A novel current-based CCD clock driver

Abstract

Charge-Coupled Device (CCD) detectors are widely used in astronomy for generating measurable electrical charge from incident photons. The CCD reading process involves a charge transfer stage, in which the charge collected in each pixel is transferred to the output amplifiers. This process is performed by applying a set of clock waveforms to each pixel electrode of the detector. Parameters signals such as voltage excursion, rise/fall times and rise/fall rates, are related directly to the performance of the charge transfer process. Due to the capacitive nature of CCD pixels, the existing clock drivers are not the best suited for the task, because the waveform control is performed by a voltage amplifier. This architecture does not allow to accurately set the rise/fall rate of the voltage signal, since its settling always follows the dynamic response of the amplifier. This paper studies the use of a current-based CCD clock driver implemented through the improved Howland current source (IHCS). This idea takes advantage of the capacitive impedance of CCD pixels, and allows one to accurately set the rise/fall rate of the CCD driving signals, and in consequence, improve the performance of the charge transfer process. A CCD clock driver is designed, built and tested.