Chapter 3 ADC and DAC

Selecting The Antialias Filter

  • The Chebyshev optimizes the roll-off, the Butterworth optimizes the passband flatness, and the Bessel optimizes the step response.

  • 2 common ways for information to be encoded in an analog waveform, only two methods are common:

    • time domain encoding
      • time domain encoding uses the shape of the waveform to store information
        • Example: electrocardiogram of EKG, Images.
    • frequency domain encoding
      • the information is contained in sinusoidal waves that combine to form the signal
        • Example: Audio / Music
        • changes the phase of the various sinusoids, looks different on scope, but sounds the same
        • We need antialias filter with a sharp cutoff, such as a Chebyshev, Elliptic, or Butterworth.
        • Step response is not important.

Multirate Data Conversion

  • Digital recorder example:
    • Capture the frequencies between about 100 and 3000 hertz with 40 kHz noise
  • Analog solution:
    • 8-pole low-pass Chebyshev at 3 kHz, then sample at 8 kHz
    • the DAC reconstructs the analog signal at 8 kHz with a zeroth order hold.
  • Multirate techniques:
    • sample the data at 64 kHz
    • remove these unusable frequencies in software, by using a digital low-pass filter at 3 kHz
    • resample the digital signal from 64 kHz to 8 kHz by simply discarding every seven out of eight samples, a procedure called decimation.
    • 8 kHz data converter to 64 kHz with interpolation.
    • Everything above 3 kHz is then removed with a digital low-pass filter.
    • a simple RC network is all that is required to produce the final voice signal.
  • The advantages for multirate data conversion:
    • mass produced products
    • digital filters outperform analog filters by hundreds of times in key areas.

Single Bit Data Conversion

  • Delta modulation
    • Use a comparator, if input voltage is bigger than the voltage on the capacitor, output 1 and feedback to capacitor to increase the voltage
    • if input voltage is smaller than the voltage on the capacitor, output 0 and feedback to capacitor to decrease the voltage
  • Continuously Variable Slope Delta (CVSD) modulator
    • If detected 4 consecutive 0/1 increase the charging/ discharging rate.
    • At the receiver, the analog signal is reconstructed by incorporating a syllabic filter that is identical to the one in the transmission circuit.