Analog Sensing 101
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Transcript of Analog Sensing 101
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Analog Sensing 101WITH P14452
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Agenda
Analog Signal Characteristics Common Problems with A/D Conversion
Clipping Small Signals Aliasing
Analog Signal Conditioning “Digital” Sensors
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Analog Signal Characteristics
Two Key Parameters Voltage Range
i.e. +/- 50V Frequency Range
i.e. < 5 KHz
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Common Problem: Clipping
Cause: A/D Converter is unable to record
voltages outside of operating range (i.e. 0-5V)
Solution: Attenuation
Reduce the voltage range of the signal, without corrupting the information
Scale (in software) the digital representation to reflectthe true value
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Common Problem: Small Signals
Cause: Analog Signal has a voltage
range much lower than that of the A/D converter
Solution: Amplification
Increase the voltage range of the signal, without corrupting the information
Scale (in software) the digital representation to reflect the true value
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Common Problem: Aliasing
Cause: Violation of the Nyquist limit by
Incorrect sampling frequency High Frequency Signal Noise
Solution: Low Pass Filter
Remove all frequencies above the frequency range of the sensor before capture of the signal
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Analog Signal Conditioning Process:
Attenuate Large Signal Amplify Small Signal Convert Differential Signal to Single
Ended Signal Apply DC Bias Remove High Frequencies Convert to Digital Value Scale Digital Value according to:
Attenuation Amplification DC Bias Conversion Factor ( i.e. Celsius / mV )
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“Digital” Sensors What are they?
Analog Sensor with Signal Conditioning and A/D conversion in the same package
Advantages Standard Communication Protocol
i.e. I2C, SPI, CAN Preset to Optimally Condition and
Sample Minimized PCB Requirements Inexpensive Accurate
Disadvantages Software Requirement Typically Fixed Sampling Rate
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Case Study: Temperature Sensor
ADT7301 Temperature Range
-40 C to 150 C Accuracy
+/- 0.5 C Precision
13-bit 0.03125 C / 1 bit increase
Max Sampling Rate 83.33 Hz
Interface SPI 2.7-5.25V Supply
Cost: ~$1.21
OMEGA TTIN-18 Type T Thermocouple Temperature Range
-185 C to 300 C Additional Components Needed
Instrumentation Amplifier INA128 - $11.30
Low Pass Filter 3 Op-AMPS
LM741 - $0.70 Analog to Digital Converter
Cost: $21 + $11.30 + $2.10 = $33.40
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Case Study: Pressure SensorHoneywell SSCDANN150PG2A3 Pressure Range
0 – 150 PSIG Accuracy
+/- 1% (2% band) Precision
14-bit 0.0092 PSI / 1 bit increase
Max Sampling Rate ~2 kHz
Interface I2C 3.3V or 5V Supply
Cost: ~$36
Ashcroft G17M0242F2200# Pressure Range
0 – 200 PSI Accuracy
+/- 0.5% Voltage Supply: 9-24V Additional Components Needed
Attenuator (2 Op-amps) ~$1.40 Instrumentation Amplifier ~$11.30 Low Pass Filter (3 Opamps) ~$2.10 Analog to Digital Converter
Cost: $175.50 + $20.80 = $196.30
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Case Study: Vibration SensorFreescale Xtrinsic MMA8451Q 3-axis Vibration Range
+/- 8g Accuracy
+/- 1% (2% band) Precision
14-bit 0.000977g / bit
Max Sampling Rate =< 800Hz
Interface I2C 1.95V to 3.6V Supply
Cost: ~$1.67
Omega ACC310 1-axis Vibration Range
+/- 75g Precision: 100mV/g Output Range: +/- 7.5V Voltage Source: 24-36V Additional Components Needed
Attenuator (2 Op-amps) ~$1.40 Instrumentation Amplifier ~$11.30 Low Pass Filter (3 Opamps) ~$2.10 Analog to Digital Converter
Cost: $175 + $20.80 = $195.80