In practice, an ADC is usually in form of an integrated circuit (IC). ADC0808 and ADC0809 are two typical examples of 8-bit ADC with 8-channel multiplexer using successive approximation method for its conversion.

ADC0809National Semiconductor

-Vref

Start Clock

End of Conversion

8-bit OutputOutputLatchBuffer

OutputEnable

256R Resistor Ladder

S.A.R.

8-bit ADC

Control & Timing

Switch Tree

Comparator

8 ChannelsMultiplexing

Switches

AddressLatch andDecoder

8 Analog Inputs

3-bit Address

Address LatchEnable

VCC GND +Vref

Block Diagram

When this ADC is connected to a computer, the sequence of operation is listed below:

1. The computer reads the EOC to check the ADC is busy or not.2. If the ADC is not busy when the computer selects the input

channel and send out the “Start” signal. Otherwise, step (1) is repeated.

3. The computer monitors the EOC.4. When the EOC is activated, the computer reads the digital

output.

When there is more than one ADCs being linked to the computer, they can be connected in parallel. Using the ‘output enable’ can do the selection of ADC output.

• Used when a continuous analog signal is required.

• Signal from DAC can be smoothed by a Low pass filter

Digital to Analog ConvertersCommon Applications

0 bit

nth bit

n bit DAC011010010101010100101101010101011111100101000010101010111110011010101010101010101010111010101011110011000100101010101010001111

Digital Input

Filter

Piece-wise Continuous Output

Analog Continuous Output

DAC Performance SpecificationsDAC Performance Specifications

• Resolution• Reference Voltages• Settling Time• Linearity• Speed• Errors

• Resolution: is the amount of variance in output voltage for every change of the LSB in the digital input.

• How closely can we approximate the desired output

• A common DAC has a 8 - 12 bit Resolution

ResolutionResolution

NLSBVV2

Resolution Ref N = Number of bits

Example: Calculate the resolution of an 8-bit DAC.

Solution: Resolution = 8 bits

Percentage resolution =

%391.0%1002561%100

21

8

ResolutionResolution

Better Resolution(3 bit)Poor Resolution(1 bit)

Vout

Desired Analog signal

Approximate output

2 V

olt.

Leve

ls

Digital Input0 0

1

Digital Input

Vout

Desired Analog signal

Approximate output

8 V

olt.

Leve

ls

000

001

010

011

100

101

110

111110

101

100

011

010

001

000

• Reference Voltage: A specified voltage used to determine how each digital input will be assigned to each voltage division.

• Types:– Non-multiplier: internal, fixed, and defined by

manufacturer– Multiplier: external, variable, user specified

Reference VoltageReference Voltage

Reference VoltageReference Voltage

Assume 2 bit DAC

Non-Multiplier: (Vref = C)

Digital Input

Multiplier: (Vref = Asin(wt))

43C

2C

4C

0

Voltage

00

01 01

00

10 10

11

0

Voltage

Digital Input

43A

2A

4A

00 00

01 01

10 10

11

• Settling Time: The time required for the input signal voltage to settle to the expected output voltage(within +/- VLSB).

• Any change in the input state will not be reflected in the output state immediately. There is a time lag, between the two events.

Settling TimeSettling Time

Settling TimeSettling Time

Analog Output Voltage

Expected Voltage

+VLSB

-VLSB

Settling time Time

• Linearity: is the difference between the desired analog output and the actual output over the full range of expected values.

• Ideally, a DAC should produce a linear relationship between a digital input and the analog output, this is not always the case.

LinearityLinearity

Digital to Analog Converters-Performance Specifications

--LinearityLinearity

Linearity(Ideal Case)

Digital Input

Perfect Agreement

Desired/Approximate Output

Ana

log

Out

put V

olta

ge

NON-Linearity(Real World)

Ana

log

Out

put V

olta

ge

Digital Input

Desired Output

Miss-alignment

Approximate output

• Speed: Rate of conversion of a single digital input to its analog equivalent

• Conversion Rate – Depends on clock speed of input signal– Depends on settling time of converter

SpeedSpeed

• Non-linearity– Differential– Integral

• Gain• Offset

ErrorsErrors

• Generic use• Circuit Components• Digital Audio • Function Generators/Oscilloscopes• Motor Controllers

Digital to Analog Converters-Common ApplicationsCommon Applications

• Voltage controlled Amplifier– digital input, External Reference Voltage as control

• Digitally operated attenuator– External Reference Voltage as input, digital control

• Programmable Filters– Digitally controlled cutoff frequencies

Digital to Analog Converters-Common Applications

--Circuit ComponentsCircuit Components

• CD Players• MP3 Players• Digital Telephone/Answering Machines

Digital to Analog Converters-Common Applications

--Digital AudioDigital Audio

1 2 3

Digital to Analog Converters-Common Applications

--Function GeneratorsFunction Generators

• Digital Oscilloscopes– Digital Input– Analog Ouput

• Signal Generators– Sine wave generation– Square wave generation– Triangle wave generation– Random noise generation

1 2

• Cruise Control• Valve Control • Motor Control

Motor ControllersMotor Controllers

1 2 3

Summary Selection of ADCThe parameters used in selecting an ADC are very similar to those considered for a DAC selection.

• Error/Accuracy: Quantizing error represents the difference between an actual analog value and its digital representation. Ideally, the quantizing error should not be greater than ± ½LSB.

• Resolution: V to cause 1 bit change in output• Output Voltage Range Input Voltage Range• Output Settling Time Conversion Time• Output Coding (usually binary)

Binary-coded decimal, or BCD, is a method of using binary digits to represent the decimal digits 0 through 9. A decimal digit is represented by four binary digits, as shown below:

BCD Decimal 0000 0 0001 1 0010 2 0011 3 0100 4 0101 5 0110 6 0111 7 1000 8 1001 9

It should be noted in the table above that the BCD coding is thebinary equivalent of the decimal digit. However, BCD and binary are not the same.

For example,

4910 in binary is 1100012,

but 4910 in BCD is 01001001BCD.

Each decimal digit is converted to its binary equivalent.

To measure an AC voltage at a particular instant in time, it is necessary to sample the waveform with a ‘sample and hold’(S/H) circuit.

Hold

Sample

Input Output to

ADC

Sampling• A continuous signal y(t) could be represented by a

set of samples yi, i = 1, . . . , N, taken at discrete intervals of time ΔT (sampling interval)

• The switch is closed fS times per second, where sampling frequency fS = 1/(ΔT )

Sampling

• Nyquist sampling theorem: A continuous signal can be represented by, and reconstituted from, a set of sample values provided that the number of samples per second is at least twice the highest frequency present in the signal

fS ≥ 2fMAX• If f < 2fMAX, the sampling components occupy the same

frequency range as the original signal and it is impossible to filter them out and reconstitute the signal

• Aliasing error occur when the original signal is sampled at sampling rate below the Nyquist rate

Sampling• Here a sine wave of period 1 s, i.e. frequency 1 Hz, is being sampled

approximately once every second, i.e. the sampling frequency is below the Nyquist minimum of 2 samples/second. The diagram shows that it is possible to reconstruct an entirely different sine wave of far lower frequency from the sample values. This is referred to as the ‘alias’ of the original signal;

Sample-and-Hold Circuit• The operation of analog-to-digital conversion can take up to a

few milliseconds; it is necessary therefore to hold the output of the sampler constant at the sampled value while the conversion takes place.

• This is done using a sample-and-hold device• In the sample state the output signal follows the input signal;

in the hold state the output signal is held constant at the value of the input signal at the instant of time the hold command is sent

Sample-and-Hold Circuit