What are the common sources of noise in measurement systems? Suggest suitable methods for minimizing noise.

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## Noise in Measurement Systems

**Noise** is any unwanted signal that corrupts the measured signal and reduces the signal-to-noise ratio, $SNR = 20\log(V_s/V_n)$ dB.

### Sources of noise
```mermaid
flowchart TD
  N[Noise sources] --> I[Inherent or internal]
  N --> E[External or interference]
  I --> I1[Thermal noise]
  I --> I2[Shot noise]
  I --> I3[Flicker noise]
  E --> E1[Power-line hum 50 Hz]
  E --> E2[Electromagnetic and radio interference]
  E --> E3[Ground loops]
  E --> E4[Crosstalk and switching spikes]
```

| Source | Cause |
|---|---|
| Thermal (Johnson) | Random motion of electrons in resistors, $V_n=\sqrt{4kTRB}$ |
| Shot | Discrete charge flow across junctions |
| Flicker (1/f) | Low-frequency device imperfections |
| Power line | 50 Hz mains coupling (capacitive or inductive) |
| EMI / RFI | Motors, relays, radio transmitters |
| Ground loop | Different ground potentials |
| Crosstalk | Coupling between adjacent wires |

### Methods to minimize noise
1. **Shielding** of cables and sensors, grounded at one point.
2. **Twisted-pair wiring** so induced noise cancels.
3. **Differential measurement** and an instrumentation amplifier with high CMRR.
4. **Filtering:** low-pass, notch (50 Hz) and band-pass filters.
5. **Proper grounding** (single point) and star ground to avoid loops.
6. **Isolation** using optocouplers or isolation amplifiers.
7. **Averaging** of repeated samples and lock-in techniques.
8. Keep signal cables away from power lines; use short leads and low source resistance.

### Exam tip
Classify as internal and external, give 3 sources and at least 5 remedies.

Noise in Measurement Systems

Sources of noise

Methods to minimize noise

Exam tip

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