1629 Sensor Selection Process

1629.1 Learning Objectives

By the end of this chapter, you will be able to:

Define application requirements: Translate project needs into measurable sensor specifications
Compare components systematically: Create comparison tables for objective evaluation
Calculate power consumption: Compute average current with duty cycling for battery life estimation
Perform weighted ranking: Score and rank components based on prioritized criteria
Understand specification trade-offs: Balance range vs sensitivity, accuracy vs power, analog vs digital

1629.2 Prerequisites

Before diving into this chapter, you should be familiar with:

Specification Sheet Fundamentals: Understanding datasheet sections and vocabulary
Accelerometer Case Study: Experience reading real-world datasheets

1629.3 Common Misconception: “Typical” Specifications

“Typical” Specifications are NOT Guaranteed Performance

The Misconception: Many developers assume “Typical” values in datasheets represent guaranteed performance across all devices. This leads to designs that work with some units but fail with others.

The Reality (Quantified): Datasheets use three specifications with very different meanings:

Specification Type	What It Means	Example (Temperature Sensor)	Guarantee Level
Typical (Typ)	Median or average value from production testing	Accuracy: +/-0.5C (typ)	0% guarantee - Marketing value only
Minimum/Maximum	Worst-case limits at specified conditions	Accuracy: +/-2.0C (max)	100% guarantee - Every unit meets this
Conditions	Test environment	At 25C, 3.3V supply	Only valid under these conditions

Real-World Impact:

Example 1: Temperature Sensor Selection

Datasheet shows: “Accuracy: +/-0.5C (typ), +/-2.0C (max)”
Developer designs for +/-0.5C requirement
Result: 50% of production units exceed +/-0.5C error
Cost: $25,000 product recall for 5,000 units

Example 2: Current Consumption Miscalculation

Sensor spec: “Sleep current: 2 uA (typ), 10 uA (max)”
Battery life calculation uses 2 uA
Result: Battery life is 5x shorter than predicted (10 uA / 2 uA)
Expected 5 years -> Actual 1 year
Cost: Customer dissatisfaction, warranty claims

Example 3: ADC Resolution Reality

ADC spec: “INL: +/-1 LSB (typ), +/-4 LSB (max)”
Developer assumes 1 LSB effective resolution
Result: 20% of units have 4x worse linearity error
12-bit ADC (4096 codes) effectively becomes 10-bit (1024 codes) in worst case

The Fix:

Always design for “Max” specifications, not “Typ”
Add margin: Use 80% of “Max” limit for safety
Request distribution data: Ask manufacturer for +/-1 sigma, +/-2 sigma, +/-3 sigma specifications
Test early: Validate with multiple production units, not just samples

Quantified Best Practice:

Safety margin = 20% beyond worst-case spec
Example: Max current = 10 uA -> Design for 12 uA (10 x 1.2)
This accounts for temperature derating, aging, and manufacturing spread

Remember: “Typical” is marketing. “Maximum” is engineering. Design for maximum.

1629.4 Sensor Selection Process

1629.4.1 Step 1: Define Requirements

Requirements Checklist:

Requirement Category	Questions to Answer
Measurement	What physical quantity? What range? What accuracy?
Power	Battery or mains? Target battery life? Duty cycle possible?
Interface	What MCU interfaces available? I2C, SPI, Analog?
Environment	Temperature range? Humidity? Vibration?
Physical	Size constraints? Weight limits? Mounting method?
Cost	Target unit cost? Volume pricing considerations?

1629.4.2 Step 2: Compare Specifications

Key Comparison Criteria:

Criteria Category	What to Compare	Why It Matters
Range	Measurement min/max	Must cover expected values
Accuracy	+/-% error	Determines measurement quality
Power	Active/sleep current	Battery life impact
Interface	Analog, I2C, SPI, UART	MCU compatibility
Temperature	Operating range	Environmental suitability
Package	Size, pin count	PCB footprint
Cost	Unit price	Total system cost

Example: Comparing Three Accelerometers

Specification	ADXL335	MMA8452Q	LIS3DH
Manufacturer	Analog Devices	NXP	STMicroelectronics
Range	+/-3g	+/-2g	+/-2g
Interface	Analog	I2C (digital)	I2C (digital)
Accuracy	+/-2.0%	+/-1.5%	+/-1.0%
Active Current	350 uA	165 uA	11 uA
Sleep Current	0.1 uA	2 uA	0.5 uA
Resolution	Depends on ADC	12-bit	12-bit
Noise Density	150 ug/sqrt(Hz)	99 ug/sqrt(Hz)	100 ug/sqrt(Hz)
Max Sample Rate	1600 Hz	800 Hz	5376 Hz
Package	LGA (3x3x1 mm)	QFN (3x3x1 mm)	LGA (3x3x1 mm)
Price	$3.50	$2.20	$1.80

Best in category marked with star: LIS3DH wins on accuracy, active current, sample rate, and price.

1629.4.3 Step 3: Calculate Power Consumption

Power Consumption Calculation Example:

For a sensor sampling once per second with 1% duty cycle (10ms active, 990ms sleep):

Average Current = (Active Current x Duty Cycle) + (Sleep Current x (1 - Duty Cycle))

LIS3DH Example:
  Active: 11 uA x 0.01 = 0.11 uA
  Sleep:  0.5 uA x 0.99 = 0.495 uA
  Total: 0.605 uA average

Battery Life (CR2032 = 220 mAh):
  Lifetime = 220,000 uAh / 0.605 uA = 363,636 hours = 41 years!

1629.4.4 Step 4: Score and Rank

Sensor Ranking Decision Tree:

Weighted Scoring Example:

Criteria	Weight	ADXL335	MMA8452Q	LIS3DH
Accuracy	30%	6	8	10
Power	30%	5	7	10
Cost	25%	5	8	10
Interface	15%	7	9	9
Weighted Total	100%	5.6	7.8	9.85

Winner: LIS3DH - Best overall score across prioritized criteria.

1629.5 Accelerometer Measurement Calculations

For an analog accelerometer with these specs:

Sensitivity: 800 mV/g
Supply: 3.0V
ADC: 10-bit (0-1023)

Key Formulas:

Calculation	Formula	Example (+/-2g sensor)
Zero-g Output	Vdd / 2	3.0V / 2 = 1.5V
Output at +1g	(Vdd/2) + (Sensitivity x g)	1.5V + (0.8V x 1) = 2.3V
Output at -1g	(Vdd/2) - (Sensitivity x g)	1.5V - (0.8V x 1) = 0.7V
ADC Value	(Vout / Vdd) x 1023	(2.3V / 3.0V) x 1023 = 784
Resolution	(Vdd / 1023) / Sensitivity	(3.0V / 1023) / 0.8 = 3.66 mg/LSB

Example Measurement Table:

Acceleration	Output Voltage	ADC Value (10-bit)	Notes
0g (rest)	1.5V	512	Center/zero point
+0.5g	1.9V	648	Gentle tilt
+1.0g (vertical)	2.3V	784	Standing upright
+2.0g (max)	3.1V	1023 (clipped!)	Maximum range
-1.0g	0.7V	239	Upside down
-2.0g (min)	-0.1V	0 (clipped!)	Minimum range

Hands-on Lab: Spec Sheet Analysis

1629.5.1 Objective

Practice reading and interpreting sensor datasheets to select appropriate components for a wearable fitness tracker.

1629.5.2 Scenario

You are designing a wearable fitness tracker with the following features:

Activity Recognition: Detect walking, running, cycling
Step Counting: Count steps accurately
Heart Rate Monitoring: Measure heart rate
Battery Life: Must last 7+ days on single charge

1629.5.3 Requirements

System Requirements:

Aspect	Requirement
Sensors Needed	Accelerometer, gyroscope, heart rate (PPG)
Accelerometer Range	+/-8g (for running impacts)
Gyroscope Range	+/-2000 dps (degrees per second)
Heart Rate	40-220 BPM
Sample Rate	100 Hz (accelerometer), 25 Hz (HR)
Power Budget	< 2 mA average (3.7V, 150mAh battery)
Size	Must fit in 20mm x 15mm x 3mm package
Temperature	0C to 50C (wearable)
Cost Target	< $5 total for sensors

1629.5.4 Task

Follow this systematic approach to select sensors:

Step 1: Create Comparison Table

Research 3-4 accelerometer datasheets and extract these specifications:

Spec	Sensor A	Sensor B	Sensor C	Your Requirement
Range	+/-__g	+/-__g	+/-__g	+/-8g
Accuracy	+/-__%	+/-__%	+/-__%	< +/-2%
Active Current	__ uA	__ uA	__ uA	Low as possible
Sleep Current	__ uA	__ uA	__ uA	< 5 uA
Interface	__	__	__	I2C preferred
Sample Rate	__ Hz	__ Hz	__ Hz	>= 100 Hz
Package Size	__ mm	__ mm	__ mm	< 20x15 mm
Price	$__	$__	$__	< $5

Step 2: Calculate Power Consumption

For each sensor, calculate average current with 1% duty cycle (100 Hz sampling = 10ms active per second):

Average Current = (I_active x 0.01) + (I_sleep x 0.99)

Example:
  Sensor: 150 uA active, 2 uA sleep
  Avg = (150 x 0.01) + (2 x 0.99) = 1.5 + 1.98 = 3.48 uA

Step 3: Estimate Battery Life

With 150 mAh battery at 3.7V:

Battery Life (hours) = Battery Capacity / Average Current
                     = 150,000 uAh / 3.48 uA
                     = 43,103 hours = 1,796 days = 4.9 years

Step 4: Score and Rank

Assign weights and score each sensor (0-10 scale):

Criteria	Weight	Sensor A Score	Sensor B Score	Sensor C Score
Accuracy	30%	__	__	__
Power	30%	__	__	__
Cost	40%	__	__	__
Total	100%	__	__	__

1629.5.5 Expected Deliverable

A one-page report that includes:

Comparison Table: All candidate sensors with key specs
Power Analysis: Average current and battery life calculations for each
Ranking: Weighted scores showing trade-offs
Recommendation: Final sensor choice with justification (2-3 sentences)

Example Justification: > “Selected the LIS3DH accelerometer due to ultra-low 11 uA active current (15x lower than competitors), providing 4+ year battery life on a single charge. Despite being the cheapest option at $1.80, it offers the best +/-1.0% accuracy. The I2C interface simplifies MCU integration and the 5376 Hz max sample rate exceeds our 100 Hz requirement with plenty of headroom.”

1629.6 Spec Sheet Analysis Framework

A spec sheet analysis framework for IoT helps systematically compare and select sensors based on electrical characteristics, performance specifications, mechanical constraints, and environmental requirements.

1629.6.1 Framework Components

1. Specification Data Structures:

ElectricalCharacteristics: Supply voltage, current consumption (active/sleep/shutdown)
PerformanceSpecifications: Range, sensitivity, resolution, accuracy, bandwidth
MechanicalSpecifications: Package dimensions, pin count, footprint
EnvironmentalSpecifications: Temperature range, humidity, shock/vibration ratings

2. Key Calculations:

Calculation	Formula	Use
LSB Value	Vref / (2^bits)	ADC resolution
Measurement Resolution	LSB / Sensitivity	Minimum detectable change
Dynamic Range	6.02 x bits + 1.76 dB	Signal range
Average Power	P_active x duty + P_sleep x (1 - duty)	Battery life

3. Battery Life Estimation:

Lifetime = (Capacity x Efficiency) / Average Current

Where:
  Capacity = Battery mAh
  Efficiency = ~85% (typical)
  Average Current = Weighted sum of modes

4. Component Ranking:

Weighted scoring across multiple criteria
Normalized values (0-1 scale)
Configurable weights for power, resolution, accuracy, cost, size
Sorted rankings (highest score wins)

1629.7 Knowledge Check

Quiz: Sensor Selection

Question: A datasheet lists “Supply Current: 50 uA (sleep), 12 mA (active)” for a wireless sensor. The device sleeps 99% of the time and is active 1%. What is the average current consumption?

Explanation: Average current = (duty_cycle_sleep x I_sleep) + (duty_cycle_active x I_active) = (0.99 x 50uA) + (0.01 x 12mA) = 49.5uA + 120uA = 169.5uA approximately 170uA. Even though sleep is 99% of time, active current contributes significantly because it’s 240x higher. With 2000 mAh battery: lifetime = 2000 mAh / 0.170 mA = 11,765 hours = 490 days approximately 16 months. Battery capacity must support peak current (12 mA), but lifetime is determined by average current. This calculation is critical for battery-powered IoT device design.

1629.8 Summary

Key Takeaways:

Define requirements first - Clear criteria enable objective comparison
- Measurement type, range, accuracy
- Power budget and battery life target
- Interface compatibility
- Environmental conditions
- Cost constraints
Create structured comparison tables - Extract key specs for all candidates
Calculate power consumption accurately
- Use duty cycling for realistic estimates
- Account for all operating modes
- Design for worst-case (max) values, not typical
Use weighted scoring - Prioritize criteria by project importance
- Assign weights based on application needs
- Score objectively (0-10 scale)
- Calculate weighted totals
Document your decision - Justify selection with quantitative reasoning

1629.9 What’s Next

Continue to Automotive Sensor Applications to explore industry-specific sensor requirements for safety-critical systems including seat occupancy detection, airbag deployment, tire pressure monitoring, and adaptive cruise control.

Related Chapters:

Specification Sheet Fundamentals - Basic datasheet concepts
Accelerometer Case Study - Detailed datasheet analysis
Automotive Applications - Safety-critical sensors
Energy Management - Power optimization techniques