a FEATURES 4–20 mA, 0–20 mA Output Ranges Precalibrated Input Ranges: 0 V to 2 V, 0 V to 10 V Precision Voltage Reference Programmable to 2.000 V or 10.000 V Single or Dual Supply Operation Wide Power Supply Range: +4.5 V to +36 V Wide Output Compliance Input Buffer Amplifier Open-Loop Alarm Optional External Pass Transistor to Reduce Self-Heating Errors 0.002% typ Nonlinearity
4–20 mA Transmitter AD694* FUNCTIONAL BLOCK DIAGRAM
The AD694 is a monolithic current transmitter that accepts high level signal inputs to drive a standard 4–20 mA current loop for the control of valves, actuators, and other devices commonly used in process control. The input signal is buffered by an input amplifier that can be used to scale the input signal or buffer the output from a current mode DAC. Precalibrated input spans of 0 V to 2 V and 0 V to 10 V are selected by simple pin strapping; other spans may be programmed with external resistor.
1. The AD694 is a complete voltage in to 4–20 mA out current transmitter. 2. Pin programmable input ranges are pre-calibrated at 0 V to 2 V and 0 V to 10 V. 3. The input amplifier may be configured to buffer and scale the input voltage, or to serve as an output amplifier for current output DACs. 4. The output voltage compliance extends to within 2 V of the positive supply and below common. When operated with a 5 V supply, the output voltage compliance extends 30 V below common. 5. The AD694 interfaces directly to 8-, 10-, and 12-bit single supply CMOS and bipolar DACs. 6. The 4 mA zero current may be switched on and off with a TTL control pin, allowing 0–20 mA operation. 7. An open collector alarm warns of loop failure due to open wires or noncompliance of the output stage. 8. A monitored output is provided to drive an external pass transistor. The feature off-loads power dissipation to extend the temperature range of operation and minimize self-heating error.
The output stage compliance extends to within 2 V of VS and its special design allows the output voltage to extend below common in dual supply operation. An alarm warns of an open 4-to20 mA loop or noncompliance of the output stage. Active laser trimming of the AD694’s thin film resistors results in high levels of accuracy without the need for additional adjustments and calibration. An external pass transistor may be used with the AD694 to off-load power dissipation, extending the temperature range of operation. The AD694 is the ideal building block for systems requiring noise immune 4–20 mA signal transmission to operate valves, actuators, and other control devices, as well as for the transmission of process parameters such as pressure, temperature, or flow. It is recommended as a replacement for discrete designs in a variety of applications in industrial process control, factory automation, and system monitoring. The AD694 is available in hermetically sealed, 16-pin cerdip and plastic SOIC, specified over the –40°C to +85°C industrial temperature range, and in a 16-pin plastic DIP, specified over the 0°C to +70°C temperature range. *Protected by U.S. Patents: 30,586; 4,250,445; 4,857,862.
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