EXTENDING THE CAPABILITIES OF AD694 TRANSMITTER IN BRIDGE CONNECTED SENSOR APPLICATIONS
Evgeni Dimitrov, Mitio Mitev · 2005
The 4-20mA analog current interface has a wide application in industrial electronics because of high signal/noise ratio, especially in high noise environment. The interest in this interface doesn’t decrease over the years. A proof of this is the development of new ICs such as AD693, AD694 and XTR101 as well as embedding extra digital communication capabilities (for example the HART protocol, some HONEYWELL solutions etc.). This paper examines the subject of using the AD694 transmitter for converting the voltage of bridge connected sensor into current. It is proposed a three-wire interface scheme, which satisfies the requirements for higher measuring bridge supply current. A variety of suitable preamplifier schemes are presented for amplification of the bridge signal to the levels required by the AD694 transmitter. The bridge measurement scheme power supply stabilisation is achieved by using the reference voltage source built into AD694. It provides a capability to supply the bridge sensor by stabilized voltage source or by current generator. Experimental data are collected and provided by powering a single strain gauge using each of these power supply methods. The force sensors measure both stretching and pressure, and have bipolar readout. This puts the requirement to shift the zero line of the transmitter output current. In the proposed schematic solution this is achieved by applying the necessary signal shift in the preamplifier stage. A suitable method is proposed to derive the reference voltage, for the zero line shift, from the internal reference source. This is achieved in current generator mode as well as in voltage source mode. A temperature range measurements is foreseen for the schematic proposed.