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Analog Sensing 101: Understanding Signal Characteristics and A/D Conversion Challenges

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This comprehensive guide delves into analog sensing, focusing on signal characteristics and common issues with A/D conversion. It addresses key problems such as clipping, small signals, and aliasing, providing effective solutions like attenuation, amplification, and low-pass filtering. The document also explains the process of analog signal conditioning, highlighting the role of digital sensors and their benefits, including standard communication protocols. Various case studies on temperature, pressure, and vibration sensors are included, showcasing practical applications and associated costs.

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Analog Sensing 101: Understanding Signal Characteristics and A/D Conversion Challenges

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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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