Teledyne 2002 instruction manual Theory of Operation, Piezoresistive Sensor, Dual Sensor Emodiment

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 1x10-4 Torr. The piezoresistive device is a direct force sensor which provides pressure indication from 1000 Torr down to less than 1 Torr. The thin film Pirani device is a thermal conductivity sensor that provides pressure indication from 100 Torr down to less than 1x10-4 Torr. The two decade overlap in measurement range is convenient for smooth transition either descending or ascending in pressure. Both sensors are small micro machined die that are bonded to a Au coated Al2O3 preform (stress isolation) which in turn is bonded to a TO-8 header. The header is resistance welded into a 316 stainless steel envelope as shown in Figure 5.1.

5.1.Piezoresistive Sensor

Cable

Connector

	Electronics
	Board
Top Cap	(EEPROM)

TO-8

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