Electronics & Circuits · Study deck
Sampling Design: Product Scenarios
A doorbell, thermometer, and ECG monitor do not need the same sample rate.
Voltage Vera is your guide for this deck.
After studying this chapter
Learning objectives
You will be able to:
- Explain: Hardware filters can support lower sampling rates when their passband and stopband are specified against the source spectrum; a filter alone does not guarantee alias-free sampling.
- Explain: If the sampling rate is too low, different signals can produce the same apparent pattern, and later processing cannot recover the missing changes.
- Explain: Violating this rule causes aliasing, where high-frequency content masquerades as lower frequencies, producing silently corrupted data that looks plausible but is wrong.
- Explain: Final optimization: A verified 40% compression reduction would leave ~228 GB, within a 256 GB card's nominal capacity.
Major section
Start With the Decision
A doorbell, thermometer, and ECG monitor do not need the same sample rate.
- Each product must preserve its fastest useful change without wasting power or storage.
Major section
For Kids: Meet the Sensor Squad!
ECG records electrical activity from the heart.
- A sample is one reading; the sample rate says how many readings are taken each second.
- A temperature trend can be sampled slowly, while an ECG waveform needs enough samples to retain its shape.
- the microcontroller said, "I cannot watch continuously like you, Sammy.
Major section
For Kids: Meet the Sensor Squad! (continued)
In Max's photos, the wheel looked like it was barely moving -- or even going backwards!
- If the sampling rate is too low, different signals can produce the same apparent pattern, and later processing cannot recover the missing changes.
- I can only take snapshots -- like photographs.
- But the wheel was spinning 10 times per second!
Major section
For Kids: Meet the Sensor Squad! (continued)
Sampling is like taking photos of a moving object -- take photos too slowly and you miss the action!
- Temperature Terry was watching a spinning wheel. "I can see it spinning smoothly, going round and round!".
- the LED was confused. "That is like when car wheels look like they spin backwards in movies!".
- "Exactly!" said ADC Andy. "That is called ALIASING.
Major section
For Kids: Meet the Sensor Squad! (continued)
Max is not taking photos fast enough, so the fast spinning LOOKS like slow spinning.".
- the battery asked, "How fast does Max need to take photos?".
- Andy did some math. "The wheel spins 10 times per second.
- Blinking slowly is like undersampling -- you miss what is really happening!
Major section
Doorbell Sampling Design
Scenario: You're designing a smart doorbell that records 5-second audio clips when the button is pressed.
- You need to select the ADC sampling rate for the ESP32 and calculate storage requirements.
- Anti-aliasing filter: specify its transition band and stopband attenuation; a 6 kHz RC corner alone is not sufficient.
- Hardware filters can support lower sampling rates when their passband and stopband are specified against the source spectrum; a filter alone does not guarantee alias-free sampling.
Major section
Deep dive: Adaptive Multi-Channel Sampling
The system must optimize storage/power by adapting sampling rates.
- Ch3: 200 kHz (ESP32 limit = 83 kHz → requires external ADC).
- Ch4: 100 kHz (requires external ADC).
- Ch3: 200,000 × 2 × 86,400 = 34.6 GB/day (exceeds 30-day budget!).
- Variable rate: Ch3 records only at night (bats), Ch2 only at dawn/dusk (birds).
Major section
Deep dive: Adaptive Multi-Channel Sampling (continued)
Final optimization: A verified 40% compression reduction would leave ~228 GB, within a 256 GB card's nominal capacity.
- Key lesson: Multi-channel systems with diverse sampling rates require careful storage/power budgeting.
- Anti-aliasing filters are mandatory for each channel.
- Adaptive/triggered recording is essential for battery-powered deployments.
Major section
Key Takeaway
The Nyquist theorem is the single most important rule for digitizing analog signals: always sample at least twice the highest frequency present.
- Violating this rule causes aliasing, where high-frequency content masquerades as lower frequencies, producing silently corrupted data that looks plausible but is wrong.
- In battery-powered IoT devices, balancing sampling rate against power consumption is a critical design decision.
Deck summary
Key takeaways
A doorbell, thermometer, and ECG monitor do not need the same sample rate.
- ECG records electrical activity from the heart.
- In Max's photos, the wheel looked like it was barely moving -- or even going backwards!
- Sampling is like taking photos of a moving object -- take photos too slowly and you miss the action!
- Max is not taking photos fast enough, so the fast spinning LOOKS like slow spinning.".
Retrieval practice
Recall check 1 of 3
Voltage Vera says: answer from memory, then check your reasoning.
Q1The doorbell’s uncompressed audio exceeds its flash budget. Why is halving the sample rate a poor fix for the stated input bandwidth?
Show answer
Answer: B The worked example identifies aliasing when the sample rate is reduced without narrowing the signal.
Retrieval practice
Recall check 2 of 3
Voltage Vera says: answer from memory, then check your reasoning.
Q2A home sensor tracks slowly changing room temperature. What supports a slower sampling plan than an audio recorder?
Show answer
Answer: B The temperature example sets sampling from its change rate and measurement purpose.
Retrieval practice
Recall check 3 of 3
Voltage Vera says: answer from memory, then check your reasoning.
Q3A recorder samples slow temperature and fast audio on separate channels. Which filter plan protects both measurements?
Show answer
Answer: C Each channel needs its own alias margin before conversion.
Q4What discrete values are produced at each sampling-clock instant?
Show answer
Answer: D ADC samples belong in Capture samples because they are the discrete values produced at each clock instant.
Print reference
Answers
Answer key.
- B · The worked example identifies aliasing when the sample rate is reduced without narrowing the signal.
- B · The temperature example sets sampling from its change rate and measurement purpose.
- C · Each channel needs its own alias margin before conversion.
- D · ADC samples belong in Capture samples because they are the discrete values produced at each clock instant.