Emerson 755A - Manual (Page 63)

Instruction Manual
245364-V
May 2002
Rosemount Analytical Inc. A Division of Emerson Process Management Circuit Analysis 5-9
Model 755A
a. Pressure Compensation Circuit
This circuit provides a pressure-corrected
output signal conditioned in the range of 0
to 10VDC. The circuit solves the
following equation:
Vo = k (Vx/Vz)
where
Vo = the corrected output signal
Vx = the amplified detector-output
signal, which includes a pressure
factor
Vz = the pressure signal derived
from: (a) The pressure sensor and
associated amplifiers, and (b) the
positive reference voltage power
supply
k = the constant that is characteristic
of the circuit
Circuit function is such that, assuming a
constant % oxygen concentration in the
sample stream, the output from the
analog divider U6 (and thus also the
conditioned output from amplifier U8) will
remain constant regardless of pressure
variations within the specified range.
b. Pressure Signal Circuit
Refer to Figure 5-8 on page 5-10. The
pressure signal circuit consists of the
pressure sensor (transducer) and two
associated amplifiers. The sensor
provides a voltage output that is
proportional to pressure. This signal is
ratioed and combined with the negative
voltage reference so as to provide a zero-
based signal at the output of the first
amplifier, where it is available for
measurement and display for calibration
and setup purposes.
The second amplifier adds the 10V
reference back ;into the pressure signal.
The output from the second amplifier is
then appropriately attenuated to provide
signal Vz for analog divider U6. Signal Vz
is also made available to the display and
measurement circuit for calibration
purposes.
c. Positive and Negative Reference
Voltage Circuits
These circuits provide precisely controlled
voltages that set overall system
performance and accuracy for the oxygen
measurement system. While the absolute
accuracy is of importance, more important
yet is the ability of these circuits to remain
stable despite temperature variations that
may occur during normal instrument
usage.