Welch Allyn Atlas Service Manual page 95

Appendix A 10/14/99
1. >=10mmHg pressure for 295 seconds
2. >=15mmHg pressure for 175 seconds
3. >295mmHg pressure for 0.5 seconds.
Software detected overpressure faults are considered application faults. The user is warned of a fault with
an audible alarm and a 'Check blood pressure cuff' message on the CRT display. NIBP is not disabled for
this type of fault. In the event of a fault, the drive signals to the NIBP pump and valve are opened.
Two hardware faults are detected, pressure over 330mmHg (nominal trip point 314mmHg, and pressure
over 15mmHg for three minutes (13.3mmHg nominal trip point). These faults are considered more serious
(since software should have detected and corrected this condition). The user is notified with a 'BP
SYSTEM FAULT' message, and NIBP is disabled. A redundant safety transistor is opened to ensure the
NIBP pump is off and the valve is open.
The primary and safety transducer outputs are continuously digitized. The outputs of the transducers are
checked vs. each other, and if they disagree, an 'NIBP Fault Message' is declared and the NIBP system is
disabled. The outputs of the transducers are linearly proportional to the supply voltage (supply current for
the safety transducer). The transducers use unique reference voltages to ensure that a fault in one
reference will not cause an equivalent gain error in both transducers.
The A/D also has redundant checks. Two reference voltages (derived from the primary transducer
reference supply) are measured, and the A/D gain and zero is checked. In addition, a unique reference is
digitized, and compared vs. expected results. An error in any of these A/D measurements will again cause
an 'NIBP Fault Message', and the NIBP system will be disabled.
4.2 Primary Transducer - Amplifier:
The primary pressure transducer is a fully calibrated and compensated transducer with built in voltage
amplification. The output of the transducer is proportional to the supply voltage. With a 5V supply, the
output of the transducer is:
0mmHg = 0.5V.
300mmHg = 4V.
Op amp U204A is used to level shift the output of the transducer such that the nominal voltage for
0mmHg is set to 0.75V. The CPU monitors the digitized zero pressure voltage, and any offset is corrected.
This correction comes from the summation of an error correction signal through op amp U204B. The CPU
will output a pulse width modulated signal at 76.8KHz. This signal is RC filtered to provide ad DC
voltage at U204-5. This signal can adjust the offset seen at the A/D converter by +/-20mmHg.