Transcript Slide 1

Electronics and Noise, Ch. 14
and 16, Senturia
• What determines the performance of a sensor?
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Minimum Detectable Signal? (What does this mean?)
Precision?
Accuracy?
Frequency Response?
Dynamic Range? (What does this mean?)
Look at the ADXL 204
• Sensitivity
– What is the significance?
• Noise Specs?
– What is the signficance?
• Anything on accuracy or precision?
–?
Plan
• Start with elecronics – review op-amp
circuits
• Talk about noise in general
• Do some examples using specs for
particular op-amps.
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.
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??)
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.
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.
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.
4kT=4.1x10-21 at 300 K