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Reference Manual
00809-0100-4514, Rev BA
January 2008
4-3
Rosemount 1154
Figure 4-2. Electrical Block
Diagram.
THE δ-CELL
™
SENSOR Process pressure is transmitted through an isolating diaphragm and silicone
oil fill fluid to a sensing diaphragm in the center of the
δ-Cell. The reference
pressure is transmitted in like manner to the other side of the sensing
diaphragm.
The position of the sensing diaphragm is detected by the capacitance plates
on both sides of the sensing diaphragm. The capacitance between the
sensing diaphragm and either capacitor plate is approximately 150 pf. The
sensor is driven through transformer windings by an oscillator at roughly 32
kHz and 30 V
p-p
.
DEMODULATOR 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
ref
(the sum of the dc currents through two
transformer windings) is controlled to be 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
ADJUSTMENT
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.
DEMODULATOR
CURRENT
DETECTOR
OSC.
CONTROL
AMP.
OSCILLATOR
SENSOR
VOLTAGE
REGULATOR
CURR.
CONTROL
AMP.
CURRENT
LIMITER
REVERSE
POLARITY
PROTECTION
CURRENT
CONTROL
-
+
TEST
SIGNAL
+
-
I
diff
fV
pp–
C
2
C
1
–()=