Oscillometric Amplifier - Spacelabs 90217 Service Manual

Ambulatory Blood Pressure Monitor 90217 Service Manual

Oscillometric Amplifier

Gain, Offset, and Filtering
The oscillometric amplifier is DC coupled. A D/A converter provides large amounts of DC offset to
the amplifier to prevent the large static pressure component of the waveform from over driving the
amplifier. It provides gain (x64), DC offset, and high frequency filtering (3dB point = approximately
80 Hz). The oscillometric filtering that was present in earlier designs is now done by software.
RAM Protect and Reset Circuit
When the +5 volt power supply begins to drop, the RAM protect circuit asserts a reset signal to the
processor that protects RAM data during power collapse. This same circuit provides a start up
processor reset signal at power up. Reset time is a product of the reset R-C time constant and the
hold off time necessary for the power converter to reach +5 volts (the reset time constant is 350
msec).
A/D Voltage References
The A/D reference (+ADR) is generated from a LM4041-1.2 band gap reference. Its reference
output of 1.2 volts is amplified to 4.608 volts by a gain adjustable amplifier (+ADR = +4.608 volts,
adjustable).
30 Second Pulse
The real-time clock produces a pulse every minute with a 30 second duration. This pulse's leading
and trailing edges are conditioned into separate pulses and applied through a diode to the
processor's WAKE_UP line to awaken it every 30 seconds.
Second Pulse
The SECONDS line goes to the LCD where it blinks the colon and changes the LCD polarity.
Cable Connected
If a cable is connected to the RS-232 communications connector with the power switch ON, the
power converter activates and awakens the processor. A cable connected condition prevents the
power converter from going down as a result of a shutdown fault generated by the watch dog
timer, but cannot prevent an over-pressure shutdown. Once a cable has been connected, the
processor goes into a listening mode, awaiting instructions from the RS-232 port.
These instructions adhere to the ABP communications protocol.
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