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Microelectronics in consumer products > ADC > Flashcards

ADC Flashcards

1
Q

Why do we need ADCs in microcontrollers?

A

Because many microcontrollers control and monitor the real environment, which is analogue in nature

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2
Q

Diagram showing DAC and ADC connecting to the mc?

A

DRAW..

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3
Q

How is information contained in analogue signals?

A

Information lies in the continuously changeable worth of some constituent parameter, such as amplitude, frequency or phase.

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4
Q

What is the resolution and range of analogue signals?(theoretically)

A

infinite (theoretically)

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5
Q

How to digital signals represent information?

A

Represent information content as arrangement of discrete characters

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6
Q

What is the range and resolution of digital signals?

A

Range and resolution limited by the number and type of symbols making up the pattern

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7
Q

What are the main reasons for error in ADC?

A

Sampling and Quantisation

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8
Q

Can you draw the graph of 3 bit resolution for an analogue signal showing the sampled and quantisised signals? (Goes from 0 to 1 on both axes)

A

YES OR NO?

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9
Q

What does Shannon’s sampling theorem state?

A

A signal must be sampled with a frequency of at least twice the maximum frequency of the signal itself.

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10
Q

What is the maximum measurable frequency called and so what value is this compared to the sampling frequency?

A

The nyquist limit (or frequency) (and so this is half the sampling frequency)

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11
Q

When would aliasing occur?

A

When the signal frequency is higher than the nyquist limit, because then it is more than half of the sampling frequency

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12
Q

What is aliasing?

A

The misidentification of a signal frequency, introducing error or distortion.

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13
Q

Can you draw what happens when a signal with max frequency component of 60Hz is sampled at 100Hz?

A

YES OR NO?

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14
Q

How can aliasing be prevented?

A

By using an analogue low pass filter (An ANTI-ALIASING filter) to ensure that no components with greater than half the sample frequency remain.

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15
Q

Can you draw what would happen to the signal before if it has been passed through a low pass filter to become 40Hz maximum frequency?

A

YES OR NO?

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16
Q

When are anti-aliasing filters only really required?

A

When the analogue signal has significant frequency components above the Nyquist frequency

17
Q

Can you provide and example of when anti-aliasing filters would be used?

A

When sampling a high-quality music signal (Fmax

18
Q

Can you provide an example of when anti-aliasing filters would not be required?

A

Signals from light or temperature sensors usually have no significant components at higher frequencies and so anti-aliasing filters are not needed.

19
Q

Why do we get a quantisation error?

A

Because a digital signal only has a limited number of symbols to portray a corresponding analogue signal (which has an infinite number of levels).

20
Q

What is the resolution in quantisation?

A

The resolution is defined by the number of bits used to represent the signal.. with n bits the number of levels is 2^n. Resolution is 1/2^n

21
Q

How many levels and what resolution is a 3 bit quantisation?

A

3 bits - 2^3 levels = 8 levels

resolution = 1/2^3 = 0.125

22
Q

Can you draw graphs showing 3 bit quantisation of an analogue signal and the corresponding error graph?

A

YES or no?

23
Q

What are some other sources of error in ADC?

A
  1. Linearity of the ADC
  2. Offset (0 analogue signal does not give 0 digital signal)
  3. Inaccurate reference voltage
  4. Noise in analogue signal or sensor
24
Q

Can you draw the diagram showing the key components needed to read analogue signals into a mc?

A

YES OR NO?

25
Q

In practise, what is the range of an analogue signal?

A

The range is limited to maximal and minimal values.

26
Q

How is the input range of an ADC defined?

A

By its low reference voltage Vss (usually 0) and its high reference voltage Vdd (Vref)

27
Q

What is signal conditioning?

A

To ensure that the analogue signal lies within the input range of the ADC it must be scaled: i.e. either amplified or reduced.

28
Q

What is the most common technique for ADC called?

A

Successive approximation technique

29
Q

Can you draw the diagram showing how successive approximation works for a 4bit ADC?

A

YES OR NO?

30
Q

How is electronic implementation of successive approximation done?

A

Using a network of precision resistors or capacitors. They are configured to allow consecutive halving of the fixed voltage Vref to be switched in to an analogue comparator.

31
Q

What happens in the sample and hold process for a capacitor network?

A
  1. During the sample period the network is connected to the analogue input voltage Vin.
  2. During the hold period the analogue input is disconnected and the charge at each capacitor is examined by the comparator.
32
Q

What are 4 features of successive approximation ADC?

A
  1. The successive approximation method produces one bit for each comparison, starting with the most significant bit (MSB)
  2. Each step requires 2 clock cycles: one to change the reference voltage and one to make the comparison -> 16 ADC clock cycles in all for an 8bit ADC.
  3. The conversion is done by an ingenious network of switched capacitors:
    - - must be slow enough for charge to redistribute on capacitors
    - - must be fast enough for charge not to leak away
    - - hence ADC clock must be around 1MHz, regardless of frequency of clock for the rest of the mc, which may run faster.
  4. Multiple channel ADCs are usually implemented by multiplexing the inputs to a single ADC unit.
33
Q

What is the cheapest way to buy an ADC? And is this always the best option?

A

Cheaper to buy a mc with a built in ADC than to buy an ADC on its own. Sometimes a separate ADC is better, e.g. if the analogue signal needs to be converted close to the signal source (or use a cheap mc as a separate ADC).

34
Q

What are the 11 features of the SAM3X8E ADC module?

A
  1. (software or hardware) Trigger to initiate conversion
  2. (comparison) Window function
  3. (Programmable) Offset
  4. (Programmable) Gain
  5. (Separate) Power supply for module (2-3.6V)
  6. 1MHz Conversion rate. (programmable)
  7. 10 or 12 bit Resolution (selectable)
  8. Single ended (16 channels) or differential (8 channels) Input mode
  9. 16 channels via integrated Multiplexer
  10. External reference voltage
  11. Sleep and standby modes
35
Q

Is all the functionality of the SAM3XE available with the arduino due?

A

No, only the basic functionality is accessible on the arduino due.

36
Q

What are 3 features of the ADC with the Arduino Due?

A
  1. 12 ADC channels available
  2. all inputs configured single ended
  3. initiate conversion and read values using analogRead()
  4. Default resolution is 10 bits but can be changed to 12 bits using analogResolution()
  5. Analogue reference voltage fixed at 3.3V.
    - - analogue input range 0..3.3V
    - - input voltage > 3.3V will damage the mc
    - - an external reference voltage can be used via the AREF pin, but requires removal of resistor from the PCB.
37
Q

In successive approximation, what is the most significant bit?

A

The bit produced after the first comparison (comparing 1/2 Vdd with Vin to see if the voltage is in the top or bottom half of the range)

Microelectronics in consumer products (14 decks)

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