et438b-3.ppt 1

R-2R Binary Ladder DAC

Vref = 10 Vdc

R= 10k

2R = 20k

Req3Req2

Req1

Circuit analysis

et438b-3.ppt 2

R-2R ladder analysis (cont.)

10k 10kV=10 Vdc

20k20k20k20k

V1 V2 V3

I1 I2 I3

I4

I I’

Voltages and

currents

R-2R network produces binary weighted currents from

2 values of resistance

et438b-3.ppt 3

Example: Find the output voltage for the R-2R

DAC shown below. The digital input is 1102.

Vref = 5 Vdc

R= 15k

2R = 30k

Rf = 15k

IT

I0I1I2

Find currents I0, I1, I2 and use formula V0 = - ITRf

Req

et438b-3.ppt 4

Commerical DACs

DAC0800 Family

Devices used in practical designs use intergrated

R-2R networks and transistor switching. TTL

compatiable.

8-bit binary code converted to 256 levels of I0Full scale value set by reference current.

1 bit change produces change of 1/256 in I0

4

2

14

Iref

Equations: D = decimal equivalent of binary input

IV

Rref

ref

ref

= I ID

ref0256

=

I

V

Rfs

ref

ref

=

255

256

Full scale output

et438b-3.ppt 5

IC DAC Example: Use OP AMP to convert current

to voltage. The reference voltage is +10 Vdc and the

Reference resistance is 5kΩ. The value of Rf = 2.5kΩ

a.) Digital input 000011012b.) Digital input 100011012

et438b-3.ppt 6

IC DAC example (cont.)

input 100011012

et438b-3.ppt 7

Analog-to-digital Conversion

Convert continuous signals to

digital values

Requires 3 steps

1.) Sample analog signal

(Nyquist rate)

Need a minimum of 2 x max

frequency of interest

Higher sample rates, easier to

reconstruct signal

2.) Hold analog sample value while

conversion in progress

3.) Convert analog value to digital

value

et438b-3.ppt 8

Types of Analog-to-Digital

Converters

Integrating

• High accuracy

• Low speed

• low cost

Not used in data acquisition systems

Tracking (Counter)

• High speed in tracking mode

• Slow conversion time (some sub-types)

• susceptible to noise

Successive Approximation

• conversion time independent of value

“Flash” Parallel

Multicomparator type of conversion

• Highest speed

• HIgh cost

et438b-3.ppt 9

Counter Analog-to-Digital (A/D)

Converters

Clock

AND

C

O

U

N

T

E

R

D

A

C

+

-

When V+ = V-

V+

V-

AB

V

V

1.) Input a constant value. Requires a sample and hold

circuit. Not shown.

2.) AND gate passes clock signal when pt. A logic high

3.) Counter incremented by signal B

4.) DAC output increases as counter output increases

Digital output

et438b-3.ppt 10

Clock

AND

C

O

U

N

T

E

R

D

A

C

+

-

input V+

V-

AB

V

Counter-type A/D converter

5.) Counter stops when DAC V exceeds input V

input V

time

input

DAC output

Conversion time depends on the input level

Tracking A/D converters us up/down counters to

minimize conversion times

et438b-3.ppt 11

Successive Approximation A/D

Converters

Counter A/D converters conversion time proportional

to the input level

Improve conversion speed using a binary search

technique.

Procedure

1. Set MSB to 1

2. Test input, Vin, against DAC output, VDAC

3. If VDAC > Vin, reset bit to 0, else bit = 1 (VDAC < Vin)

4. Move to next bit and repeat steps 1 - 3

The input is converted to digital value in n

steps, where n = the number of bits in digital signal

et438b-3.ppt 12

Successive-Approximation A/D

Converters

et438b-3.ppt 13

Example: an 8-bit successive approximation ADC is

presented with a analog value that has a value of

143. Use the successive approximation algorithm

given previously to determine the binary value.

Assuming that each test takes a single clock cycle,

determine the maximum conversion time for the ADC

if it is clocked at 4.77 MHz

et438b-3.ppt 14

Example continued

et438b-3.ppt 15

Digital Input/Output Signals

(Digital I/O)

Boolean logic level 1 = logic high 0 = logic low

Boolean levels form basis of numbering system and

computer structure.

1 and 0’s bits

groups of bits 8-bits = byte

16-bits = 2 bytes = 1 word

All collections of bits are power of 2

Addressing bits in a byte

Address location0123456

Bit0110001

Weighted number system - conversion from binary

to decimal

n = 1x27+1x26+0x25+0x24+0x23+1x22+1x21+0x20

n = 1x128+1x64+1x4+1x2 = 198

7

1

198 decimal equivalent of binary number

et438b-3.ppt 16

Digital Hardware Standards

Specifies voltage levels of logic high and logic low

current output and input levels. Makes chips from

same family compatible with each other

Transistor-Transistor Logic (TTL)

Nominal 5 Vdc logic high and 0 Vdc logic low

Practical TTL output port characteristics

Source current 200 µASink current 1.7 mA

Logic 1 Threshold voltage V>= 2.4 Vdc

Load

Switch source of current

Load

+5 Switch sink of current

et438b-3.ppt 17

Simple TTL gate Sink and Source

Vcc = + 5 Vdc

RL

Vin

Rb

IL Vin = 5 V for Logic high and

0 V for logic low

IL flows to ground when

Vin = logic 1

Switch model of gate

Load

RL

IL

Vin

When Vin = 5 Vdc Vo = 0 V

When Vin = 0 Vdc Vo = 5 V

Vo

Vo

Logic Inverter

ILL

Vcc = + 5 Vdc

input output

1 0

0 1

Load will draw current went

Vin = 5 Vdc switch (Transistor) must

sink load current

et438b-3.ppt 18

Switch model of gate - current sourcing

IL

Vin

Vo

Load

ILL

Vcc = + 5 Vdc

When Vin is logic 1 load is de-activated (switch

closed)

When Vin is logic 0 load draws current from

source

With Transistors

current

sourcing

Vin

TTL Inverter

symbol

Load

ILLsourcing

current

Load

ILL

et438b-3.ppt 19

TTL Sinking LEDs

Output byte port feeding loads from inverters (buffering)

Resistor

used to

limit current

through

gate

When input is logic 1 (+5 Vdc) output is logic 0 (ground)

Limits of TTL buffers - 1.7 mA

For high current output - use discrete transistor

or relay (electromechanical or solid state)

et438b-3.ppt 20

Solid State Relay

Ac supplies only

Optically coupled device

AC

AC

+5 Vdc

Ground

Typical Application

M

Motor Load

1/4 HP 120 Vac

120 V

Grd

+5 Vdc

Output Bit

Output bit low motor is on

Output bit high motor is offMust remember output

logic level to get

desired action in device

et438b-3.ppt 21

Electromechanical Relays

To output

bit

+ 12 Vdc

Typical

NPN transistor

To high power

load AC or DC

diode

voltage

spike

protector

Relay coil

dc resistance

load to transistor

Resistor

sized to allow

TTL level to turn

on relay

Relay interface using TTL gate (open collector)

Lower current handling ability

+ 12 Vdc

Typical

To output

bit

LS7406 chip

To high power

loads AC or DC

Current limited

by the TTL

chip

et438b-3.ppt 22

Digital Input Interface

Limit current and change voltage levels

Using mechanical switches

+ 5 Vdc

2.2 k

Mechanical

switch

To

data

acquisition

board

Input port

constructed of TTL

chips

Resistor

sized to limit

current to

value

below port

sinking limit

Digital interface for a

byte of inputs.

+ 5 Vdc

All resistors sized to limit sink

current

Switches can be either

momentary contact or

toggle

et438b-3.ppt 23

Digital Interface

Non-TTL Levels

+24 Vdc

Optocoupler

2.2k Typical2.2k Typical

+ 5 Vdc

TTL inverter

gate (7404 or 7414)

To TTL

input

R

Switch closed - LED de-energized

Switch open - LED energized

LED and Optical transistor integrated on same chip

energizing LED causes optical transistor to conduct

Transistor off - inverter input +5 Vdc output 0 V

Transistor on - inverter input 0 Vdc output 5 V

et438b-3.ppt 24

Example: determine the logic levels and currents in

the digital interface circuit shown below

Assume that the optocoupler diode has an on-state

voltage drop of 1.4 V and the optical transistor has a

on-state drop of 0.4 V.

+24 Vdc

2.2k 2.2k

+ 5 Vdc

To TTL

input

R = 100

Logic

open switch = 0 = 0 V

close switch = 1 = 5 V

et438b-3.ppt 25

Example (cont.)

et438b-3.ppt 26

Relay Interface Example

+24Vdc

Rb2N3904

Q1

Relay

Coil model

500 Ω

10 mH

Size Rb such that

Q1 will activate with

a TTL input

Vs

Transistor Parameters

hfe = 200 (nominal)

Vce(sat) = 0.2 V

Vbe(sat) = 0.8 V

Ib

Ic

Assume transistor is in saturation and compute value of

Ic

+

+

--Vbe

Vce

et438b-3.ppt 27

Relay Interface Example (cont.)

+24Vdc

Rb2N3904

Q1

Relay

Coil model

500 Ω

10 mH

Vs

D1

Freewheeling

diode

Diode D1 provides a path for the current induced

when the transistor is switched off. If also clamps the

induced voltage to the forward drop of the diode.