Analog to Digital Converters (ADC)

1Technician Series

©Paul GodinCreated April 2008Last Edit Mar 2015ADC 1.1

Introduction

◊ Analog to digital conversion is an important aspect of digital electronics.

◊ ADCs allow the use of real-world values with the advantages of digital electronics.

◊ There are many examples of ADC converters used in everyday applications.

Name a few examples of ADC applications

ADC 1.2

Advantages of Digital Values

◊ Relatively less sensitive to distortion (noise and losses)◊ Can be reproduced much more accurately◊ Much easier to reconstruct a signal◊ More storage options◊ Can be processed mathematically and logically◊ Easier to standardize◊ Systems are easier to design (fewer voltage / current issues)◊ Digital systems can be made small (low current)◊ Display options

ADC 1.3

Challenges with ADC

◊ Converting an analog value to digital values comes with disadvantages:◊ It takes time to convert a signal from Analog to Digital,

and then to process that signal. May be too slow for some applications.

◊ Never 100% reproduction…always a series of discrete values.

◊ Requires more complex circuit design◊ More faithful reproduction requires more bit of

resolution.◊ Requires other circuit elements such as oscillators and

memory systems.

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ADC FUNDAMENTALS

ADC 1.5

Sampling

◊ Voltage signals are comprised of amplitude over time.

◊ The analog signal must be converted to its digital value at specific periods of time.

◊ Sampling is the process of taking a digital value at regular time intervals.

◊ The digital values must be stored if they are to be processed.

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Sampling

Time

AC Value

Digital Values at timed intervals

Sampling Pulses

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Sampling

◊ Increasing the number of binary values representing a voltage value improves its voltage resolution. This is called quantization. The greater the number of bits available, the greater the quantization level.

◊ Increasing the sampling frequency improves the time resolution. The more samples taken over time the more accurate the representation of the signal.

ADC 1.8

Nyquist

◊ The sampling frequency must be greater than the highest frequency component of the analog signal.

◊ The Nyquist frequency has a value of twice the highest analog frequency.

Where:◊ fsample is the sampling frequency

◊ fA(MAX) is the maximum analog frequency

)MAX(Asample f2f

ADC 1.9

Sampling Issues

AC Value

Digital Value

Properly Sampled Under-sampled

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Sampling Error (animated)

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Sampling Rates

◊ Sampling rates are selected based on:◊ application◊ requirements◊ standards

◊ As an example, an exterior thermometer needn’t be sampled at the same rate as an audio application.

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Audio Application of ADC

◊ When music is digitized for CDs the sampling frequency is 44.1 kHz (48 kHz for professional recording).

◊ According to the Nyquist frequency, 44.1kHz is acceptable for up to 22 kHz. Since most audio equipment functions at less than 20 kHz (and is at the upper limit of human hearing), the 44.1 kHz sampling rate is acceptable.

◊ The bit depth is 16 for CD audio (up to 24 for DVD-Audio).

ADC 1.13

Notes on mp3

◊ MP3 audio files refer to their quality as a bit rate. Typical mp3 bit rates are 128kbps and 192kbps (maximum is 320kbps according to standards).

◊ MP3 is an encoding format used to compress and reduce the file size. The file follows protocols and contains various elements such as headers, file information, the compressed data, bit rate type and other information.

◊ MP3s do not have a bit depth per say, more of a dynamic compression of data based on an analysis of the sound. The result is a high compression rate.

◊ For comparison, bit rate for uncompressed audio (CD) recording is 44.1kHz sampling x 16 bits x 2 channels (1411.2 kbps).

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Digitizing Voice

◊ Human voice for applications such as telephone conversations need not be sampled at a similar rate and bit depth as music.

◊ Typically, 8 bits at 8 kHz sampling rate is used (64kbps).

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ASYNCHRONOUS ADC

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Asynchronous ADC

◊ ADCs can be constructed from comparators.

◊ A comparator is an op amp configuration where the voltages of two inputs are compared. ◊ If the “+” input is greater than the “-” input, the output is

a logic high.VDD

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Comparator-Based ADC

2-bit “weighted” ADC

Analog In

VDD

Digital Out

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Flash ADC

3-bit Flash ADC

Analog In

VDD

Digital Out

Priority Encoder

Enable

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Flash ADC

◊ Flash ADCs are very fast and can convert data at high frequencies.

◊ The major disadvantage to flash ADCs is the complexity of the circuits. ◊ One op amp is required for each output value (minus one

for all zero). This means that:◊ an 8-bit Flash ADC requires 255 op amps◊ a 12-bit Flash ADC requires 4095 op amps◊ a 16-bit flash ADC requires 65,535 op amps

ADC 1.20

Hold/Store

◊ ADCs need to store measured values between the sampling pulses. ◊ Converting a stored digital signal back to analog requires a

similar clocking frequency (time needs to be reproduced). AD conversion represents a series of values at specific instances of time.

◊ The Sample and Hold creates the output “ladder” effect.

ADC 1.21

Flash ADC with Sample/Hold

3-bit Flash ADC

Analog In

VDD

Dig

ital O

ut

Priority Encoder

Enable

Dig

ital O

ut

Latch Circuit

Clock

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Basic ADC

◊ A basic ADC contains:◊ differential analog inputs (VREF) for

scaling◊ Analog signal input (VIN)◊ Output Enable for tristate-able outputs

(OE)◊ Start of Conversion input (SOC) to

trigger the analog signal read cycle.◊ End of Conversion output (EOC) to

indicate that the conversion is complete, the data is on the data bus and a new input may be applied.

◊ Digital output (D0~D7).

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Dual Slope ADC

◊ Also known as Counter-Ramp or Digital Ramp ADC

◊ A dual slope ADC is commonly used in measurement instruments (such as DVM’s).

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Dual Slope

ChargeCapacitor Cycle

Counter Cycle Counts from 0 to max

Discharge

Max Count / Restart Count

Count Reset

CountDisplay

CountDisplay

VReference

ADC 1.25

Dual Slope Function

◊ The Dual Slope ADC functions in this manner:◊ When an analog value is applied the capacitor begins to

charge in a linear manner and the oscillator passes to the counter.

◊ The counter continues to count until it reaches a predetermined value. Once this value is reached the count stops and the counter is reset. The control logic switches the input to the first comparator to a reference voltage, providing a discharge path for the capacitor.

◊ As the capacitor discharges the counter counts.◊ When the capacitor voltage reaches the reference

voltage the count stops and the value is stored in the register.

ADC 1.26

Dual Slope

◊ The Dual Slope method takes time for the conversion to occur. Each additional bit improves resolution but also adds a significant bit to the counter, costing considerable time. This type of ADC is therefore unsuitable for rapidly changing analog input.

◊ Each clocking pulse increments the counter by one. It takes (2N-1) clock cycles times the clock period for an output to be produced.

ADC 1.27

Dual Slope

◊ The Dual Slope method is accurate and requires less circuitry than other methods. Since it uses the same clock input for both phases of conversion, a drift in the clocking frequency will not affect the accuracy of the output.

◊ The Dual Slope is best suited for applications where the measured value is relatively stable such as DC voltage measurements.

ADC 1.28

Successive-Approximation ADC

◊ The Successive-Approximation ADC is one of the most popular types in use today. It has a relatively simple configuration and an excellent conversion rate.

◊ If an IC such as a microcontroller has an integrated ADC, it is typically this type.

ADC 1.29

Successive-Approximation ADC

Input

Digital Output

Oscillator

Control Logic

Approximation Register

Output Register

Digital to Analog

Converter

Voltage Comparator EOC

ADC 1.30

SOC

Successive-Approximation ADC◊ The SAC ADC functions in this manner:

◊ The approximation register is reset to all zero.◊ When a voltage is applied to the input the approximation

register’s most significant bit is changed from a 1 to a 0. The digital output of the register is converted back to analog through the DAC and is compared to the applied analog voltage. If the value is too low the 1 is left at the MSB. The next MSB is incremented, the output converted to analog and again compared to the analog input. Each bit is successively incremented and the output value compared.

◊ If the voltage from the DAC becomes higher than the applied analog value the bit is reset to 0 and the next MSB is incremented and compared.

◊ The process continues in this manner until the LSB value is reached. At the LSB, if the applied value makes the DAC output voltage higher the bit is reset to 0. The ADC has completed its process. It stores the value to the output register and provides an EOC output to indicate there is a value in the register.

ADC 1.31

SAC ADC Conversion Time

◊ Theoretically each step in the comparison process takes a clock edge. It therefore takes a SAC ADC approximately the same amount of clock edges as the number of bits it handles.

If a SAC ADC has an output of 8 bits and an input clocking frequency of 500 kHz, it takes approximately:

8•(1/500kHz) = 8•2µs=16µs

◊ In actual practice it may take more than one clock edge per step, but this is still faster than some other methods.

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The ADC08◊ The ADC08 family is a relatively popular SAC ADC.

D0 to D7 Digital Output-VIN

+VIN

GNDAnalog GNDDigital

ClkIN

ClkOUTRD

CS

Vref/2

INTR

WR

A D

VDD

ADC 1.33

ADC08

◊ +VIN and –VIN : Differential analog voltage.

◊ Vref/2 : Used to change the input voltage range. Normally at 2.5V when VDD = 5V, if 1.5V is applied the input range is 3.0 Volts and the resolution is changed accordingly.

◊ ClkIN: Input clock. External clocking edges can be provided to the ADC.

◊ ClkOUT: Output Clock. This ADC has an internal clocking circuit that requires external connection to an RC.

T = 1.1RCTypical values: 10kΩ & 150ρF

ADC 1.34

ADC08

◊ CS’ : Chip Select (input), tri-states the digital output for bus applications

◊ RD’: Read enable (input), enables the output from the Approximation register to the output register.

◊ WR’: Write enable (input), used to request the start of a new conversion.

◊ INTR: Interrupt, output high when the ADC is in the process of converting an input. Used to signal microprocessors or microcontrollers. Conversion time is approximately 100µs.

ADC 1.35

ADC08

◊ Questions:◊ What is the purpose of two grounds?

◊ How would the device be configured for an input of:◊ 0 to 5 Volts◊ 0 to 3 Volts◊ -2.5 to +2.5 Volts

◊ What is the purpose of the Vref/2 input?

ADC 1.36

SAC ADC

◊ The SAC ADC is a fast, accurate device.

◊ It has few disadvantages over other methods.◊ It requires a high clock frequency as each bit takes up an

edge.◊ It is not overly complex, and can be scaled more easily.

ADC 1.37

Other Methods of ADC

◊ Up/Down Ramp◊ Similar to the Ramp ADC but instead of starting the count

at zero for each measurement it increments the counter up or down based on the direction of the analog input. The time it takes for conversion is variable.

◊ Voltage-Frequency ADC◊ This device utilizes a Voltage Controlled Oscillator (VCO).

The higher the input voltage the higher the output frequency of the VCO. The ADC enables the output of the VCO for a specific period of time and the output of the oscillator is connected to a counter. The result of the count is supplied as the digital value.

ADC 1.38

Other Methods of ADC

◊ Sigma/Delta◊ Creates an internal bitstream based upon the difference

(Delta) between the analog input and a feedback comparator. The running total (Sigma) is reported as the digitized value. Operates at a high internal frequency.

See http://www.analog.com/Analog_Root/static/techSupport/designTools/interactiveTools/sdtutorial/sdtutorial.html for more information.

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Errors

◊ Analog to Digital systems have several possible sources of error.

◊ The errors can be the result of:◊ Quantization◊ Noise◊ Slew◊ Under-Sampling

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Quantization Error

◊ Quantization errors are a normal occurrence for ADCs. The input voltage will commonly have a value that is between the LSB voltage step and must be rounded up or down.

◊ For instance, if the resolution is 20mV per bit and a value of 10 mV is applied, the ADC will interpret this as either 0v or 20 mV, depending on the ADC design.

ADC 1.41

Quantization

◊ There are several additional quantization issues:

◊ If the total number of bits representing the amplitude of the signal is too small the quantization error increases.

◊ If the voltage applied to the ADC is not properly compensated then the full range of output values is not used.

ADC 1.42

Quantization and Sampling

◊ Sampling-Related Quantization Errors

◊ Relatively low sampling frequencies can lead to quantization errors

◊ If the sampling frequency is slightly different from the analog signal’s frequency (or a harmonic..a factor of the frequency), then the values may not indicate all the peaks and valleys, leading to other harmonics (also see aliasing).

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Not quite perfect...lower sample rate

Above: original signal with samplesBelow: imperfections in the digital output quantization

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Not quite perfect...higher sample rate

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Noise

◊ Noise is a factor in analog communications and it causes problems in ADC.

◊ The sources for noise are varied and can be somewhat complex.

◊ Sources include:◊ External Sources◊ Internal circuit sources◊ Aliasing

ADC 1.46

Aliasing

◊ Higher quality ADC (and DAC) will contain anti-aliasing filters to remove frequencies that are aliasing (“posing”) as signal.

◊ Aliasing occurs when additional frequencies are inadvertently produced in the AD process.

◊ For instance, if there are some frequencies present in the original signal that are higher than the Nyquist sample rate, a different lower frequency may appear on the conversion.

◊ Aliasing also occurs with adequate sampling frequencies.

ADC 1.47

Aliasing due to Inadequate Sampling Rate

ADC 1.48

Aliasing with an Adequate Sampling Rate

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Noise Sources

◊ Analog values are susceptible to noise (an advantage of digital electronics!). This noise causes problems for the AD conversion, as the noise will be included in the conversion process.

◊ There are external and internal sources of noise.◊ External: use filters and proper techniques to minimize

noise (such as decoupling, isolation, etc)◊ Internal: switching noise occurs with AD converters. Use

filters, decoupling and ground isolation. Thermal noise (aka white noise) is caused by a variety of reasons.◊ Other solutions include processing the signal (DSP),

injecting additional white noise to help average out the noise and ignoring the LSB.

ADC 1.50

Filter

◊ The analog input to an ADC should contain a low-pass filter to ensure high frequencies cannot pass and create aliasing problems.

◊ Advanced filtering techniques are also employed. These include DSP (Digital Signal Processing) to mathematically analyze and adjust the digital values.

ADC 1.51

Gain Error

◊ A Gain Error occurs when the ADC produces an output at a different quantization level than desired. The output is linear but the steps are either larger or smaller than they should be. The resolution is poor and there is the possibility of clipping (the binary number stops changing with a change in analog input). The voltage values will be inaccurately represented.

Red: IdealBlue: Error

Green: Error

Input Volts

Binary Code

ADC 1.52

Gain Error

Red: IdealBlue: Error

Green: Error

Typical Causes: •wrong VDD/VEE/Vreference voltage to the ADC•Improper Analog input voltage

ADC 1.53

Offset Error

◊ An Offset Error occurs when the output has the same voltage per step but the starting voltage is different.

Red: IdealBlue: Error

Green: Error

Input Volts

Binary Code

ADC 1.54

Offset Error

Red: IdealBlue: Error

Green: Error

Typical Causes: •Improper ground reference•Improper analog voltage range

ADC 1.55

Non-Linearity: Non-Monotonic

◊ A Monotonic Error occurs when the individual voltage steps are non-linear.

Red: IdealBlue: Error

Input Volts

Binary Code

ADC 1.56

Non-Linearity: Non-Monotonic

Red: IdealBlue: Error

Typical Causes: •defective ADC•wrong outputs measured•significant noise on ac input

ADC 1.57

Slew

◊ Slew represents the maximum rate of change of a signal. Slew rate is measured in V/t.

Black: ideal binary step from ADCGrey: binary step from ADC with slew

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Analog Sample & Hold

◊ The AD conversion process may be adversely affected when an analog signal changes voltage during conversion.

◊ Analog sample & hold circuits are able to capture the analog value and retain it, allowing the ADC to perform the conversion without input changes.

Analog In

Digitally controlled switch

StorageCapacitor

Analogto ADC

ADC 1.59

Specification Sheet

◊ View the specification sheet for the ADC08 from National Instruments.

◊ Definitions: ◊ Common-Mode noise is noise that occurs on both

conductors at the same time.◊ Ratiometric means measures the ratio, or takes ratio into

account when measuring.

ADC 1.60

DSP

◊ Digital Signal Processors perform operations on the digitized signals including:◊ removing interference, noise and other frequency and

transmission effects◊ encoding the values for transmission◊ reconstruct signals by enhancing or decreasing specific

frequencies

◊ DSPs are complex devices that rely on programming code (algorithms) to perform operations.

ADC 1.61

ADC-DAC with DSP

FilterFilter ADCADC DSPDSP

DACDACDSPDSP FilterFilter

ADC 1.62

DSP

◊ Analog signals contain noise. Noise can be filtered but sometimes the filtering process can eliminate wanted frequencies.

◊ The better means of removing noise is to take an average, as illustrated below as the red line.

Analog Signal with Noise

ADC 1.63

©Paul R. Godinprgodin°@ gmail.com

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