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EET260: A/D and D/A converters

EET260: A/D and D/A converters. A/D conversion. We will consider process and circuits required to convert an analog waveform into a digital output. 0110 0111 1000 1010 1100 1110 1111 1111 1111 1110 1100 1010 0111 0101 0011. Analog-to-digital conversion. A/D conversion.

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EET260: A/D and D/A converters

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  1. EET260: A/D and D/A converters

  2. A/D conversion • We will consider process and circuits required to convert an analog waveform into a digital output. 0110 0111 1000 1010 1100 1110 1111 1111 1111 1110 1100 1010 0111 0101 0011 Analog-to-digital conversion

  3. A/D conversion • A modern A/D converter is a single-chip IC which performs the following processes. [ADC IC picture] 0110 0111 1000 1010 1100 1110 1111 1111 1111 1110 1100 1010 0111 0101 0011 Anti-aliasing filter Sample-and-hold(S/H) circuit A/D converter

  4. Sampling and aliasing • Sampling is basically a multiplication process, and it gives rise to sidebands just like AM modulation does.  analog input sampled output sampling pulses

  5. Sampling and aliasing (frequency) • The frequency spectrum resulting from sampling looks like: • Decreasing the sampling frequency ( fs), we get: • If we continue to decrease fs to a value less than the Nyquist rate, aliasing (overlapping of the spectra) occurs: We get a carrier and upper and lower sidebands at multiples of the sampling frequency: fs, 2fs, 3fs ,etc.

  6. Sampling and aliasing • To eliminate the problem, a low-pass filter called an anti-aliasing filter is included at the input of the A/D converter to block any frequencies higher than one-half the sampling frequency. Anti-aliasing filter Sample-and-hold(S/H) circuit A/D converter

  7. Sample-and-hold circuit • A/D conversion begins with sampling, which is carried out by a sample-and-hold (S/H) circuit. • The S/H circuit takes a precise measurement of the analog voltage at specified intervals. Anti-aliasing filter Sample-and-hold(S/H) circuit A/D converter

  8. Sample-and-hold circuit • A sample-and-hold (S/H) circuit accepts the analog input signal and passes it through, unchanged, during its sampling mode. In the hold mode, the amplifier remembers or memorizes a particular voltage level at the instant of sampling. The output is a fixed DC level whose amplitude is the value at the sampling time.

  9. Conversion • The constant S/H output during the sampling interval permits accurate quantization. • The last step is the conversion from an analog voltage into a binary number. • Two common converters are: • Successive-approximation converter • Flash converter Anti-aliasing filter Sample-and-hold(S/H) circuit A/D converter

  10. Comparator • Comparators are found in both types of A/D converters. • A comparator compares two analog inputs and produces a binary output. +Vcc= +5 V A + C  B -Vcc= 0 V

  11. Successive-approximation converter • This converter contains an 8-bit successive-approximations register (SAR).

  12. Successive-approximation converter • Special logic in the register causes each bit to be turned on one at a time from MSB to LSB until the closest binary value is stored in the register. • At each clock cycle, a comparison is made. • If the D/A output is greater than the analog input, that bit is turned off (set to 0) • If the D/A output is less than the analog input, that bit is left on (set to 1). • Process repeats until 8 bits are checked.

  13. Successive-approximation converter • If the clock frequency is 200-kHz, how long does it take to complete the conversion for an 8-bit D/A converter?

  14. Successive-approximation converter • Successive-approximation converters are fast and consistent. • Conversion times range from 0.25 to 200 ms and 8-, 10-, 12-, and 16-bit versions are available.

  15. Flash converter • A flash converter uses a large resistive voltage divider and multiple analog comparators.

  16. Flash converter • The encoder logic circuit converts the 7-bit input from the comparators into a 3-bit binary output. • The flash converter produces an output as fast as the comparators can switch and the signals can be translated to binary levels by the logic circuits. • Flash converters are the fastest type of A/D converter.

  17. Flash converter • The number of comparators is equal to 2N – 1, where N is the number of desired output bits.

  18. Flash converter • Flash A/D converters are complicated and expensive but are the best choice for high-speed conversions. • Conversion speeds < 100 ns are typical. • Speed less than 0.5 ns are possible. • 6-, 8-, 10-bit flash converters are available.

  19. Digital-to-analog (D/A) conversion • We will consider a circuit which convert a digital signal into and analog output. D/Aconversion Analog-to-digital conversion

  20. Digital-to-analog (D/A) conversion • One of the most popular D/A circuits is the R-2R ladder shown below

  21. Vo b0 b1 b2 b3 Vref R-2R ladder DAC • The output voltage is given .

  22. Vo b0 b1 b2 b3 Vref Example Problem 1 Assume the R-2R ladder DAC circuit below as the following values: R = 50 k and Rf = 100 k. Assume Vref= 1 V. Determine the D/A converter output for the following binary inputs. a. 00002 b. 00012 c. 01012 d. 11112 e. What is the resolution of this D/A converter.

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