Hill-Rom Advanta P1600, Service Manual

Page: 167 / 429

Theory of Operation
Chapter 3: Theory of Operation
Advanta™ Bed Service Manual (man168ra) Page 3 - 29
3
Sensor Control Board—P/N 44740
General Description
The pressure sensing circuit provides the interface between the Advanta™ Bed
ZoneAire® Sleep Surface multiple air volume and the control circuit.
The pressure sensing circuit consists of six air pressure transducers and a
common mode reference voltage input, which are multiplexed and amplified
sequentially. The DC offsets are corrected by a digital-to-analog (D/A)
converter whose output negates the offsets of each transducer and reference
amplified signal. An eight-bit, analog-to-digital (A/D) converter configured for
nine bit accuracy converts the amplified signal to a digital form compatible
with the serial interface requirements. Included in the circuit are the following:
• 64-word EEPROM, which stores the required D/A DC offset compensation
byte for the reference and each transducer
• the A/D converter output corresponding to the reference input
• the A/D converter output at zero psi for each transducer
• the A/D converter output at one psi for each transducer.
This design allows all circuit gain, offset, and calibration parameters to remain
on board, thereby enabling characteristic independence from the mating
control processor circuit.
Input Protection
Diodes D21-D26 and D30 are transient voltage suppressors used to divert
input electrostatic discharge (ESD) currents to ground. They have a breakdown
voltage between 6.4 and 7 volts. The voltage at the input pins of the Schmitt-
Trigger U9 and NAND gate U7 cannot exceed the supply voltage by more than
1.5 volts nor can it drop below the ground potential by 1.5 volts. Therefore,
any voltage build-up across the voltage suppressors is clamped by diodes D3-
D14, D28, and D29 through current-limiting resistors R52, R54, R56, R58,
R60, R62, and R64. The inherent hysteresis of U9 provides clean logic levels
and sharp transitions to the circuit’s logic devices. U7 provides a clean logic
level to the select line of multiplexers U1 and U10. Diode D27 is also a
transient suppressor with a breakdown voltage between 20 and 24.4 volts.