DigitaltoAnalog Conversion

1
Digital-to-Analog Conversion
2
Op-Amp Review

• For Negative Feedback,
• 1. Inputs are high impedance
• 2. Input voltages ( and -) seek
  same potential
• 3. Voltage gain is Av -Rf / Rin

3
Definition

• Digital-to-Analog Converter -
• Circuit that utilizes digital information
  to generate an analog output. The output
  can take the form of a voltage or a current.

4
Binary Weighted DAC
VCC
100 kW
50 kW
25 kW
12.5 kW
10 kW
-
Vout

Max Vout -7.5 V
Min Vout 0.0 V
5
R/2R Ladder DAC
6
DAC Techniques

• Binary Weighted DAC - Uses switches
  precision resistors - Difficult to achieve
  high density due to large resistance
  values
• R/2R Ladder - Most common method -
  Keeps resistance values low - Uses
  intricate interconnections

7
DAC Specifications

• Resolution -
• Refers to the smallest subdivisions of the
  full-scale output that are possible given the
  number of bits in the input word. (aka step-size)

8
DAC Specifications

• Accuracy -
• A comparison of the actual output with the
  expected output. Generally specified as a
  percentage of the full scale output.

10 Vfs , 0.2 accuracy 20 mV
9
DAC Specifications
Vref The full scale (or maximum) output
voltage Vref- The minimum output voltage
10
DAC Specifications

• Speed -
• Output settling time. Time required for
  output to settle after an input word change.

11
DAC Example

• Given a 2-bit DAC with an 8 V full scale_at_ 0.2
  accuracy, find resolution andaccuracy in terms
  of voltage.

12
DAC Example
8 V
6 V
4 V
2 V
00
01
10
11
13
Analog-to-Digital Conversion
14
Definition

• Analog-to-Digital Converter -
• Circuit that converts an analog voltage into a
  digital value.

15
Analog to Digital Conversion Basics
16
(No Transcript)
17
Digital Ramp ADC
18
(No Transcript)
19
Successive Approximation ADC (SAC)
20
(No Transcript)
21
(No Transcript)
22
Flash ADC
8 V
Vin
5 V
Encoding
Q1
Q0
23
ADC Specifications

• Resolution -
• Also referred to as quantizing error. Error
  between actual analog value and its digital
  representation.

24
ADC Specifications

• Accuracy -
• A comparison of the actual output with the
  expected output. Same as for DAC.

25
ADC Specifications

• Conversion Time -
• Time required to digitize each sample.
  Function of conversion method and number of bits.

26
ADC Techniques

• Parallel Comparator (Flash) - Very
  high-speed conversion
• Dual Slope - Slow Speed, High Resolution
• Successive Approximation - High Speed, Medium
  Resolution

27
Sample and Hold

• Used when input signal is quicklyChanging
  (high frequency applications).

28
Sampling Rate

• Frequency of samples.(e.g. 44 kHz or every
  22.7 ms)

Each sample is quantizedinto a digital
number.(e.g. 12 bits)
29
ADC and DAC Conversion