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Section 16: MP-205 Service Manual; 044540A-0296
16.10 LED Driver
16.10.1 References
16.11 Reset Schmitt Trigger
16.12 High Resolution A/D Converter
16.13 Input Filter
16-4
The Red filter circuit components consist of U1A6-8, R19–21, R31, R34–37,
C22, C29–30, and C37–38. The IR filter circuit components consist of
U1A10-12, R22–24, R32–33, R38–40, C23–24, C31–32, and C39 . The IR filter
is identical to the Red filter except for the component values in the last stage.
The level shifter moves the reference for the signal back to ground. The level
shifter (U1A9) selects the desired signal and shifts the signal reference from 2.2
volts reference to ground, or 0 volts. This circuit has a gain of two and an
intentional offset of 80–120 millivolts at the output.
The LED driver circuit generates regulated and programmable currents for
driving the sensor LED' s. The circuit switches the currents in the proper phases
of the LED strobe cycle. The IR and Red currents can be programmed
independently. The LED currents are generated by forcing a
programmable-voltage across R9, then switching the resulting current through
the LED' s.
The LED driver is almost entirely contained within the Oxichip U1 circuit. This
driver also has an over-current detection feature that shuts down all LED drive if
the average current exceeds 44 milliamps. The over-current trip level is set by
passing the LED current through R10 and C9 and comparing the voltage drop to
V
0.3 volts. Components CR5 and CR6 serve as ESD-protection diodes. See
DD-
Figure 17-2.
There are three voltage references on the Oxichip U1 circuit. The 3.125 volt
reference is used for Rcal stimulus and the high resolution A/D converter
reference. It is a low-impedance output. The 2.2 volt buffered reference is used
as the signal ground for the photodetector cathode and is a low-impedance
output. The 2.2 volt reference is used for signal ground in the rest of the circuit.
It is a high-impedance output and is filtered by C3.
The power-on reset function is accomplished with a Schmitt Trigger inside the
Oxichip U1 circuit and external components R45 and C44. The Schmitt Trigger
supplies an active high reset to the processor on RST. Diode CR2 protects the
Oxichip circuit from the discharge of C44.
The oximetry module uses a crystal semiconductor CS5102A 16-bit A/D
converter for the conversion of the filtered optical signals and for RCAL
measurement. This A/D converter is shown in Figure 17-2 as U2.
The input filter to U2 is through the level shifter circuit, which consists of R18
and C13. The Rcal filter circuit consists of R17, C20 and C21.
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Troubleshooting
