Analog-DigitalELCTEC-131

Analog-to-Digital Conversion

Uses a circuit that converts an analog signal at its input to a digital code.

Called an A-to-D converter, A/D converter, or ADC.

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Unipolar ADC

Converts positive input voltages.

Generates a 2n-bit binary code for any given input voltage.

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Unipolar ADC Code Equation

Va = analog input voltage to be sampled.

FS = Full scale range of input voltage.

n = number of bits in the output code.

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n2 FSV code a ×=

Unipolar ADC Code Equation

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Unipolar ADC Output Codes

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Nominal Voltage of Input Step (volts) Range (volts) Output Code0.0 0.0 - 0.5 0001.0 0.5 - 1.5 0012.0 1.5 - 2.5 0103.0 2.5 - 3.5 0114.0 3.5 - 4.5 1005.0 4.5 - 5.5 1016.0 5.5 - 6.5 1107.0 6.5 - 8.0 111

Bipolar ADC (Offset Binary Coding)

Used to represent positive and negative input voltages.

Output code an unsigned binary number.

◦ Numbers below 0 V are negative.

◦ Numbers above 0 V are positive.

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Bipolar ADC (Offset Binary Coding)

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Bipolar ADC Code Equation

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222

FSV

offset 2 FSV code

a

a

nn

n

+

×=

+

×=

Bipolar ADC Output Codes

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- 4.0 - 4.0 to - 3.5 000- 3.0 - 3.5 to - 2.5 001- 2.0 - 2.5 to - 1.5 010- 1.0 - 1.5 to - 0.5 011

0 - 0.5 to + 0.5 100+ 1.0 + 0.5 to + 1.5 101+ 2.0 + 1.5 to + 2.5 110+ 3.0 + 2.5 to + 4.0 111

Bipolar ADC (2’s Complement Coding)

Uses a 2’s complement number system.

Most significant bit (MSB) is the sign bit.

◦ MSB = ‘0’ sign positive.

◦ MSB = ‘1’ sign negative.

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2’s Complement Output Codes

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Nominal Voltage of Input Step (volts) Range (volts) Output Code- 4.0 - 4.0 to - 3.5 100- 3.0 - 3.5 to - 2.5 101- 2.0 - 2.5 to - 1.5 110- 1.0 - 1.5 to - 0.5 111

0 - 0.5 to + 0.5 000+ 1.0 + 0.5 to + 1.5 001+ 2.0 + 1.5 to + 2.5 010+ 3.0 + 2.5 to + 4.0 011

Flash ADC

Uses a resistive voltage divider, comparators, and a priority encoder to produce a digital code.

Conversion occurs in one clock cycle (fastest conversion time).

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Flash ADC

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Flash ADC

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Disadvantage of Flash ADC

Requires 2n resistors and 2n – 1 comparators for an n-bit output.

For any large number of bits, the circuit becomes overly complex.

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Successive Approximation ADC

The most widely used ADC.

Finds the digital representation using a “binary search.”

Also called a SAR.

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Binary Search

1. Set the MSB of the digital representation to 1, all other bits to 0.

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Binary Search2. Compare the analog value produced in the

first step to the voltage being converted.

a) If the test voltage is higher than the voltage being converted, reset the MSB and set the second MSB.

b) If the test voltage is less than the voltage being converted, leave the MSB set and set the second MSB.

3. Repeat Steps 2, 2A, and 2B until all the bits have been tested.

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Binary Search

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Binary Search – 4

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Binary Search

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SAR - ADC Characteristics

Final answer is always less than the input voltage.

Conversion always requires a fixed number of clock cycles.

Conversion requires n clock cycles where n is the number of bits in the digital representation.

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Dual Slope ADC

Based on an integrator, a circuit whose output is the accumulated sum of all previous input values.

Circuit relies on storing charge representing current flow in a capacitor.

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Dual Slope ADC Characteristics

High accuracy.

Relatively slow conversion time.

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Dual Slope ADC Characteristics

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Dual Slope ADC Characteristics

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ADC0808 IC ADC

Successive approximation ADC.

Able to convert analog information from up to 8 (multiplexed) channels.

Can form the basis of a data acquisition network.

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ADC0808 IC ADC

START conversion with HIGH pulse.

Conversion process driven by the clock.

End-of-conversion indicated by a HIGH on EOC.

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ADC0808 IC ADC

Making OE HIGH allows the digital output to be read.

When OE inactive, outputs in Hi-Z state.

ADC0808 IC ADC

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