Emerson 1154, 00809-0100-4514 manual Demodulator, Linearity Adjustment

Reference Manual

00809-0100-4514, Rev BA January 2008

Rosemount 1154

Figure 4-2. Electrical Block

Diagram.

SENSOR	DEMODULATOR	CURRENT			-
		DETECTOR
				TEST
			CURRENT		+
			LIMITER	+
OSCILLATOR
				SIGNAL
			CURR.	-
	OSC.
			CONTROL
	CONTROL	VOLTAGE		REVERSE
			AMP.
	AMP.	REGULATOR
				POLARITY
				PROTECTION
			CURRENT
			CONTROL

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 Vp-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, Iref (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.,

Idiff = fVp – p(C2–C1)

LINEARITY ADJUSTMENT

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.

4-3