Discuss the operation of weighted

resistor DAC circuit and R/2R DAC. Include figures and detailed explanation

with relevant applications.

A

digital to analog converter (DAC) is required to convert digital signals into

analog signals. A digital binary number is converted into an analog voltage or

current, giving an output voltage or current relative to the binary input. (Kuphaldt,

2000)

Block diagram of a DAC.

Digital

to analog converters are used in many applications, one of the most common uses

is for audio, in telephones and mobile phones. Digital information is converted

into audio signals. It is also used for speakers when playing music, headphones

etc.

Another

common application is display devices, mobile phones, televisions, HMI’s (Human

Machine Interface) where colours and shades are generated from digital video

signals. (Futureelectronics.com,

2018)

A

weighted resistor digital to analog converter uses a circuit similar to the

inverting summing op-amp circuit but with different resistor values instead of

equal. In a 4 input circuit, there will be five resistors, RF, R1, R2, R3 and

R4. Resistors 1 to 4 set at multiple powers of two, R1 = R, R2 = 2R, R3 = 4R

and R4 = 8R. Thus giving each digital signal exactly half the effect on the

output as the signal before it. With 4 binary signals there is a potential of

16 different analog output voltages.

(ElProCus –

Electronic Projects for Engineering Students, 2018)

Using

the formula;

Where V1, V2, V3 and V4 are the binary inputs all equal

to 10V the analog output voltages can be calculated. (Kuphaldt, 2000)

Inputs

Output

V1

V2

V3

V4

0

0

0

0

0V

0

0

0

1

1.25V

0

0

1

0

2.5V

0

0

1

1

3.75V

0

1

0

0

5V

0

1

0

1

6.25V

0

1

1

0

7.5V

0

1

1

1

8.75V

1

0

0

0

10V

1

0

0

1

11.25V

1

0

1

0

12.5V

1

0

1

1

13.75V

1

1

0

0

15V

1

1

0

1

16.25V

1

1

1

0

17.5V

1

1

1

1

18.75V

From

the above table we can see V1 = 10V, V2 = 5V, V3 = 2.5V and V4 = 1.25V. Showing

that the combination of the four binary inputs in this circuit can give a range

of different values from 0V – 18.75V.

With the weighted resistor digital to analog converter,

as the amount of digital binary signals increase, the amount of resistors

increases, and also the value of the resistors. This means that with a high

amount of digital binary signals a large range of resistors is required, this

can also reduce the accuracy of the circuit. (ElProCus – Electronic Projects for

Engineering Students, 2018). An advantage of the circuit is the low amount of

resistors required compared to other DACs, as only one resistor is required in

each network.

The R/2R DAC is an

alternative converter to the binary weighted resistor DAC which uses fewer

unique resistor values. Instead it uses a repeating structure of resistor

values R and 2R. This allows the circuit to be produced easier than the binary

weighted resistor, especially if the amount of digital binary signals is a

large number as the resistors required would all either be R or 2R instead of

R, 2R, 4R, 8R etc. however it does mean that there is an extra resistor

required in each network. The R/2R resistor could even be constructed only

using one value of resistor if desired, 2R can be made with 2 x R in series, or

R can be made with 2 x 2R in parallel. (Kuphaldt, 2000)

(ElProCus –

Electronic Projects for Engineering Students, 2018)

Discuss operation of

successive approximation ADC using examples, figures and applications of each.

The successive approximation analog to digital converter

is made up of four sub-circuits, sample and hold amplifier (SHA), analog

comparator, digital to analog converter (DAC) and successive approximation

register (SAR). (Analog.com,

2018)

The circuit uses the SAR to count by trying all the

values of the bits starting with the most significant bit (MSB) and finishing

with the least significant bit (LSB) if the DAC output is larger than the

analog input, then the bit that it is counting is switched off, if the DAC

output is smaller than the analog input then that bit is left switched on and

it moves onto the next most significant bit. It continues this process until

all of the bits within the successive approximation register have been tested. (Analog.com,

2018)

Successive approximation

ADCs are used heavily for data-acquisition systems, and was also first utilised

in pulse code modulation systems in the 1940s. Pulse code modulation systems

are used in digital audio recording systems, voice mails, telephones, radios,

digital video recording systems. (ElProCus – Electronic Projects for

Engineering Students, 2018) It is also

utilised in ultrasonic welding systems for visual displays, the power of the

system is digitally measured and converted to an analog output 0-10V, which can

then be monitored as a percentage where 0.1V = 1%.

The advantage of this circuits counting strategy is it

allows it to process the data quickly, allowing for faster results than other

analog to digital converters such as the digital ramp ADC circuit. This can be

seen by comparing the outputs of the successive approximation ADC and the

outputs of the digital ramp ADC circuit.

Digital ramp ADC circuit outputs.

The higher the input analog voltage, the longer the time

is between updates. Whereas the smaller the input analog voltage the shorter

the time is between updates.

By comparing the outputs of the successive approximation

ADC it can be noted that the outputs all occur at regular intervals thus giving

faster results.

Successive

approximation ADC outputs. (Kuphaldt, 2000)