《现代传感器》课件Lecture3

Inexpensive circuits are not of much use, expensive anyway!
Department of Optical Engineering
Zhejiang University
Limitations
Limitations to resistance measurement
Zhejiang University
Spec Sheet
Nominal Pressure Range
(mbar)
100
200
400 1000
Sensitivity
(mV/mb ar)
0.5
0.25
0.12 0.06
Linearity
(%FSO)
<1
Bridge Resistance
Chip Size
How to get a voltage signal out of the sensor?
Need a constant current source
The easiest way to build a current source: voltage divider
Condition: load R>>sensor R
Lock-in: bias the sensor and trigger the sampling, get the low noise signal
Disadvantage: clocked switch inject noise charge into circuit
Very accurate capacitance measurement still requires expensive precision circuitry
Zhejiang University
Limitations to measurement of capacitance
Stray Capacitance
Appear as additional capacitances in the measuring circuit
Wires moving about with respect to ground, causing capacitance fluctuations
Resistors have much smaller temp coeff. than caps: 0.3ppm/ºC v.s. 200ppm /ºC
Department of Optical Engineering
Zhejiang University
Capacitance measuring circuits
Department of Optical Engineering
Zhejiang University
Introduction to Sensors
Electronics
The electronics which go along with the physical sensor element are very important:
Modern Sensors Lecture 3
X. Wu
Department of Optical Engineering
Zhejiang University
Lecture 3 Basic Intent
Review some background on electrical measurement of sensor outputs
(k) (mm3)
Diaphragm Size (mm2)
5.6
3x3x1
2.2 x 2.2 x1
2 x 2 1.5 x 1.5 1.1 x 1.1
0.8 x 0.8
Department of Optical Engineering
Zhejiang University
BASIC PROCESSING STEPS
A substantial hassle:
providing an AC bias Converting the AC for microprocessor interface
Use clock signal or integrated clock/sampling circuit
Modulated signal creates an opportunity for use of some advanced sampling and processing techniques
Department of Optical Engineering
Zhejiang University
Theoretical background:
piezoresistance
A piezoresistor: a device which exhibits a change in resistance when it is strained.
Impedance:
Very much like a resistor at AC, may measure capacitance by building voltage divider circuits, use either resistor or capacitor as the load resistance
Pressure and Temperature Sensor Cluster
Diffusion (n-type and p-type) Metallization Anisotropic back-side etching (by using
well known chemicals as TMAH or KOH)
Semiconductor resistors on the front side transduce this tension into resistance changes by means of the piezoresistive effect. .
Department of Optical Engineering
Some details regarding the behavior of simple passive filters and operational amplifiers
Provide an overview of piezoresistive devices. Some examples are worked out using this sensing technique
Department of Optical Engineering
Zhejiang University
A Review of Lecture 2
Characteristics of sensors
Transfer function Sensitivity Dynamic Range Hysteresis Temperature Coefficient Linearity Accuracy Noise Resolution Bandwidth
Lead resistace -> 4-wire configuration
Output impedance
The measuring network resistance places a lower limit on the value of a resistance which may across the output terminals
An example: 10K thermister+1M load, if connected to an 1K measuring instrument
-> output voltage would be reduced by ~90%
Department of Optical Engineering
Piezoresistive devices – an
Silicon-based
overview
Specific advantages are:
High sensitivity, >0.5mV/mbar
Good linearity at constant temperature
Ability to track pressure changes without signal hysteresis, up to the destructive limit
Limit the performance, cost, and range of applicability
If carried out properly, the design can improve the characteristics of the entire device,
Focus on basic techniques fro processing the signals most typically produced by a sensor
These effects are due to pressureinduced vibrations in the positions of objects, referred to as microphonics.
Department of Optical Engineering
Zhejiang University
Most sensor act like passive device
Resistive Capacitive Inductive
Department of Optical Engineering
Zhejiang University
Resistive Sensor Circuits
Resistive sensors obey Ohm’s law
Department of Optical Engineering
Zhejiang University
Structure and Assembly
Principle of Operation
Deformation by applied pressure causes high levels of mechanical tension at the edge of diaphragm
There are two components of the piezoresistive effect
the geometric component the resistive component.
The geometric component of piezoresistivity:
a strained element undergoes a change in dimension. These changes in cross sectional area and length affect the resistance of the device.
Shortcoming: small signal might need some amplification
Department of Optical Engineering
Zhejiang University
Capacitance measuring circuits
Many sensors respond to physical signals by producing a change in capacitwenku.baidu.comnce
Department of Optical Engineering
Zhejiang University
Inductance measurement circuits
Impedance: iL -> essentially resistive elements
Inductive sensors generally require expensive techniques for the fabrication of the sensor mechanical structure: 3D structure