High-speed Cherry Hooper flash analog-to-digital converter

Abstract

PURPOSE : The 60 GHz unlicensed band is being utilized for high speed wireless networks with data rates in the gigabit range. In order to successfully make use of these high speed signals in a digital system, a high speed analog-to-digital converter (ADC) is required. This paper presents the use of a common collector (CC) input tree and Cherry Hooper differential amplifier to enable analog-to-digital conversion at high frequencies. APPROACH : The CC input tree is designed to separate the input Miller capacitance of each comparator stage. The CC stages are biased to obtain bandwidth speeds higher than the comparator stages while using less current than the comparator stages. The Cherry Hooper differential amplifier is modified to accommodate the low breakdown voltages of the technology node and used as a comparator. The comparator stages are biased to obtain a high output voltage swing and have a small signal bandwidth up to 29 GHz. Simulations were performed using foundry development kits to verify circuit operation. A two-bit ADC was prototyped in IBM’s 130 nm silicon-germanium (SiGe) bipolar complementary metal–oxide–semiconductor (BiCMOS) 8HP technology node. Measurements were carried out on test printed circuit boards (PCBs) and compared with simulation results. FINDINGS : The use of the added CC input tree showed a simulated bandwidth improvement of approximately 3.23 times when compared to a basic flash architecture for a two-bit ADC. Measured results showed an effective number of bits (ENOB) of 1.18, from DC up to 2 GHz, whereas the simulated result was 1.5. The maximum measured integral non-linearity (INL) and differential nonlinearity (DNL) was 0.33 least significant bits (LSBs). The prototype ADC had a figure of merit of 42 pJ/sample. ORIGINALITY / VALUE : The prototype ADC results showed that the group delay for the Cherry Hooper comparator plays a critical role in ADC performance for high frequency input signals. For minimal component variation the group delay between channels deviate from each other, causing incorrect output codes. The prototype ADC had a low gain which reduced the comparator performance. The two-bit CC Cherry Hooper ADC is capable of achieving an ENOB close to 1.18 for frequencies up to 2 GHz, with 180 mW total power consumption.