A highly digital microbolometer ROIC employing a novel event-based readout and two-step time to digital converters
Abbasi, Shahbaz (2019) A highly digital microbolometer ROIC employing a novel event-based readout and two-step time to digital converters. [Thesis]
Uncooled infrared imaging systems are a light weight and low cost alternative to their cooled counterparts. Uncooled microbolometer IR focal plane arrays (IRFPAs) for applications such as medical imaging, thermography, night vision, surveillance and industrial process control have recently been under focus. These systems have small pixel pitches (< 25µm) and require power eﬃciency, low noise equivalent temperature diﬀerence (NETD) (< 50 mK) and adequate scene dynamic range (> 250 K). Low NETD demands excellent microbolometer and readout noise performance. If sensitive analog circuits, driving long metal interconnects, are part of the predigitization readout channel, this necessitates the use of power consuming buﬀers, potentially in conjunction with noise cancellation circuits that result in power and area overhead. Thus re-thinking at the architectural level is crucial to meet these demands. Accordingly, in this thesis a column-parallel readout architecture for frame synchronous microbolometer imagers is proposed that enables low power operation by employing a time mode digitizer. The proposed readout circuit is based on a bridge type detector network with active and reference microbolometers and employs a capacitive transimpedance ampliﬁer (CTIA) incorporating a novel two-step integration mechanism. By using a modiﬁed reset scheme in the CTIA, a forward ramp is initiated at the input side followed by the conventional backward integrated ramp at the output. This extends the measurement interval and improves signal-to-noise ratio (SNR). A synchronous counter based TDC measures this interval providing robust digitization. This technique also provides a way of compensating for self-heating eﬀects. Being highly digital, the proposed architecture oﬀers robust frontend processing and achieves a per channel power consumption of 66 µW, which is considerably lower than the most recently reported designs, while maintaining better than 10mK readout NETD.
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