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Reference Manual
00809-0100-4514, Rev BA
January 2008
Rosemount 1154
4-4
OSCILLATOR The oscillator has a frequency 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 is used as a feedback control circuit and
controls the oscillator drive voltage such that:
VOLTAGE REGULATOR The transmitter uses a zener diode, transistor, and resistors to provide a
constant voltage of 6.4 V dc for the reference and 7 V dc for the oscillator.
ZERO AND SPAN
ADJUSTMENTS
Zero adjustment components consist of a potentiometer and resistor that
develop a separate adjustable current that sums with the sensor current. The
coarse zero switch switches resistors into the circuit as needed.
Span adjustment is performed with a potentiometer which determines the
amount of loop current which is sensed and fed back to the current control
amplifier.
CURRENT CONTROL The current control amplifier consists of an integrated circuit, two transistors,
and associated components. The IC reference voltage is established at the
junction of a resistor network. The current control amplifier drives the current
control to a level such that the current detector feeds back a signal equal to
the sum of the zero current and the variable sensor current.
CURRENT LIMIT A current limiter prevents the output current from exceeding 30 mA in an
overpressure condition.
REVERSE POLARITY
PROTECTION
A zener diode provides reverse polarity protection.
fV
pp–
I
ref
C
1
C
2
+
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