Theory; Pressure Amplifier - Spacelabs 90217 Service Manual

Contents

Pressure Amplifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Oscillometric Amplifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
90217 Block Diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Digital Circuitry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Unit Power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Software Flow Chart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Pressure Amplifier
The pressure amplifier monitors the voltage produced across the pressure transducer. This
voltage is proportional to the pressure in the arm cuff.
The pressure transducer circuits require temperature compensation to account for changes in
sensitivity. This is accomplished by using a reference current to provide a constant current source
into the bridge. Any change in bridge resistance due to temperature will change the bridge voltage
in an amount that compensates for any change in sensitivity.
Voltage across the transducer is amplified differentially and turned into a single ended voltage that
is amplified, offset and sent to the processor's A/D converter, the oscillometric amplifier and the
overpressure detector.
Offset Adjust
Both transducer offset and operational amplifier offset are nulled out using a pressure offset
adjustment. Minor variations in the offset are tracked and compensated for in software.
Gain Adjust
Changes in gain are compensated for with a gain adjustment. The voltage gain to the A/D
converter is +15 mV/mmHg, and the voltage is offset by approximately 0.09 volts. This 0.09 volts is
inserted to prevent the A/D converter signal from going negative during drifts in the offsets. The
0.09 volt offset is subtracted in software.

Theory

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