Analog Sensing 101WITH 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

i.e. < 5 KHz

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

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

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

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 )

“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

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

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

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