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3. Charge-coupled Array Detector

A charge-coupled array detector is to be used for obtaining atomic emission spectra. What readout ADC is appropriate?

Analysis of the problem: Because of the wide dynamic range required for analysis of trace components in the presence of high matrix concentrations, scientific CCDs tend to use large pixels, deep full-well depths, and slow readouts. An example is the E2V Technologies CCD42-90 (note: this is a PDF file even if the file name doesn't include the .pdf suffix). Superficially, it's not hugely different from the CMOS array example previously:

2048×4608 pixels, each 13.5 μm square.

Full well capacity: 100,000 electrons.

Output at 100,000 electrons: 0.45 V.

Typical quantum efficiency across the visible spectrum: 80%.

Dark current and read noise: 3 e-/pixel + 1 e-/pixel/hr if cooled to -100°C

Recommended clock speed (pixel read rate): 50 kHz - 3 MHz

Output read by 2 amplifer channels (so reading the entire array requires 95 s = 1 min, 35 s if each pixel is read at optimum frequency).

Questions and choices:

  1. Given the recommended readout time, what is the "optimum frequency" for readout?
  2. How many conversions per second must each ADC perform?
  3. How many bits of resolution are justified?
  4. What type of ADC do you recommend be employed?
  5. For a SINGLE READ CYCLE, what is the signal-to-noise ratio of an intensity measurement?
  6. Comparing this CCD to the CMOS detector in Problem 1, give an example of a measurement problem where the CCD is preferable, and an example of a measurement problem where the CMOS detector is preferable.

Go to next page for answers.



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