The opportunity to jab yourself in the eye with a tiny computer display is one step closer, thanks to the ongoing work with opto-electronic contact lenses taking place at the

By Albert O’Grady & Jim Ryan
An analog transmitter is a field-mounted device that senses a physical parameter such as pressure or temperature and generates a current proportional to the measured variable in the standard range, 4 to 20 mA. Providing the output as a current in a twisted-pair loop has many advantages: the measurement signal is insensitive to noise and is not affected by changes in loop resistance; transmitters meeting the standard are interchangeable; and the power required to energize the transmitter circuits can be derived from a remotely supplied loop voltage. Figure 1 shows a conventional transmitter circuit, consisting of a power supply, a current-manipulating transmitter, and a receiving controller.