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Analog Sensing 101

Analog Sensing 101. With P14452. Agenda. Analog Signal Characteristics Common Problems with A/D Conversion Clipping Small Signals Aliasing Analog Signal Conditioning “Digital” Sensors. Analog Signal Characteristics. Two Key Parameters Voltage Range i.e. +/- 50V Frequency Range

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Analog Sensing 101

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  1. Analog Sensing 101 With P14452

  2. Agenda • Analog Signal Characteristics • Common Problems with A/D Conversion • Clipping • Small Signals • Aliasing • Analog Signal Conditioning • “Digital” Sensors

  3. Analog Signal Characteristics • Two Key Parameters • Voltage Range • i.e. +/- 50V • Frequency Range • i.e. < 5 KHz

  4. 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

  5. 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

  6. 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

  7. 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 )

  8. “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

  9. Case Study: Temperature Sensor ADT7301 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 • 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

  10. Case Study: Pressure Sensor Honeywell SSCDANN150PG2A3 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 • 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

  11. Case Study: Vibration Sensor FreescaleXtrinsicMMA8451Q 3-axis 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 • 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

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