Condition Your Signals

In a previous Tips column, I explained the need to place anti-alias filters between signal sources and data-acquisition (DAQ) equipment (see Useful Links). But, those signal paths may require additional signal conditioners — available from many companies as boards and modules — to ensure high-quality signals reach digitizing instruments.

Not all signals exist in a form DAQ systems can process. Suppose you have millivolt-level signals from thermocouples and 4-20 mA current signals from transducers, but your DAQ equipment requires 0-10V at its inputs. Typical signal conditioners accept such analog inputs and produce a standard output, say 0-10V, which goes to an analog-to-digital converter.

In addition to converting a voltage or current to a standard output, a signal-conditioning module should filter 50-Hz or 60-Hz signals radiated by power lines. Many times engineers forget to properly shield wires between a sensor and DAQ equipment or they cannot retrofit systems with shielded wires. So, power-line-frequency signals couple into the sensor's leads and produce "noisy" measurements. Even well-shielded systems can pick up line-frequency noise.

A signal conditioner can also isolate input signals from output signals and eliminate the effects of ground loop. According to Bill McGovern, sales manager for North America at Dataforth, engineers should look for three-way isolation that separates an input signal from the output signal so that no current flows between them. The conditioner's power connection also remains isolated from the sensor and output sides of the circuit (Fig. 1). This type of signal conditioner inherently isolates sensor

	This block diagram shows the three isolated sections in a signal-conditioner module. users can choose modules with a range of capabilities.

channels from one another.

Even with three-way isolation modules, engineers can run into trouble. McGovern noted at times engineers connect sensor, DAQ-equipment, and power-source grounds and doing so defeats the isolation they needed to achieve. "When they see more noise, they must look for and eliminate any common grounds. That often makes engineers scratch their heads because they think the more grounds, the better."

When vendors tout isolated DAQ equipment, ensure you know what they isolate. In some cases, they may simply isolate the equipment itself from a USB connection to a PC or an Ethernet connection to a host computer. That type of isolation does nothing for analog signals.

Keep signal-conditioner protection capabilities in mind, too. Equipment failures and errors in wiring sensors can send high voltages down sensor lines, so a signal conditioner should withstand and recover from a voltage overload. To be safe, look for signal conditioners that offer a continuous overload rating because overloads can exist for some time before you discover them. The ground between an outdoor motor and nearby equipment panel, for example, can provide a path for a transient signal to reach DAQ-equipment inputs.

For more discussions on this and other columns, join our Electronics/Test Forum at http://rbi.ims.ca/4930-518.