And a fundamental understanding of how analog signal transmission works must first begin with a discussion of electrical basics. To understand the ways in which an analog signal is transmitted over a circuit, it is first important to understand the relationships that make analog signal transmission possible. In the early 19th century, Jean Baptiste Joseph Fourier, a French mathematician and physicist, discovered that ac signals could be defined in terms of sine waves. Most data acquisition signals can be described as analog, digital, or pulse. In most control applications, analog signals range continuously over a specified current or voltage range, such as 4-20 mA dc or 0 to 5 V dc.
Transduction is the process of changing energy from one form into another.
Hence, a transducer is a device that converts physical energy into an electrical voltage or current signal for transmission. There are many different forms of analog electrical transducers. Transmission channels may be wires or coaxial cables. If the current source is of good quality, current loops tend to be less sensitive to noise pickup by electromagnetic interference than voltage-based signals.