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Rosemount 1152
Figure 4-2. The -Cell.
Lead Wires
Isolating
Diaphragm
Rosemount 1152DP, HP, GP
DEMODULATOR
LINEARITY
ADJUSTMENT
OSCILLATOR
4-4
Capacitor
Plates
Sensing
Diaphragm
Rigid
Insulation
Silicone
Oil
Welded Seals
Evacuated Absolute
Reference
The demodulator consists of a diode bridge that rectifies the ac signal from
the sensor cell to a dc signal.
The oscillator driving current, I
transformer windings) is kept a constant by an integrated circuit amplifier.
The dc current through a third transformer winding is a current directly
proportional to pressure; i.e.,
The diode bridge and span temperature compensating thermistor are located
inside the sensor module. The effect of the thermistor is controlled by
resistors located in the electronics housing.
Linearity is adjusted by a variable-resistance network, capacitor, and diodes.
The currents generated through this part of the circuit are summed into the
inputs of the oscillator control circuit. This provides a programmed correction
that raises the oscillator peak-to-peak voltage to compensate for first-order
nonlinearity of capacitance as a function of pressure.
The oscillator frequency is determined by the capacitance of the sensing
element and the inductance of the transformer windings.
The sensing element capacitance is variable. Therefore, the frequency is
variable about a nominal value of 32 kHz.
An integrated circuit amplifier acts as a feedback control circuit and controls
the oscillator drive voltage such that:
Lead Wires
Isolating
Diaphragm
Rosemount 1152AP
(the sum of the dc currents through two
ref
(
)
I
=
fV
C
–
C
diff
p p
2
1
–
I
ref
fV
=
--------------------
p p
–
C
C
+
1
2
Reference Manual
00809-0100-4235, Rev BA
April 2007
Capacitor
Plates
Sensing
Diaphragm
Rigid
Insulation
Silicone
Oil
Welded Seals
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