Smiths Medical BCI 3304, Руководство по обслуживанию

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Chapter 2: Oximeter Circuit Description
2-6 3304 Service Manual
Signal Processing
The differential transconductance amplifier formed by U1 and U2B
converts the photodetector’s current output to a voltage at TP10
(V-AMB). Amplifier U3B offsets the signal at TP10 so the signal baseline
is at 3.4 VDC, allowing a wider signal range for the negative-going
pulses at TP10.
Comparator U30A is used to inform microprocessor if U2B is saturated
by excess of ambient light.
V-AMB is passed through blocking capacitor C19 to remove the signal’s
DC component. The signal is then buffered and amplified by U2A.
Gain is defined by the first channel of digital potentiometer U6. The
output of U2A is routed to the integrator-filter U4A, controlled by the
analog switch U5. The signal at TP12 is then passed to the ADC for
measurement.
Display Board
The display board contains the LED display driver chips, the 7-segment
displays, the LED bar graph and the indicator lights. The display board
connects to the main board through a fourteen-conductor ribbon
cable. The display driver chips U1 and U2 are connected in a data chain
and receive synchronous data from the microprocessor’s TXS and CKS
lines. The data is loaded with the I/O strobe DISP-LD.
U1 and U2 generate electrical noise that may cause inaccurate
measurements from the ADC. For this reason, U1 and U2 are turned off
for 2 out of 8 msec during the data acquisition time. The signal /BLANK
turns off U1 and U2.
U1 controls the 7-segment displays and the LED bar graph according to
the data it receives from the TXS serial channel. U2 controls alarm and
other signal LED bars. The brightness of the displays is controlled by the
I-SET input at U1-18 and U2-18.
Green LED DS14 is directly connected to the charger section to indicate
that Charger is plugged to the wall.