Rosemount 3152 - Oscillator

Rosemount 3150 Series
Reference Manual
00809-0100-
4835 Rev BD
July 2016
4-5
OSCILLATOR
VOLTAGE REGULATOR
CURRENT CONTROL
CURRENT LIMIT
REVERSE POLARITY
PROTECTION
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 110kHz. An operational amplifier acts
as a feedback control circuit and controls the oscillator drive voltage
such that:
21
CC
I
fV
ref
pp
+
=
−
The transmitter uses a zener diode, transistors, associated resistors
and capacitors to provide a constant reference voltage of 3.2 Vdc and a
regulated voltage of 7.4 Vdc for the oscillator and amplifiers
.
The current control amplifier consists of two operational amplifiers, two
transistors, and associated components. The first amplifier provides an
adjustable gain output proportional to the sum of the differential sensor
current and a zero adjustment current. This output is supplied to the
second amplifier, which controls the current in the 4-20mA loop
proportionally.
The current limiter prevents output current from exceeding 30mA
nominal in an overpressure condition. Conversely, minimum output is
limited to 3 mA nominal. Both the minimum and maximum current limits
may vary slightly depending upon sensor pressure range code and
associated calibration.
A diode provides reverse polarity protection.