I stage.
Programmable Gain I (DC Gain)
Programmable Gain I block is also controlled by the 63C09 processor through the Control
Register. U11A is configured as noninvering amplifier with the gain selected by state of
signals DCG0 and DCG1 (See ). This stage amplifies both AC component of the signal and
it’s DC pedestal.
Subtraction and Programmable Gain II (AC Gain)
In this block U11B is configured as an inverting summing amplifier. Two inputs to the
amplifier are red/infrared signal and output of the Digital to Analog Converter. This
configuration allows the output signal of the DAC to be subtracted from the red/infrared
signal. Subtraction function is used to get rid of most of the signal’s DC pedestal. The circuit
then amplifies the resulting signal according to the gain set with signals ACG0 and ACG1.