An adaptive approach to on-chip CMOS ramp generation for high resolution single-slope ADCs

Many image sensors employ column-parallel ADCs in their readout structures. Single-slope ADCs are ideally suited for these multi-channel applications due to their simplicity, low power and small overall area. The ramp generator, shared by all the converters in the readout architecture, is a key element that has a direct effect in the transfer characteristic of single-slope ADCs. Because a digital counter is inherently present in this conversion scheme, one common practice is to use a digital-to-analog converter driven by the counter to generate the ramp. Given the direct relationship between the DAC and the ADC transfer characteristics, one of the main issues is to ensure a sufficient linearity of the DAC, with special emphasis on its monotonicity. Very often, in particular when medium to high resolutions are aimed, this requires calibration of the DAC, which must be repeated every once in a while to account for temperature, process, power supply, and aging variations. This paper presents an inherently monotonic ramp generator with high levels of linearity and stability against any expected source of variations, combined with a very efficient realization and an inherent automatic adaptability to different resolutions. The ramp generator has been designed using radiation hardening by design (RHBD) techniques, allowing its use in space applications.