Chapter4 Analog - Analog input subsystem Property being...

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Analog input subsystem input transducer signal conditioning sample analog to digital conv. Property being measured Digital value to CPU convert “property” to electrical voltage/current produce convenient voltage/current levels over range of interest hold value during conversion
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Analog output subsystem digital to analog conv signal conditioning output transducer/ actuator Property being controlled Digital value from CPU convert binary code to an analog voltage/current produce convenient voltage/current levels over range of interest convert electrical signal to mechanical or other property
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Multichannel analog input subsystem input transducer signal conditioning sample Analog to Digital Conv. Property1 Digital value mux input transducer signal conditioning Property2 input transducer signal conditioning PropertyN Select channel
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Analog subsystem properties Accuracy : degree to which measured value differs from true value Resolution /precision: degree to which two conditions can be distinguished Related to #bits in digital value Range : minimum to maximum “useful” value Linearity : y = Ax + B (correction req’d if not linear) piecewise linear approximation over different ranges Repeatability : same measurement for a given value affected by hysteresis or other phenomena Stability : value changes other than due to the property being measured (eg. T affecting P)
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Analog to digital conversion errors May need to correct in software Offset error Gain error Nonlinearity error - Unequal distances between transition points Quantization error: Difference between digital Usually want ½ LSB
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Transducers Convert physical quantity to electrical signal Self-generating – generates voltage/current signal Non-self-generating – other property change (ex. R) Examples: Force/stress (strain gage) Temperature (thermocouple, thermistor, semicond.) Pressure Humidity (gypsum block) Smoke Light (phototransistor, photoconductive cell) Acceleration (accelerometer) Flow Position (potentiometer, displacement)
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Temperature sensors Thermocouple – “Seeback EMF produced by heating junction of dissimilar metals ( μ V) Thermistor – mix of materials in ceramic Metal conductor: )] ( 1 [ 0 0 T T R R t + = α + V - [ ] To T t e R R / 1 / 1 0 = β •Negative temperature coefficient: R^ with Tv •Linear over small range •Positive temp. coefficient: R^ with T^
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- + Vcc ≈V BE V BE V BE Semiconductor temperature sensor = Is Ic q kT V BE ln Base-emitter voltage approximately proportional to T T V BE
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Analog Devices AD590 Temperature Transducer IC generates current proportional to temperature Generated current I T is linear: 1 μ a/ o K Example: Design a temperature monitor with output in the range [0v. .4v] over temperature range [-20 o C . . +60 o C] (Use summing amplifier) I T
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Strain Gage Measure stress by measuring change or
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Chapter4 Analog - Analog input subsystem Property being...

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