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Understanding Sensor Performance in Electronics and Noise Circuits

This chapter explores the factors influencing sensor performance such as detectable signals, precision, accuracy, frequency response, dynamic range, sensitivity, and noise specifications. It delves into the importance of accuracy, precision, and noise considerations, including interference and random noise. The text delves into thermal noise and its impact on energy storage elements in electrical circuits. Examples and specifications for op-amp circuits are discussed, along with limitations like slew-rate and strategies for minimizing noise in MEMS sensors.

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Understanding Sensor Performance in Electronics and Noise Circuits

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  1. Electronics and Noise, Ch. 14 and 16, Senturia • What determines the performance of a sensor? • Minimum Detectable Signal? (What does this mean?) • Precision? • Accuracy? • Frequency Response? • Dynamic Range? (What does this mean?)

  2. Look at the ADXL 204 • Sensitivity • What is the significance? • Noise Specs? • What is the signficance? • Anything on accuracy or precision? • ?

  3. Plan • Start with elecronics – review op-amp circuits • Talk about noise in general • Do some examples using specs for particular op-amps.

  4. Slew-Rate • Another limitation, in addition to all the others, that comes from properties of the op-amp circuit: • Slew-Rate: • The rate at which the output voltage can change • Typically measured in V/ms (at the output) • It is another spec. for op-amps • Typically 0.5-1,000 V/ms • Sometimes this needs to be large for driving something like an electrostatic actuator.

  5. Noise, Ch. 16, Senturia • Noise often limits performance of MEMS sensors and other devices (oscillators, filters, for example). • What we often think of as noise can be divided into 2 (or more) parts. • 1. Interference. • 2. Random noise. • 3. Drift, aging effects… (random noise??)

  6. Interference • Definintion: Unwanted sensitivity to external or internal disturbances. • Electrical, thermal, mechanical, optical… • Examples. • Electrical: Capacitive coupling to 60 Hz, radio waves, driving voltage to output … • Mechanical: Sensitivity to vibration… • Optical: Sensitivity to ambient light. • Thermal: Sensitivity to temperature (very common!) • System design critical (Senturia has examples) • References: Keithley, Low-Level measurements + others.

  7. Random Noise • Thermal noise. • Dissipative processes result in fluctuations. • Energy storage elements have a non-zero fluctuating amount of energy stored. • Shot noise. • Current consists of discrete particles. • Flicker noise (1/f noise). • Mostly capture and release of carriers from traps in electrical circuits. Many physical mechanisms, generally.

  8. Thermal Noise • Statistical mechanics -> average energy of a particle = 3/2 kBT. (1/2 KBT for each degree of freedom (x, y, z)) • Mass with 1 degree of freedom -> ½ kBT <-> inductor! • Inductor has on average ½ kBT of energy. • The capacitor is also an energy storage element with one degree of freedom. If connected to its environment with a resistor (or almost anything else) it has an average stored energy of ½ kBT! This does not depend on the size of the resistor or capacitor! • Spring (capacitor) also has ½ kBT. • This is characteristic of thermal noise.

  9. 4kT=4.1x10-21 at 300 K

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