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 Sensor

Honeywell 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 Sensor

Freescale 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