NELLCOR PURITAN BENNETT Symphony N-3000 Service Manual page 96

Technical Supplement
S- 6
Components R30 and C24 filter any high-frequency noise on VCCI that is
beyond the bandwidth of U32. Finally, Q13, R104, and R105 turn the LEDs
off between pulses.
e. Differential Input Amplifier
Input op amp U23 converts the differential current to a voltage. One side of
the output is coupled through capacitor C48 while the other output is used as
a reference that is switched by U24 onto R78 only during one channel on/off
cycle. This AC couples only one channel.
Low frequency noise on the other channel is eliminated also since the LED-off
output for both channels is the same. This switched AC coupling removes the
DC without resulting in channel crosstalk. The resulting AC-coupled signal
and reference are switched through U25 to the input of differential amplifier
U21 when the patient module is plugged in.
If the cable is plugged in, U25 selects the output of the preamp and ground as
the input to U21. AC coupling is performed to keep crosstalk from the
preamped AC signal. Differential amplifier U21 has a programmable gain of
1, 4, 16, or 64, depending upon the state of PD0 and PD1.
f. Synchronous Demodulator and Gain
If calibration is not selected, the output of U21 is directed through U26 to
filter U27 to eliminate high-frequency noise. Amplifier U28 is either an
inverting or noninverting amplifier, depending on the state of switch U26.
U3 samples the output to demodulate the two channels. This sample-and-
hold is then amplified by U5 or U6. The gain is 1 or 16, depending upon the
state of DM2 from the U10 controller. The other half of U5 (or U6) drives the
linear opto-isolator to create a current output proportional to the input
voltage. This current has an effective gain of 1, 2, 4, or 8, controlled by DM0
and DM1. U13 converts the current output of the linear opto-isolator (U6) to
a voltage and then to a digital value.
g. Auto Calibration
Switch U26 selects either the differential amplifier (U21) output or the
internal calibration signals as an input to the U27-based filter. If the
calibration input is selected, U24 can select either a zero or test input.
The zero input connects the filter input to ground so the system can calibrate
the no-signal output of the two channels. The test input connects the LED
current waveform to the filter so the demodulator has an output proportional
to the programmed LED current.
h. Nonisolated Power Supply
The power supply creates ±5 volt from the system battery supply. The input
voltage is 8–16 volts, which is filtered and regulated by U19 to create the
+5 volts. U20 converts this input to a regulated -7 volt output that is
regulated to -5 volts by U18.