IoT Lab (IT-705) - Important Questions

For a $12$-bit ADC with reference voltage $V_{ref}=3.3\,\text{V}$:

(a) Calculate the resolution in volts per LSB.

(b) Calculate the ADC code for an input voltage $V_{in}=1.65\,\text{V}$. Use the formula

$$\text{Code} = \left\lfloor \frac{V_{in}}{V_{ref}} \times \left(2^{N} - 1\right) \right\rfloor$$

where $N$ is the ADC bit depth.

(c) State the maximum quantization error in volts and express it as a percentage of full-scale.

Explain the important considerations when interfacing an analog sensor to a microcontroller ADC. In your answer discuss the following factors and why each matters: input impedance, source impedance, ADC sample-and-hold operation, input capacitance, buffering (op-amp or unity buffer), and protection against over-voltage/transients.

Design a signal conditioning circuit for a thermistor-based temperature sensor whose resistance varies from $10\,\text{k}\Omega$ to $100\,\text{k}\Omega$ over the operating range. Show a practical topology (voltage divider, buffer, and amplifier/offset stage) and calculate component values to map the sensor output linearly into the ADC input range $0\text{–}3.3\,\text{V}$. Explain choices for input impedance, filter time-constant and protection.

A sensor signal has useful bandwidth from $0$ to $3\,\text{kHz}$. Determine the minimum sampling frequency according to Nyquist. Propose an anti-aliasing low-pass filter (specify type and cut-off frequency) and justify the selected filter order and cut-off to limit aliasing while preserving signal fidelity.

Write firmware pseudocode to read an analog sensor using the MCU ADC and publish readings to an MQTT broker. Your answer must include:

• ADC sampling rate selection and justification.

• A filtering/averaging strategy (e.g., moving average or decimation) and why it is used.

• Topic structure (example topic strings) and QoS selection with justification.

• Handling of ADC over-range and transient spikes.

Explain design trade-offs for sampling frequency, network bandwidth and power considerations.

A $10$-bit ADC has a full-scale input range of $0\text{–}5\,\text{V}$.

(a) Calculate the theoretical SNR due to quantization noise in dB.

(b) If the measured SNR is $46\,\text{dB}$, calculate the Effective Number of Bits (ENOB). Use the relation for ideal quantization SNR and the conversion formula for ENOB.

Explain the operation of a sample-and-hold (S/H) stage in an ADC. Describe the roles of the hold capacitor, aperture time and acquisition time. Show the equation for the charging of the hold capacitor from a source with source resistance $R_s$ and capacitance $C_{hold}$:

$$V_{C}(t) = V_{in} \left(1 - e^{-t/\left(R_s C_{hold}\right)}\right)$$

and explain how this governs the required acquisition time for a given resolution.

Describe basic analog front-end techniques to reduce noise and interference in sensor signals prior to ADC conversion. For each technique below give one practical circuit example and the context where it is used:

• Passive RC low-pass filtering

• Active (op-amp) filtering

• Differential measurement with instrumentation amplifier for common-mode rejection

• Shielding and grounding practices