Analog to Digital Conversion[ADC]

mohammad iqbal� iqbal.umk@gmail.com

Teknik Elektro – Universitas Muria Kudus

Latihan: ADC

• Jika saya menggunakan ADC 10-bit dengan menggunakan nilai tegangan referensi 5 Volt1. Berapa kemungkinan nilai pada output ADC?2. Berapa mVolt resolusi (perbandingan input dan

output) dari ADC tersebut?3. Berapakah nilai output ADC, jika tegangan input

adalah a) 1 Voltb) 2 Voltc) 4 Volt

4. Jika diketahui frekuensi clock yg digunakan adalah 500kHz sedangkan waktu konversi adalah 70 kali clock, berapakah total waktu konversinya?

Latihan: ADC

• Jika saya menggunakan ADC 10-bit dengan menggunakan nilai tegangan referensi 5 Volt

1. kemungkinan nilai pada output ADC = 2n = 210 = 10242. resolusi (perbandingan input dan output) dari ADC =

Vrange/2n 2= 5/1024 = 4,883mV3. nilai output ADC, jika tegangan input adalah

a) 1 Volt � output ADC = 204 = 00 1100 1100 (biner)b) 2 Volt � output ADC = 409 = 01 1001 1001 (biner)c) 4 Volt � output ADC = 819 = 11 0011 0011 (biner)

4. Jika diketahui frekuensi clock yg digunakan adalah 500kHz sedangkan waktu konversi adalah 70 kali clock, maka total waktu konversinya = 70/f = 70/(500.103) = 140us

Analog Signals

Analog signals – directly measurable quantities in terms of some other quantity

Examples:• Thermometer – mercury height rises as

temperature rises• Car Speedometer – Needle moves farther

right as you accelerate• Stereo – Volume increases as you turn the

knob.

Digital Signals

Digital Signals – have only two states. For digital computers, we refer to binary states, 0 and 1. “1” can be on, “0” can be off.

Examples:• Light switch can be either on or off• Door to a room is either open or closed

Examples of A/D Applications

• Microphones - convert pressure waves in the air into varying electrical signals

• Strain Gages - resistance changes with applied strain

• Thermocouple - temperature measuring device converts thermal energy to electric energy

• Voltmeters• Digital Multimeters

Just what does an A/D converter DO?

• Converts analog signals into binary words

Analog � Digital Conversion 2-Step Process:

• Quantizing - breaking down analog value is a set of finite states

• Encoding - assigning a digital word or number to each state and matching it to the input signal

Step 1: Quantizing

Example: You have 0-10V signals. Separate them into a set of discrete states with 1.25V increments. (How did we get 1.25V? See next slide…)

Output States

Discrete Voltage Ranges (V)

0 0.00-1.25

1 1.25-2.50

2 2.50-3.75

3 3.75-5.00

4 5.00-6.25

5 6.25-7.50

6 7.50-8.75

7 8.75-10.0

Quantizing

The number of possible states that the converter can output is:

N=2n

where n is the number of bits in the AD converter

Example: For a 3 bit A/D converter, N=23=8.

Analog quantization size:Q=(Vmax-Vmin)/N = (10V – 0V)/8 = 1.25V

Encoding

• Here we assign the digital value (binary number) to each state for the computer to read.

Output States

Output Binary Equivalent

0 000

1 001

2 010

3 011

4 100

5 101

6 110

7 111

Accuracy of A/D Conversion

There are two ways to best improve accuracy of A/D conversion:

• increasing the resolution which improves the accuracy in measuring the amplitude of the analog signal.

• increasing the sampling rate which increases the maximum frequency that can be measured.

Resolution

• Resolution (number of discrete values the converter can produce) = Analog Quantization size (Q)(Q) = Vrange / 2n, where Vrange is the range of analog voltages which can be represented

• limited by signal-to-noise ratio (should be around 6dB)

• In our previous example: Q = 1.25V, if we used a 2-bit converter, then the resolution would be 10/2^2 = 2.50V.

Sampling Rate

Frequency at which ADC evaluates analog signal. As we see in the second picture, evaluating the signal more often more accurately depicts the ADC signal.

Aliasing

• Occurs when the input signal is changing much faster than the sample rate.

For example, a 2 kHz sine wave being sampled at 1.5 kHz would be reconstructed as a 500 Hz (the aliased signal) sine wave.

Nyquist Rule:• Use a sampling frequency at least twice as high

as the maximum frequency in the signal to avoid aliasing.

Overall Better Accuracy

• Increasing both the sampling rate and the resolution you can obtain better accuracy in your AD signals.

ADC Basic Principle

• The basic principle of operation is to use the comparator principle to determine whether or not to turn on a particular bit of the binary number output.

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

ADC Various Approaches

• 3 Basic Types

– Digital-Ramp ADC

– Successive Approximation ADC– Flash ADC

Digital-Ramp ADC

• Conversion from analog to digital form inherently involves comparator action where the value of the analog voltage at some point in time is compared with some standard.

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

Digital-Ramp ADC

Digital-Ramp ADC

• The output of the DAC is applied to the other terminal of the comparator.

• Since the output of the DAC is increasing with the counter, it will trigger the comparator at some point when its voltage exceeds the analog input.

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

Successive approximation ADC

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

Successive approximation ADC

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

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

Successive Approximation Example

• 10 bit resolution or 0.0009765625V of Vref

• Vin= .6 volts• Vref=1volts• Find the digital value of

Vin

Successive Approximation

• MSB (bit 9)– Divided Vref by 2– Compare Vref /2 with Vin

– If Vin is greater than Vref /2 , turn MSB on (1)– If Vin is less than Vref /2 , turn MSB off (0)– Vin =0.6V and V=0.5

– Since Vin>V, MSB = 1 (on)

Successive Approximation

• Next Calculate MSB-1 (bit 8)– Compare Vin=0.6 V to V=Vref/2 + Vref/4= 0.5+0.25 =0.75V– Since 0.6<0.75, MSB is turned off

• Calculate MSB-2 (bit 7)– Go back to the last voltage that caused it to be turned on

(Bit 9) and add it to Vref/8, and compare with Vin

– Compare Vin with (0.5+Vref/8)=0.625– Since 0.6<0.625, MSB is turned off

Successive Approximation

• Calculate the state of MSB-3 (bit 6)– Go to the last bit that caused it to be turned on

(In this case MSB-1) and add it to Vref/16, and compare it to Vin

– Compare Vin to V= 0.5 + Vref/16= 0.5625

– Since 0.6>0.5625, MSB-3=1 (turned on)

Successive Approximation• This process continues for all the

remaining bits.

Flash ADC

• It is the fastest type of ADC available, but requires a comparator for each value of output.(63 for 6-bit, 255 for 8-bit, etc.)

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

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

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

Flash ADC

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

ADC080x, 8-Bit µP Compatible A/D Converters

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

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

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

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

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

ADC080x Features• Compatible with 8080 µP

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

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

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

MOS and TTL voltage level specifications

• Works with 2.5V (LM336) voltage reference

• On-chip clock generator• 0V to 5V analog input voltage range

with single 5V supply• No zero adjust required

ADC080x, interfacing