University of Twente Student Theses
A TDC-assisted SAR ADC
Kosters, Jelmer Bas (2017) A TDC-assisted SAR ADC.
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| Abstract: | For medium-speed, medium-resolution analog-to-digital conversion, a successive-approximation-register analog-to-digital converter (SAR ADC) is a popular choice due to its low figure of merit (FOM). Recent developments focus on hybrid architectures to improve the state-of-the-art. In this thesis, an improvement to the SAR algorithm is proposed utilizing a TDC, to be used in a so-called TDC-assisted SAR ADC. By measuring the speed of the comparator during conversion steps, information about the sampled signal can be extracted and used to skip unnecessary steps in the SAR algorithm. The logarithmic voltage-to-time behavior of the comparator, in combination with the exponential scaled binary voltages quantized by the SAR algorithm, result in linearly spaced timings related to each SAR bit. Several algorithms that leverage the extra information are proposed, of which one is further researched. Based on calculations, it can decrease the average number of conversion steps in a 10-bit SAR ADC from 10 to 3, drastically improving the figure of merit due to reduced power consumption. A comparator and 9-stage TDC are designed in a 65nm technology to test the feasibility of the idea. Additionally, a possible SAR logic implementation for the algorithm is given. Simulations show that the comparator requires 60fJ per comparison, and has an expected logarithmic decision speed. The TDC consumes 40fJ per comparison, and can be calibrated to match the LSB decision time of the comparator in multiple corners or mismatch scenarios. Examining the non-ideal voltage-to-time-to-digital behavior of the circuit, an input-dependent effective number of conversion steps is expected. Based on these results, expected FOMs are provided when possibly implemented in a future TDC-assisted SAR ADC. |
| Item Type: | Essay (Master) |
| Faculty: | EEMCS: Electrical Engineering, Mathematics and Computer Science |
| Subject: | 53 electrotechnology |
| Programme: | Electrical Engineering MSc (60353) |
| Link to this item: | https://purl.utwente.nl/essays/74090 |
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