Abstract
Time-to-Digital Converters (TDC) are popular circuits in many applications, where high resolution time measurements are required, for example, in Positron Emission Tomography (PET). Besides its resolution, the TDC's linearity is also an important performance indicator, therefore calibration circuits usually play an important role on TDCs architectures. This paper presents an all-digital TDC implemented using Structured Datapath to reduce the need for calibration circuitry and cells custom design, without compromising the TDC's linearity. The proposed design is fully implementable using a Hardware Description Language (HDL) and enables a complete design flow automation, reducing both development time and system's complexity. The TDC is based on a Delay Locked Loop (DLL) paired with a coarse counter to increase measurement range. The proposed architecture and the design approach have proven to be efficient in developing a high resolution TDC with high linearity. The proposed TDC was implemented in TSMC 0.18 CMOS technology process achieving a resolution of 180ps, with Differential Non-Linearity (DNL) and Integral Non-Linearity (INL) under 0.6 LSB.
Highlights
Time-to-Digital Converters (TDC) are devices used to convert the difference between the arrival time of two signals to a digital value [1], [2]
This paper proposes an all-digital TDC architecture based on a Delay Locked Loop (DLL) that takes advantage of the computer aided design (CAD) tools Structured Data Path feature, implemented in Cadence Innovus, to achieve high linearity
DESIGN OF THE ALL-DIGITAL TDC From the analysis presented in the previous Section, it can be concluded that the available architectures offer great resolutions, high linearity and large measurement range
Summary
Time-to-Digital Converters (TDC) are devices used to convert the difference between the arrival time of two signals to a digital value [1], [2]. TDCs have long been used in physics experiments for Time-of-Flight (TOF) measurements, and laser range finders [1]–[5]. In CMOS image sensors, TDCs are used in combination with Analog-to-Digital Converters (ADC) to shorten the conversion time without decreasing the dynamic range of the ADC [6]. With the appearance of LiDAR systems in the automotive industry, the high resolutions achieved by TDCs enable for higher precision in object detection and tracking [5]. Time-based accelerometers benefit from the use of a TDC, as it enables to further enhance the overall system resolution due to a more precise pull-in time measurement [7]
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