Signal Conditioning—Getting What You Expect from Your Sensors

When PC-based data acquisition (A/D) boards first became available, many people assumed that you just plugged your PC board into its slot, clipped on some test leads, and started taking measurements. Taking truly valid measurements can prove difficult, however. Factors such as long lead-wire distances, noisy environments, and the types of sensors used are significant to the quality of your data. When evaluating the validity of the measurements taken by your data acquisition system, of primary consideration is the quality of the signal conditioning provided.

Traditional signal conditioning modules can add quite a bit to the cost of a data acquisition system, especially for systems with large point counts. Many customers ask me if it is really worth the added expense to include signal conditioning in their DAS system. For some customers, signal conditioning is essential; for others, it is not worth the expense; and for some, it’ll be a judgment call whether they need it or not. A good understanding of what signal conditioning modules do will help you decide whether or not you need them.

What Do Signal Conditioning Modules Do?

Signal conditioners take your real-world signal (which could range from a few millivolts up to 450VAC) and transform it into a well-mannered voltage range readable by your A/D converter board. This can drastically improve resolution by utilizing a significant part of your A/D board’s full-scale range. Signal conditioning modules also perform many additional functions:

• Part of the signal conditioning process involves converting differential signals to single-ended. Nearly all signals coming into a signal conditioner will be differential, but voltages going out are single-ended, sharing a common ground. In many cases, this conversion function will double the number of channels readable by your analog input board.
• In addition to conditioning your signal, signal conditioning modules isolate your signal from electrical noise. Take, for instance, one of our customers whose task was to monitor the temperature of photocopier drums which were going through a coating process. Signal conditioning modules made it possible for the system to accurately read a very small thermocouple voltage while rejecting a large amount of noise from the galvanic process.
• Signal conditioners offer overvoltage protection. The lines carrying your signals are susceptible to picking up transient voltages which could damage or destroy your computer.
• Signal conditioning is essential for transducers which require voltage or current excitation. Strain gauges, for example, require an excitation voltage to power the resistor bridge network; potentiometer/resistance measurements need a known voltage source; RTDs require a precision current source.

‘All-In-One’ Remote Signal Conditioning Modules

Before the advent of “all-in-one” remote DAS modules like our CyMOD™ and ADAM™ series, data acquisition professionals relied on analog-to-analog signal conditioners which interfaced to a data acquisition board. The board in your PC would then perform the A/D conversion. Our intelligent data acquisition and control modules have consolidated these functions. They can:

• interface with your transducer like analog signal conditioners,
• perform an A/D conversion with 16-bit accuracy, and
• transmit the data back to your PC across twisted-pair cable.

Multi-drop RS-485 networks allow you to interface a very large number of points in this manner. Many CyMOD and ADAM modules allow you to handle multiple points with just one module (unlike the 5B series signal conditioners, which require one module per point, plus a module mounting rack).

Speed is one area where these newer signal conditioners are limited. Conversion rates are typically only several samples per second, so they will not be able to replace your traditional A/D board in applications where you need high-speed response, like frequency measurement. However, many of our traditional analog signal conditioners have low-pass filters set for 3 to 8Hz, in which case you’re not sacrificing anything to go with the all-in-one CyMOD or ADAM series signal conditioners.

A/D Boards with Onboard Signal Conditioning

Our “Direct Connect” and UPC-series boards both offer onboard signal conditioning for a variety of sensors, along with an A/D converter. This type of solution, allowing sensors to be connected directly to the A/D board, is very useful for high-frequency signals or any application where large numbers of samples per second are desired.

Designed to accept a wide variety of signals, the instruNet allows you to place one or more units a great distance from your PC, and still achieve high-speed, high-accuracy data acquisition. It is a unique fusion of flexible signal conditioning circuitry, high-speed A/D, & remote (near-the-sensor) I/O.

Anti-Aliasing: Filter Out Sources of Error

The Nyquist Theorem states that an input signal should be sampled at a rate of at least twice the highest frequency component of the expected signal. For example, a 100kHz board can accurately sample signals to 50kHz. But, if your signal has high-frequency components at or near your sampling rate, it’s possible to read a high-frequency signal as a much lower frequency one.

On the other hand, many signals have false high-frequency components, usually caused by noise on the system, which must be identified & eliminated, or your data will be inaccurate.

One common way of filtering out these high-frequency components is by oversampling. But using the oversampling method can get expensive: you’ll need a high-performance A/D board, more memory, and a high-bandwidth bus.

A more economical but still effective method is to use an anti-aliasing filter. These low-pass filters will only pass signals to the A/D converter that are at or below the filter’s cutoff frequency — all higher frequencies will be eliminated. Our DSC 816 has anti-aliasing circuitry, signal conditioning, A/D conversion, & optical isolation built right onto the board.