Electronics Interview Questions and Answers Part-3

21. What is Sampling?

Sampling is the process of converting a continuous-time signal to a discrete-time signal, in order for computers to process the data digitally.

The signals we use in the real world, such as our voices, are called "analog" signals.  To process these signals in computers, we need to convert the signals to the "digital" form.  While an analog signal is continuous in both time and amplitude, a digital signal is discrete in both time and amplitude.  To convert a signal from continuous-time to discrete-time, a process called sampling is used.

Sampling

• If the sampling is not proper we can't reconstruct the original wave.
• While reconstructing the signal with fewer samples, some unwanted components will present at the output. This problem is called Aliasing.
• This will happen when high-frequency components are sampled at a low-frequency rate.

22. Sampling Theorem

The Sampling Theorem states that a signal can be exactly reproduced if it is sampled at a frequency is greater than twice the maximum frequency in the signal.

Sampling frequency fs  ≥ 2fmax

23. Why do we need a Bootstrap capacitor while using NMOS as a High side switch?

Turning NMOS completely on as a high-side switch requires a high enough VGS, that is, a voltage higher than the drain voltage. Normally, because the drain voltage is Vin (input voltage), representing the highest voltage in the circuit, any higher voltage must be supplied externally. To generate that high voltage, a bootstrap circuit is used.

Here is an example:

MOSFET BOOTSTRAPPING

24. Why Power sequencing is required?

Power supply sequencing is an important aspect to consider when designing with FPGA. Typically, FPGA vendors specify power -sequencing requirements because an FPGA can require anywhere from 3 to 10 rails. By following the recommended power sequence, excessive current draw during startup can be avoided, which in turn prevents damage to devices.

Some general power sequencing methods are given below

1. Cascading PGOOD pin to enable pin
2. Sequencing using reset IC
3. Analog up/down sequencers
4. Digital system health monitors with PMBus interface

25. What are the specifications that need to be considered while selecting the Ferrite beads?

A Ferrite bead is a passive device that filters high-frequency noise energy over a broad frequency range. it becomes resistive over its intended frequency range and dissipates the noise energy in the form of heat.

Ferrite beads are always connected with the power supply lines and is often combining with a capacitor. This forms LPF. Ferrite beads are categorized by three response regions Inductive, Capacitive, and resistive. 

To reduce the high-frequency noise the bead must be in the resistive region.

26. What is LVDS?

LVDS (low-voltage differential signaling) is a high-speed low power digital interface. Speed > 155mbps. LVDS technology uses differential data transmission, so it is less susceptible to common mode noise.

27. Why 120E termination resistor used at the end of the CAN communication?

Terminal resistors are needed in CAN bus systems because CAN communication flows are two-way. The termination at each end absorbs the CAN signal energy, ensuring that this is not reflected from the cable ends. Such reflections would cause interference and potentially damaged signals.

For best results, the termination should match the nominal impedance of the cables, which for ISO 11898-2 (high-speed CAN) is specified at 120 Ohm.

CAN TERMINATION

28. What does a power supervisory circuit do?

1. During the power-up sequence, the device holds the microcontroller in reset until the system power has come up to the correct level.
2. Reset the controller immediately if the power drops below a nominal value either at power down or during a ‘brown-out’ condition.

29. Why do I need a power supervisory circuit need? 

Many times we use a pullup resistor for microcontroller reset pin and most of the time it's working fine. If we have more loads in our circuit and all the loads power on at the same time noise may inject to the power supply line. those noises make microcontroller to power up incorrectly. To solve this kind of problem we need a Power supervisory circuit.

POWER ON RESET

30. What is SOA of Mosfet?

SOA is a safe operating area. Every MOSFET datasheet includes an SOA plot, which describes the maximum time a MOSFET can be exposed to a specific voltage and current.