What is Signal-to-Noise Ratio (SNR) in Instrumentation Engineering Projects?
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What is Signal-to-Noise Ratio (SNR) in Instrumentation Engineering Projects?
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Definition & formula
SNR quantifies how strong a desired measurement is relative to background noise.
In power terms
SNR = P_signal / P_noise; in decibels: SNR_dB = 10·log10(P_signal/P_noise).
For amplitude ratios,
20·log10(V_signal/V_noise) is used.
A higher SNR means clearer, more repeatable measurements.
Why SNR matters in plant instrumentation
- Low SNR can cause false trips, control oscillation, bad loop tuning, and wrong diagnostics.
- Common causes of noise are EMI, ground loops, cable capacitance, quantization noise, sensor thermal noise, and mechanical vibration in the process.
- Thermocouples and resistance bridges are two types of sensors that are very easy to hack.
Design & selection guidance
- Pick sensors and transmitters that have a larger output or higher excitation when it’s safe to do so.
- Choose the A/D resolution and the noise figure of the instrument amplifier. For long runs, differential inputs and 4-20 mA loops are best.
- Use filtering (such hardware anti-alias and digital moving average), but make sure that dynamic needs are addressed.
Field troubleshooting checklist
- Make sure the grounding is right, utilize twisted pair shields, and keep power and signal routing separate.
- Install the right surge protection and common-mode rejection devices.
- If control jitter keeps happening, you can either extend the signal span (range) or add a notch or low-pass filter at the loop level.
Write down the SNR objectives in the datasheets and check them during FAT/SAT when there is noise in the plant that is similar to what you would expect. And write down the results.