5. Theory of Operation
The Model 2002 transducer is comprised of two very different sensors which provide a span of measurement extending from 1000 Torr down to less than
5.1.Piezoresistive Sensor
Cable
Connector
| Electronics |
| Board |
Top Cap | (EEPROM) |
|
HEADER
PIRANI
PiezoresistiveSENSOR
Sensor
Fig 5.1
Dual Sensor Emodiment
Figure 5.2 shows a typical schematic of a B implanted Wheatstone bridge network in a Si diaphragm inverted box type geometry. The inside of the box is evacuated during anodic bonding to a Pyrex substrate. The membrane has maximum deflection at atmosphere (or higher pressure) and the membrane resistances change value as the differential pressure is decreased during pump down. The resulting differential output is
Vo = SPV+V1
where
S is the sensitivity
P is the pressure
V is the applied bridge voltage
V1 is the no load output voltage
Since the sensitivity changes so dramatically with temperature, some correction is required for compensation. The change in output voltage