Analog-to-Digital

Converters

Prepared by:

Mohammed Al-Ghamdi, 259463

Mohammed Al-Alawi, 269380

Outline Introduction.

Types of data. Analog data.

Digital data.

Analog-to-Digital Converter (How it work). Sampling.

Resolution.

Inside Analog-to-Digital Converter. Parallel design (Flash ADC).

Digital-to-Analog Converter-based design.

Integrator-based design.

Sigma-Delta design.

Pipeline design.

Applications. Internet.

Audio CD.

Conclusion.

Introduction

Most of the data in our life are analog.

In computers, all what can be stored and dealt

with are digital data.

The solution was analog-to-digital converters.

Types of data

Analog data (All values on the time and amplitude are allowed).

Digital data (Only a few amplitude levels are allowed).

Analog-to-Digital Converter (How it work)

Sampling.

What the ADC circuit does is to take samples from the

analog signal from time to time. Each sample will be converted

into a number, based on its voltage level (as in the figure).

Analog-to-Digital Converter (How it work)

Resolution.

What the ADC does is to divide the “y” axis in “n” possible

parts between the maximum and the minimum values of the

original analog signal, and this “n” is given by the variable size. If

the variable size is too small, what will happen is that two

sampling points close to each other will have the same digital

representation, thus not corresponding exactly to the original

value found on the original analog signal, making the analog

waveform available at the DAC output to not have the best

quality.

Inside Analog-to-Digital Converter

Since Analog-to-Digital converters were invented, different

designs were made to fabricate them. The most five known

designs are:

Parallel design (Flash ADC).

Digital-to-Analog Converter-based design.

Integrator-based design.

Sigma-Delta design.

Pipeline design.

Parallel design (Flash ADC).

It works by comparing the input voltage of the analog signal to a reference voltage, which would be the maximum value achieved by the analog signal. For example, if the reference voltage is of 5 volts, this means that the peak of the analog signal would be 5 volts. On an 8-bit ADC when the input signal reached 5 volts we would find a 255 (11111111) value on the ADC output, i.e. the maximum possible value.

Digital-to-Analog Converter-based design.

There are few ways to design an analog-to-digital Converters using a DAC

as part of its circuit. We will present one of them: the ramp counter.

Vin is the analog input and Dn thru D0 are the digital outputs. The control

line found on the counter turns on the counter when it is low and stops the

counter when it is high.

The basic idea is to increase

the counter until the value

found on the counter matches

the value of the analog signal.

When this condition is met,

the value on the counter is the

digital equivalent of the

analog signal.

Integrator-based design. There are few ways of designing analog-to-digital converters using an

integrator. We will discuss one of them: the single-slope ADC.

We can see a single-slope ADC in the figure. We can notes that it is very similar to a ramp counter ADC, as it uses a counter, but instead of using a DAC, it uses a circuit called integrator, which is basically formed by a capacitor, a resistor and an operational amplifier. The MOSFET transistor makes the necessary control circuit.

The integrator produces a sawtooth waveform on its output, from zero to the maximum possible analog voltage to be sampled, set by -Vref. The minute the waveform is started, the counter starts counting from 0 to (2^n-1). When the voltage found at Vin is equal to the voltage achieved by the triangle waveform generated by the integrator, the control circuit captures the last value produced by the counter, which will be the digital correspondent of the analog sample being converted.

Sigma-Delta design.

The sigma-delta ADC – also called delta-sigma – uses a different

approach. We can divide it into two major blocks: analog modulator, which

takes the analog signal and converts it into a stream of bits, and digital

filter, which converts the serial stream from the modulator into a “usable”

digital number.

Pipeline design.

Pipeline ADC uses two or more steps. First, a coarse conversion is done. In

a second step, the difference to the input signal is determined with a digital

to analog converter (DAC). This difference is then converted finer, and the

results are combined in a last step. This type of ADC is fast, has a high

resolution and only requires a small die size.

Applications

Internet.

Internet network are connected using telephone networks, which

carry analog signals only. For that reason, a modem is required to convert the digital data in the computers into analog signals that can travel within the telephone network. Then reconverted in the destination into its original form (digital data). This modem is considered to be an ADC as a DAC.

1101... 1101...

Applications

Audio CD.

We know that music is actually sound waves (analog). So, to store

these analog data in a CD, we have to first convert them into digital storable data. Therefore, ADCs are used. In case of audio CD, a high sampling rate is used (44,100 Hz) to achieve a good sound resolution. So, when we play the audio CD, an inverse proceed is done. A DAC is used to reconvert the digital data stored in the CD back to its original format (analog data).

Conclusion.

In conclusion, we can see that ADCs play a major role in Computers Communications. The Internet network itself depends on the process of ADCs. Moreover, we saw how the process of ADC is done. In addition to that, we saw that there are many designs for ADCs. The most five known designs are the parallel design (flash ADC), the digital-to-analog converter-based design, the integrator-base design, the sigma-delta design and the pipeline design. All of them perform that same job but differ in their efficiency (speed & space storage).

DAC (DIGITAL TO ANALOG

CONVERTER)

A/D and D/A Conversion

Analog to Digital (A/D) Converter: converts a voltage signal to a binary encoded number that

is proportional to that voltage.

Digital to Analog (D/A) Converter: converts a binary encoded number to a voltage that is

proportional to that number.

We’ll assume the codes for the binary codes are signed or unsigned binary.

Many signals are easier to process via computer once they have been converted to number, i.e.

digital form.

•Analog-to-Digital Converter (ADC) converts an input analog

value to an output digital representation.

•This digital data is processed by a microprocessor and

output to a Digital-to-Analog Converter (DAC) the converts

an input binary value to an output voltage.

Continuous-Time vs. Discrete-Time Signals

•ADC (Analog-to-Digital Converter) –converts an

analog signal (voltage/current) to a digital value

•DAC (Digital-to-Analog Converter) –converts a digital

value to an analog value (voltage/current)

•Sample period –for ADC, time between each

conversion

–Typically, samples are taken at a fixed rate

•Vref (Reference Voltage) –analog signal varies

between 0 and Vref, or between +/-Vref

•Resolution –number of bits used for conversion (8

bits, 10 bits, 12 bits, 16 bits, etc).

•Conversion Time –the time it takes for a analog-to-

digital conversion

Vocabulary

Sampling Continuous-Time Signals

Types of D/A and A/D Converters

•D/A Converters

• Inverting Summer DAC

• R-2R DAC

• A/D Converters

• Successive Approximation ADC

• Flash ADC

Control logic use a counter to apply successive codes

0,1,2,3,4... to DAC (Digital-to-Analog Converter) until DAC

output is greater than Vin. This is SLOW, and have to allocate

the worst case time for each conversion, which is 2N clock

cycles for an N-bit ADC.

Initially set VDAC to ½ Vref, then see if Vin higher or lower tan

VDAC. If > ½ Vref, then next guess is between Vref and ½

Vref, else next guess is between ½ Vref and GND. Do this for

each bit of the ADC. Takes N clock cycles.

Binary Weight