Analog-to-Digital Converters

Lecture L10.5

Lab 11

Analog-to-Digital Converters

• Converts analog signals to digital signals– 8-bit: 0 – 255– 10-bit: 0 – 1023– 12-bit: 0 – 4095

• Successive Approximation

• Flash

Method of Successive Approximation

1111 1110 1101 1100 1011 1010 1001 1000 0111 0110 0101 0100 0011 0010 0001 00000V

5V

2.5V

3.75V

3.125V3.4375V Vin = 3.5V

step 1 step 2 step 3 step 4

voltage

1111 1110 1101 1100 1011 1010 1001 1000 0111 0110 0101 0100 0011 0010 0001 00000V

5V

2.5V

3.75V

3.125V3.4375V Vin = 3.5V

step 1 step 2 step 3 step 4

voltage

Control

D/A Converter

V

V

in

DA

Binary OutputC+

-

Implementing Successive Approximation

ADC08318-Bit Serial I/O A/D

Converter

Analog-to-Digital

Converter ADC0831

VCC CLK DO VREF

VIN (+) VIN (-)

GND

CS

Voltage Input +5 VDC

Xilinx XC95108 Interface

4.7 K

220 K

Figure 1. Analog-to-Digital Converter Circuit

1

2

4

3

8

7

5

6

      

  CLK

CS

DOTRI-

STATE 7 6 5 4 3 2 1 0TRI-

STATE(MSB) (LSB)

ADC0831 Timing

Xilinx XC95108 PC84 CPLD Clock Divider

Counter Q7..Q0

Shift RegisterS7..S0

Display RegisterD7..D0

Binary-to-BCDConverter

Hundreds Tens Units

0

Clock

Clock

Data

Load

Count Detect Logic

(Q7..Q4 = 10102)

7-SegmentDecoder

7-SegmentDecoder

0

4 MHz Clock

Q3

Q7Q6

Q3

Capture

7 7

VoltageDisplay

a..g a..g

dpt

1

CLK

CS

DO

ADC0831Interface

!Q3

Voltmeter Logic Block Diagram

Binary input: 101100101 shift left 1 01100101 shift left 10 1100101 shift left 101 100101 add 3 +11 : 1000 100101 shift left 1 0001 00101 shift left 10 0010 0101 shift left 100 0100 101 shift left 1000 1001 01 add 3 +11 : : 1011 1100 01 shift left 1 0111 1000 1 add 3 : +11 +11 : 1 1010 1011 1 shift left 011 0101 0111

hundreds tens units BCD: 3 5 7 Figure 4. Binary-to-BCD Conversion

C1

C2

C3

C7 C4

C8 C5

C9 C6

0 B8 B7 B6 B5 B4 B3 B2 B1 B0

P9 P8 P7 P6 P5 P4 P3 P2 P1 P0

0

P10

9-bit Binary Input

Hundreds Tens Units

BCD Output

Figure 5. 9-bit Binary-to-BCD Converter

Flash A/D Converter

• Uses analog comparators to convert an analog signal to a digital signal in a single clock cycle

• n-bit converter requires 2n – 1 comparators– 4-bit converter requires 15 comparators– 8-bit converter requires 255 comparators

• Or two 4-bit converters plus a D/A converter

MAX118

Flash A/D Converter

MAX118