MKS Baratron 127A, Instruction Manual, Page: 25 / 38

Chapter Three: Overview Circuit Description
17
Chapter Three: Overview
Circuit Description
The variable capacitance sensor consists of rigidly attached capacitive electrodes located on the
back or reference side of a metal diaphragm. The reference side is permanently evacuated and
sealed thus making the pressure measurement totally independent of the gas type or composition.
When pressure is applied to the diaphragm, its deflection produces a change in the distance
between the electrodes and diaphragm and a resultant capacitance change.
The center electrode increases its capacity at a greater rate than does the outer concentric
electrode. This imbalance of capacities caused by pressure is converted to a small DC Voltage in
the first stage diode bridge. This bridge is excited by a precision constant frequency oscillator.
The resultant signal is linearized, zeroed and amplified to produce a precise 0 to 10 VDC signal
scaled to the range of the sensor.
To produce the fine zero and span stability that this transducer possesses, the sensor and bridge
electronics are temperature controlled. This thermal enclosure will reduce the effects of ambient
temperature changes by a factor of at least 35 (that is, 35
°
C change in ambient will produce 1
°
C
change inside the thermal housing).
Sensor
Diode
Bridge
Heater
Supply
Oscillator
DC
Amplifier
Pressure
CAP
CAP
Heated Enclosure
+15 VDC Input
0 - 10 VDC
Output
Figure 6: Block Diagram of the Circuitry