The present invention relates to industrial process variable transmitters which measure a process variable and provide an output. More specifically, the present invention relates to such process variable transmitters which provide a pulse output representative of the process variable.
Industrial process variable transmitters are used to sense process variables in industrial processes. Example process variables include flow, temperature, level, pressure, etc. Such process variable transmitters typically couple to a process control loop which is used to transmit information related to the sensed process variable back to a centralized location. One type of process control loop carries electrical pulses which are related to the sensed process variable. For example, some types of flowmeters transmit flow related information using pulse output circuitry. Each pulse can be calibrated to a unit of flow. Thereby, a total amount of flow can be monitored by counting the number of pulses which are transmitted on the process control loop.
In many instances, the process variable transmitter must operate in accordance with Intrinsic Safety (IS) standards which provide design constraints on various aspects of the circuitry. Intrinsic Safety standards impose design constraints on devices to enable safe operation of electrical equipment in hazardous areas by limiting the energy available for ignition. These constraints place many limitations on circuitry and impose many engineering challenges on circuit design in an industrial process variable transmitter.