Mallinckrodt NELLCOR NPB-4000 Service Manual page 102

Section 11: Isolated Front End Functions -- Theory of Operation
11.4.2 R-wave Detector.
11.4.3 Pacemaker Detector.
11-6
The preamplifier is followed by a 0.05 Hz high-pass filter made of capacitor
C147 and resistors R248 and R249. The corner frequency can be changed by
software to 0.5 Hz, set by resistors R238 and R250 and switch U63B. If a
common mode charge on the capacitor C147 will offset the baseline to the point
where ESG signal clipping is detected, it can be quickly removed under software
control through the analog switch U63C and limiting resistor R239. The ECG
circuit is completed by an amplifier U73B set for gain of 41 and a two-pole
Chebychev low-pass active filter, U74C, with the cutoff frequency of 50 Hz and
the gain of 1.75. The U74C has a +2.5 volts offset determined by resistors R225
and R236 to convert bipolar signals to unipolar format, as required by linear
optocoupler. Thus, the total output swing of the ECG channel is about 1.5 volts.
The ECG signal is further used to detect R-wave peaks. Active bandpass filter
with U71A amplifier has the center frequency of 14 Hz and the passband of
±5 Hz. This filtering should eliminate false triggering in case of tall T-wave
following R-wave. The signal is rectified by the full-wave rectifier made of
amplifiers U71B and U71C, and dual-diode D13. This signal is used as one
input to the comparator U74D. Sample-and-hold circuit consists of amplifier
U71D, diode D17, capacitor C137, and discharge resistors R219 and R229. The
resistors ratio sets the level of the comparator which goes to the positive rail
when the next R-wave signal reaches the preset level of the previous R-wave
signal held in the capacitor. The comparator signal turns on the monostable
multivibrator U64A, which generates a 130-millisecond pulse. The pulse is sent
to SpO circuit and to analog multiplexer via divider R180, R186.
2
Pacers are detected by the dV/dt circuit made of amplifier U74A, resistor R251,
and capacitor C152. The circuit output will go to approximately ±1.4 volts when
pacer is detected. This will set off the window comparator built around
amplifier U74B and turn on monostable U64B to create an 8-millisecond pulse
sent to the analog multiplexer via divider R179, R185.