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Here are some examples of problems where one must choose an appropriate analog to digital converter. Don't peek at the answers 'til you've jousted with the problem for a while. In some cases, hints are provided as "mouseovers." Answers used ADCs available in June, 2009.

1. Video digitization

Goal: decide what ADC to use with a CMOS array in a cell phone camera.

Analysis of the problem: An example high speed CMOS sensing chip is Cypress Electronics LUPA-1300. The full 48 page chip specification is on-line. Useful information extracted from the specification:

1280×1024 pixels, each 14 μm square.

Full well capacity: 62500 electrons.

Output at 60,000 electrons: 1 V.

Typical quantum efficiency across the visible spectrum: 10%.

Dark current and read noise: 45 e-/pixel

Recommended clock speed (pixel read rate): 40 MHz

Output read by 16 parallel amplifer channels (so each channel only looks at an 80×1024 subarray).

Questions and choices:

  1. How many bits of resolution are justified?
  2. How many conversions per second must each ADC perform?
  3. What type of ADC do you recommend be employed?
  4. For a SINGLE READ CYCLE, what is the signal-to-noise ratio of an intensity measurement?
  5. If we average all the data for 1 s, what is the precision of the mean of the measurement? Recall that the standard deviation of the mean is t s/N1/2 with N the number of data averaged. Use t for 95% confidence (you can find t tables on the web).

Go to next page for answers.



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