Digital-to-AnalogDigital-to-AnalogAnalog-to-Digital Analog-to-Digital convertersconverters

H.SUDHEERH.SUDHEER

introductionintroduction most of real time quantities are analog in nature most of real time quantities are analog in nature

like Voltage, current, teparature. etc.like Voltage, current, teparature. etc. NEED OF A-D conversion NEED OF A-D conversion It is difficult to process, store and transmit the It is difficult to process, store and transmit the

analog signal without introducing a considerable analog signal without introducing a considerable error because of superimposition of noise in case error because of superimposition of noise in case of amplitude modulationof amplitude modulation

So for the above purposes it is convenient to So for the above purposes it is convenient to express variable in digital form which gives better express variable in digital form which gives better accuracy and reduces noiseaccuracy and reduces noise

Typical applications of A/D and D/A Typical applications of A/D and D/A converterconverter

- - Digital to Analog Conver- - Digital to Analog Conversionsion [DAC] [DAC]

- - Digital to Analog Conversion- - Digital to Analog Conversion

When data is in binary form, the 0's and When data is in binary form, the 0's and 1's may be of several forms such as the 1's may be of several forms such as the TTL form where the logic zero may be a TTL form where the logic zero may be a value up to 0.8 volts and the 1 may be a value up to 0.8 volts and the 1 may be a voltage from 2 to 5 volts.voltage from 2 to 5 volts.

The data can be converted to clean The data can be converted to clean digital form using gates which are digital form using gates which are designed to be on or off depending on designed to be on or off depending on the value of the incoming signal.the value of the incoming signal.

- - Digital to Analog Conversion- - Digital to Analog Conversion

Data in clean binary digital form can be Data in clean binary digital form can be converted to an analog form by using a converted to an analog form by using a summing amplifier.summing amplifier.

For example, a simple 4-bit D/A For example, a simple 4-bit D/A converter can be made with a four-input converter can be made with a four-input summing amplifier.summing amplifier.

- - Digital to Analog Conversion- - Digital to Analog Conversion

2 Basic Approaches2 Basic Approaches

Weighted Summing AmplifierWeighted Summing Amplifier R-2R Network Approach R-2R Network Approach

Weighted Sum DACWeighted Sum DAC One way to achieve D/A conversion is to One way to achieve D/A conversion is to

use a summing amplifier. use a summing amplifier. This approach is not satisfactory for a This approach is not satisfactory for a

large number of bits because it requires large number of bits because it requires too much precision in the summing too much precision in the summing resistors.resistors.

This problem is overcome in the R-2R This problem is overcome in the R-2R network DAC. network DAC.

Weighted Sum DACWeighted Sum DAC

Simple weighted DACSimple weighted DAC

R-2R Ladder DACR-2R Ladder DAC

R-2R Ladder DACR-2R Ladder DAC

R-2R Ladder DACR-2R Ladder DAC The summing amplifier with the R-2R ladder of The summing amplifier with the R-2R ladder of

resistances shown produces the output where resistances shown produces the output where the D's take the value 0 or 1.the D's take the value 0 or 1.

The digital inputs could be TTL voltages which The digital inputs could be TTL voltages which close the switches on a logical 1 and leave it close the switches on a logical 1 and leave it grounded for a logical 0.grounded for a logical 0.

This is illustrated for 4 bits, but can be This is illustrated for 4 bits, but can be extended to any number with just the extended to any number with just the resistance values R and 2R.resistance values R and 2R.

DAC0830/DAC0832DAC0830/DAC08328-Bit µP Compatible DAC8-Bit µP Compatible DAC

An advanced CMOS/Si-Cr 8-bit multiplying DAC An advanced CMOS/Si-Cr 8-bit multiplying DAC designed to interface directly with the 8080, 8048, designed to interface directly with the 8080, 8048, 8085, Z80®, and other popular microprocessors.8085, Z80®, and other popular microprocessors.

A deposited silicon-chromium R-2R resistor ladder A deposited silicon-chromium R-2R resistor ladder network divides the reference current and provides the network divides the reference current and provides the circuit with excellent temperature tracking circuit with excellent temperature tracking characteristics (0.05% of Full Scale Range maximum characteristics (0.05% of Full Scale Range maximum linearity error over temperature).linearity error over temperature).

Typical ApplicationTypical Application

Analog to Digital Analog to Digital ConversionConversion [ADC][ADC]

ADC Basic PrincipleADC Basic Principle The basic principle of operation is to use The basic principle of operation is to use

the comparator principle to determine the comparator principle to determine whether or not to turn on a particular bit whether or not to turn on a particular bit of the binary number output.of the binary number output.

It is typical for an ADC to use a digital-to-It is typical for an ADC to use a digital-to-analog converter (DAC) to determine one analog converter (DAC) to determine one of the inputs to the comparator. of the inputs to the comparator.

ADC Various ApproachesADC Various Approaches 3 Basic Types3 Basic Types

Digital-Ramp ADCDigital-Ramp ADC Successive Approximation ADCSuccessive Approximation ADC Flash ADC Flash ADC

Digital-Ramp ADCDigital-Ramp ADC Conversion from analog to digital form Conversion from analog to digital form

inherently involves comparator action inherently involves comparator action where the value of the analog voltage at where the value of the analog voltage at some point in time is compared with some point in time is compared with some standard.some standard.

A common way to do that is to apply the A common way to do that is to apply the analog voltage to one terminal of a analog voltage to one terminal of a comparator and trigger a binary counter comparator and trigger a binary counter which drives a DAC.which drives a DAC.

Digital-Ramp ADCDigital-Ramp ADC

Digital-Ramp ADCDigital-Ramp ADC The output of the DAC is applied to the The output of the DAC is applied to the

other terminal of the comparator.other terminal of the comparator. Since the output of the DAC is increasing Since the output of the DAC is increasing

with the counter, it will trigger the with the counter, it will trigger the comparator at some point when its comparator at some point when its voltage exceeds the analog input.voltage exceeds the analog input.

The transition of the comparator stops The transition of the comparator stops the binary counter, which at that point the binary counter, which at that point holds the digital value corresponding to holds the digital value corresponding to the analog voltage.the analog voltage.

Successive Successive approximation ADCapproximation ADC

Illustration of 4-bit SAC with 1 volt step size Illustration of 4-bit SAC with 1 volt step size

Successive Successive approximation ADCapproximation ADC

Much faster than the Much faster than the digital ramp ADC digital ramp ADC because it uses because it uses digital logic to digital logic to converge on the converge on the value closest to the value closest to the input voltage.input voltage.

A comparator and a A comparator and a DAC are used in the DAC are used in the process.process.

Flash ADCFlash ADC It is the fastest type of ADC It is the fastest type of ADC

available, but requires a comparator available, but requires a comparator for each value of output.for each value of output.(63 for 6-bit, 255 for 8-bit, etc.)(63 for 6-bit, 255 for 8-bit, etc.)

Such ADCs are available in IC form Such ADCs are available in IC form up to 8-bit and 10-bit flash ADCs up to 8-bit and 10-bit flash ADCs (1023 comparators) are planned. (1023 comparators) are planned.

The encoder logic executes a truth The encoder logic executes a truth table to convert the ladder of inputs table to convert the ladder of inputs to the binary number output.to the binary number output.

Illustrated is a 3-bit flash ADC with resolution 1 voltIllustrated is a 3-bit flash ADC with resolution 1 volt

Flash ADCFlash ADC The resistor net and comparators provide The resistor net and comparators provide

an input to the combinational logic circuit, an input to the combinational logic circuit, so the conversion time is just the so the conversion time is just the propagation delay through the network - propagation delay through the network - it is not limited by the clock rate or some it is not limited by the clock rate or some convergence sequence.convergence sequence.

ADC080x, 8-Bit µP Compatible ADC080x, 8-Bit µP Compatible A/D ConvertersA/D Converters

CMOS 8-bit successive approximation A/D converters CMOS 8-bit successive approximation A/D converters that use a differential potentiometer ladder—similar to that use a differential potentiometer ladder—similar to the 256R products.the 256R products.

These converters are designed to allow operation with These converters are designed to allow operation with the NSC800 and INS8080A derivative control bus with the NSC800 and INS8080A derivative control bus with TRI-STATE output latches directly driving the data bus.TRI-STATE output latches directly driving the data bus.

These A/Ds appear like memory locations or I/O ports to These A/Ds appear like memory locations or I/O ports to the microprocessor and no interfacing logic is needed.the microprocessor and no interfacing logic is needed.

Differential analog voltage inputs allow increasing the Differential analog voltage inputs allow increasing the common-mode rejection and offsetting the analog zero common-mode rejection and offsetting the analog zero input voltage value.input voltage value.

In addition, the voltage reference input can be adjusted In addition, the voltage reference input can be adjusted to allow encoding any smaller analog voltage span to to allow encoding any smaller analog voltage span to the full 8 bits of resolution.the full 8 bits of resolution.

ADC080x FeaturesADC080x Features Compatible with 8080 µP Compatible with 8080 µP

derivatives—no interfacing logic derivatives—no interfacing logic needed needed - - access time access time - - 135 ns135 ns

Easy interface to all Easy interface to all microprocessors, or operates microprocessors, or operates “stand alone”“stand alone”

Differential analog voltage inputsDifferential analog voltage inputs Logic inputs and outputs meet both Logic inputs and outputs meet both

MOS and TTL voltage level MOS and TTL voltage level specificationsspecifications

Works with 2.5V Works with 2.5V ((LM336LM336) ) voltage voltage referencereference

OnOn--chip clock generatorchip clock generator 0V to 5V analog input voltage range 0V to 5V analog input voltage range

with single 5V supplywith single 5V supply No zero adjust requiredNo zero adjust required

ADC080x, interfacingADC080x, interfacing

PORT, DEVPORT, DEV00 74C374 00 74C374 01 A/D 101 A/D 102 A/D 202 A/D 203 A/D 303 A/D 304 A/D 404 A/D 405 A/D 505 A/D 506 A/D 606 A/D 607 A/D 707 A/D 7

Q & AQ & A

That’s all for this time.That’s all for this time.