Data Conversion; Sensors

Question 1

What are the trade-offs when selecting ADC?

Answer 1

The main trade off is between resolution and speed.
High resolution devices tend to be slower.
Very fast devices have poor resolution.
Another trade off is cost.
More expensive forms of ADC tend to be better.

Question 2

What are the main types of ADC?

Answer 2

Successive Approximation.
Flash.
Delta Sigma.

Question 3

Describe Successive Approximation ADC.

Answer 3

The most common form of ADC.
The output of the DAC is compared with the voltage to be digitised.
When the conversion starts, the register sets the most significant bit to 1 and all others to zero.
Vsignal is constantly compared to Vdac. Depending on whether Vsignal is more or less than Vdac changes the binary output in a different manner.
Successive Approximation ADC is essentially an iteration process.
The conversion time is independent of input voltage.

Question 4

Describe Flash ADC.

Answer 4

Used for very rapid signal capture.
The priority encoder converts output from comparators into binary.
Each comparator compares input voltage with Vref.
If Vsignal > Vref output of that particular comparator will go high.
If Vsignal < Vref output of that particular comparator will go low.
The priority encoder gives output corresponding to highest priority input.

Question 5

Describe Delta Sigma ADC.

Answer 5

Used for high resolution.
Variant on Successive Approximation but uses a 1 bit DAC.
1 bit DAC creates a stream of hi and lo outputs which are subtracted from the input voltage.
The difference alternates rapidly but is averaged.
The comparator is used to determine the value to the DAC.
The rapidly varying value is smoothed using the digital filter.
The output is a smooth, slowly varying numerical value.