Don't overlook S/N ratio

Signal-to-noise ratio (SNR) is used as a system figure of merit and as a system design parameter in communicating information, whether for an automated process control system or for spacecraft applications. The concept of higher the SNR value the better the system or device performance is simple enough, but keeping signal levels high is often impractical and holding down electrical noise often ...

03/01/2006


Embedded Control

SNR is often expressed as a voltage ratio—converted from the original power ratio definition using the relation: power = V2 /R. The extra resistance term can be eliminated because resistance values across which signal and noise voltages are measured often are equal, thus Log10 (1) = 0.

Signal-to-noise ratio (SNR) is used as a system figure of merit and as a system design parameter in communicating information, whether for an automated process control system or for spacecraft applications. The concept of higher the SNR value the better the system or device performance is simple enough, but keeping signal levels high is often impractical and holding down electrical noise often becomes the solution.

This is no simple task since numerous noise sources reside in a system—common ones include sensor noise, signal-conditioning electronic noise, analog-to-digital conversion uncertainty (modeled as noise), and software algorithm errors in extracting data from signals (modeled as noise).

An extreme example of SNR management is the Voyager 2 spacecraft. Launched over 27 years ago and having traveled more than 7 billion miles from our sun, the spacecraft is expected to continue transmitting data until year 2020 (43 years in flight), sending trillions of data bits back to NASA with a signal transmitter power tiny compared to earthly radio and TV stations. Yet NASA successfully receives this information from billions of miles away using 1977-vintage spacecraft electronics—albeit with a 10-hr delay due to the enormous distance even with radio signals traveling near light speed (186,000 miles/second).

Voyager 2 has to contend with additional noise sources, such as transmitter internally generated, space electromagnetic, antenna, and receiver electronic noises. Extracting information from a "noisy" communication system signal requires extensive filtering and sophisticated software algorithms.

Doing the math

Math associated with calculating a total system SNR can get complicated; however, the basic definition expressed in decibels (dB) is straightforward (see boxed equation). This original definition of the SNR parameter is usually associated with audio-radio-TV communication systems. Today's automated process control systems need modern SNR definitions to characterize acquisition systems incorporating high-speed analog-to-digital converter (ADC) topologies.

ADCs in process control systems often dominate the accuracy and speed of retrieving sensor data. Typical specifications of resolution, accuracy, linearity, conversion times, sampling speeds, monotonic response, component noises, etc., remain important to system designers. However, today's high-speed ADCs with high N-bit resolutions have SNR specs that more correctly characterize the dynamic behavior of process control data acquisition modules and provide an effective tool for comparing data acquisition system behaviors.

Signal-to-noise ratio of an ideal N-bit ADC module with conversion uncertainty ofand distortion (NAD) for ADC modules. Measured values of SINAD (excluding dc) are used to determine the effective number of bits (ENOB), which is a more realistic specification that characterizes ADC module overall performance. ENOB = (SINAD-1.76)/6.02.

For example, consider a 16-bit ADC module containing preamplifiers, multiplexer, sample-and-hold, ADC, and output buffer with a measured SINAD value of 86.3 dB. Using the above formula, ENOB = 14, which means this 16-bit module has the performance of an ideal 14-bit ADC for this stated SINAD. Signal frequency, amplitudes, and sampling speed affect the FFT measurement of SINAD; hence, ENOB value. The process control designer should always consult manufacturers to determine their SINAD and ENOB measurement methods.

Bill McGovern, project manager, Dataforth Corp. www.dataforth.com





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