Abstract. Successive approximation register (SAR) analogto-digital Converters (ADC) are based on a capacitive digitalto-analog converter (CDAC) (McCreary and Gray, 1975). The capacitor mismatch in the capacitor array of the CDAC impacts the differential non-linearity (DNL) of the ADC directly. In order to achieve a transfer function without missing codes, trimming of the capacitor array becomes necessary for SAR ADCs with a resolution of more than 12 bit. This article introduces a novel digital approach for trimming. DNL measurements of an 18 bit SAR ADC show that digital trimming allows the same performance as analog trimming. Digital trimming however reduces the power consumption of the ADC, the die size and the required time for the production test.
For the operation of a successive approximation register analog-to-digital converter, an integrated reference voltage source is required. This reference has to be stable and has to have high precision. Therefore, trim mechanisms for the achievement of optimal accuracy and minimum temperature drift behavior are necessary.Generally, Kuijk and Brokaw bandgap reference topologies are suitable for these requirements. The Kuijk bandgap reference has typically a very good power supply rejection ratio (PSRR). But adequate noise performance and temperature drift behavior cause relatively high current and area consumption. In contrast to the Kuijk topology, the Brokaw reference circuit has better noise performance and temperature drift at a lower quiescent current. Depending on the topology, the Brokaw reference normally has lower PSRR.For implementation, the Brokaw reference has been chosen. It has a nominal output voltage of 1.25 V with an absolute accuracy of 600 µV after trimming. The temperature drift in the range of -55 • C to 125 • C is typical 22.8 ppm/ • C. The DC power supply rejection ratio (PSRR) is more than 84 dB over all relevant corners. The minimum power supply rejection ratio is 59 dB at approximately 1 MHz. The overall current consumption at ambient temperature is maximal 59 µA.
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