MKS Baratron 223B, Инструкция по эксплуатации, Страница: 19 / 31

Chapter Three: Overview Circuit Description
11
Chapter Three: Overview
Circuit Description
Refer to Figure 3, page 12.
The capacitance of the single electrode sensor, which increases with pressure at the P
X
port, is
part of a multivibrator circuit consisting of U1, Cx, R1, R2, R3, R4, and R5. R1 to R5 are
temperature stable devices leaving Cx as the major frequency determining component. R5 is a
selected temperature sensing resistor used to compensate for temeprature coefficients in both the
sensor and electronics. The square wave frequency (inversely proportioned to pressure) is
converted to a current at pin 1 of U2. U3A converts this current to a voltage referenced to signal
ground by passing it through R12. This negative voltage is now summed with a negative zeroing
voltage drived from U4B. U4A establishes the output gain allowing the transdcuer to output a
precise 0 to 1 VDC.
Linearizing is implemented using a unique feature of an LM331 (U2). The output of pin 2 of U2
provides a precision reference voltage which is used to zero the output of U3A by adjusting the
gain of UB4. However, the transfer charactersistic of U2 is adjusted by the amount of current
flowing from pin 2 of U2. As the rpessure signal increases from U3B, the amount of current
flowing from pin 2 of U2 decreases, lowering the gain of U2. The characteristic precisely
cancels the geometric nonlinearity of the sesnor.
An overrange signal is outputted when the sesnor diaphragm shorts becasue the frequency goes
to zero and the output saturates positive. CR1 and CR2 provide protection against reverse power
connections and C8 to C11 provide immunity to interference from RF environments.