RF-telemetry transmitter features minimal parts count

摘要

REQUIREMENTS FOR portable, short-range telemetry systems frequently include low power consumption, small size, and low cost. The circuit in Figure 1 meets these criteria and uses only three off-the-shelf ICs and a few passive components. Although dedicated to conditioning the low-level signal a strain-gauge bridge produces, the circuit can operate with almost any resistive transducer based on a Wheatstone bridge. The circuit comprises a VFC (voltage-to-frequency converter) that produces a PPM (pulse-position-modulation) output, and an OOK (on/off-keyed) RF transmitter. Based on direct digitization, the VFC in Figure 1 comprises IC{sub}1, IC{sub}2, IC{sub}(3A), and IC{sub}(3B). A Wheatstone bridge containing strain gauge R{sub}x produces an output of approximately 5 mV. An integrator stage comprising C{sub}6 and IC{sub}1, a Linear Technology LTC1250 offset-compensated, lowdrift operational amplifier, connects directly across the bridge. (Note that the value of the remaining resistors in the bridge depends on the application.)