T
HEORY
5-8
April 2000 Rosemount Analytical 245364-U
Model 755A Oxygen Analyzer
or recorder. With upscale standard gas flowing through the analyzer, the SPAN
control is adjusted for the appropriate reading.
P
RESSURE
C
OMPENSATION
C
IRCUIT
The pressure compensation circuit consists of divider U6 and associated components.
This circuit provides a pressure-corrected output signal conditioned to the range of 0
to 10 VDC. The circuit solves the follow 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
A
MPLIFIER
U8
This amplifier works in conjunction with analog divider U6, providing conditioned
output signal Vo as described in Pressure Compensation Circuit above.
A
MPLIFIER
U10
U10 is an non-inverting buffer amplifier that incorporates an anticipation arrangement
in its input network, thus providing slightly faster response time (90% of fullscale on
the readout device(s).
Potentiometer R30 provides a continuously variable adjustment of 5 to 25 seconds for
the electronic anticipation time and is factory-set for 20 seconds.
Since the anticipation network attenuates the signal, a gain of 10 is provided in U10 to
restore the signal to the desired fullscale range of 0 to 10 VDC.
The pressure-corrected signal from U10 is routed to two output circuit:
Digital Output Circuit
. The signal from U10 passes through TEST switch SW1 and a
filter circuit to an integrating analog-to-digital converter. The resulting digital signal
drives the liquid crystal display.
Analog Output Circuit
. The output from U10 is provided as an input to the recorder
output amplifier. This circuitry provides zero suppression, scale expansion, and
amplification preparatory to use for potentiometric recorder, voltage-to-current
conversion for current recorder, and/or alarm functions.
