6BChapter Three: Overview 60BGeneral Information
35
Chapter Three: Overview
General Information
The DMA transducer contains a digital interface. All communication to and from the device is
achieved using the DeviceNet network. Interfacing with your transducer is accomplished using
the DeviceNet digital communications protocol, which gives you access to all features of the
instrument.
A complete pressure transducer system requires three components to convert pressure to a linear
DC voltage output:
• Sensor
• Signal Conditioner
• Power Supply
The DMA digital transducer contains two of the required components: the sensor and signal
conditioner.
The DMA transducer power supply requirements are specified on page 25. The power is
introduced to the transducer through the 5-pin m
icro-style connector on the top of the unit (Figure
5, page
38). The host computer for the network reports the pressure measurement.
Sensor
The DMA transducer contains an absolute sensor. This pressure sensor utilizes a single-sided,
dual-electrode/AC bridge circuit design. In this design, two capacitance electrodes are deposited
upon a ceramic disc in a concentric “bull’s-eye” arrangement. The disc is positioned close to an
Inconel
®
tensioned diaphragm to form two capacitors in an AC bridge circuit. The other side of
the diaphragm is exposed to the process gas.
The diaphragm deflects with changing pressure—force per unit area—independent of the
composition of the measured gas. This deflection causes a capacitance change between the
diaphragm and the adjacent electrode assembly. When pressure is equal on both sides of the
diaphragm, the bridge is balanced. As pressure deflects the diaphragm toward the electrode, the
center capacitance changes more than the outer capacitance, causing the bridge to become
unbalanced and an AC voltage to be generated. The output signal, which is proportional to
pressure, can be provided as an analog signal (0 to 10 V) through the 9-pin Type DE-9S interface
connector, or as a digital signal through the DeviceNet communications interface. The thermal
enclosure reduces the effects of ambient temperature changes.
The zero and span stability are further increased in the DMA (45° C) and DMA (80° C)
transducers because the sensor and bridge electronics are temperature controlled.
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