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Hao SAN Yoshitaka JINGU Hiroki WADA Hiroyuki HAGIWARA Akira HAYAKAWA Haruo KOBAYASHI Tatsuji MATSUURA Kouichi YAHAGI Junya KUDOH Hideo NAKANE Masao HOTTA Toshiro TSUKADA Koichiro MASHIKO Atsushi WADA
We have designed, fabricated and measured a second-order multibit switched-capacitor complex bandpass ΔΣAD modulator to evaluate our new algorithms and architecture. We propose a new structure of a complex bandpass filter in the forward path with I, Q dynamic matching, that is equivalent to the conventional one but can be divided into two separate parts. As a result, the ΔΣ modulator, which employs our proposed complex filter can also be divided into two separate parts, and there are no signal lines crossing between the upper and lower paths formed by complex filters and feedback DACs. Therefore, the layout design of the modulator can be simplified. The two sets of signal paths and circuits in the modulator are changed between I and Q while CLK is changed between high and low by adding multiplexers. Symmetric circuits are used for I and Q paths at a certain period of time, and they are switched by multiplexers to those used for Q and I paths at another period of time. In this manner, the effect of mismatches between I and Q paths is reduced. Two nine-level quantizers and four DACs are used in the modulator for low-power implementations and higher signal-to-noise-and-distortion (SNDR), but the nonlinearities of DACs are not noise-shaped and the SNDR of the ΔΣAD modulator degrades. We have also employed a new complex bandpass data-weighted averaging (DWA) algorithm to suppress nonlinearity effects of multibit DACs in complex form to achieve high accuracy; it can be realized by just adding simple digital circuitry. To evaluate these algorithms and architecture, we have implemented a modulator using 0.18 µm CMOS technology for operation at 2.8 V power supply; it achieves a measured peak SNDR of 64.5 dB at 20 MS/s with a signal bandwidth of 78 kHz while dissipating 28.4 mW and occupying a chip area of 1.82 mm2. These experimental results demonstrate the effectiveness of the above two algorithms, and the algorithms may be extended to other complex bandpass ΔΣAD modulators for application to low-IF receivers in wireless communication systems.
Masafumi UEMORI Haruo KOBAYASHI Tomonari ICHIKAWA Atsushi WADA Koichiro MASHIKO Toshiro TSUKADA Masao HOTTA
This paper proposes a continuous-time bandpass ΔΣAD modulator architecture which performs high-accuracy AD conversion of high frequency analog signals and can be used for next-generation radio systems. We use an RF DAC inside the modulator to enable subsampling and also to make the SNDR of the continuous-time modulator insensitive to DAC sampling clock jitter. We have confirmed that this is the case by MATLAB simulation. We have also extended our modulator to multi-bit structures and show that this alleviates excess loop delay problems.
In this paper, we give a concrete example of a 10-bit video rate ADC and introduce the effect of top-down design methodology with analog-HDL from the viewpoint of utilization techniques. First, we explain that analog top-down design methodology can improve chip performance by optimizing the architecture. Next, we concretely discuss the importance of modeling and verification. Verification of the full system does not require extracting all the information for each block at the transistor level in detail. The flexible verification method that we propose can provide good and fast full chip verification. We think analog top-down disign methodology will become increasingly more important from now on because "system-on-chip" requires one chip mixed-signal system LSIs.
Hao SAN Akira HAYAKAWA Yoshitaka JINGU Hiroki WADA Hiroyuki HAGIWARA Kazuyuki KOBAYASHI Haruo KOBAYASHI Tatsuji MATSUURA Kouichi YAHAGI Junya KUDOH Hideo NAKANE Masao HOTTA Toshiro TSUKADA Koichiro MASHIKO Atsushi WADA
This paper proposes a new architecture for multibit complex bandpass ΔΣAD modulators with built-in Switched-Capacitor (SC) circuits for application to Low-IF receivers such as used for Bluetooth and WLAN. In the realization of complex bandpass ΔΣAD modulators, we face the following problems: (i) SNR of AD converter is deteriorated by mismatches between internal analog I and Q paths. (ii) Layout design becomes complicated because of signal lines crossing by complex filter and feedback from DAC for I and Q paths in the complex modulator, and this increases required chip area. We propose a new structure for a complex bandpass ΔΣAD modulator which can be completely divided into two paths without layout crossing, and solves the problems mentioned above. The two parts of signal paths and circuits in the modulator are changed for I and Q while CLK is changed for High/Low by adding multiplexers. Symmetric circuits are used for I and Q paths at a certain timing, and they are switched by multiplexers to those used for Q and I paths at another timing. Therefore the influence from mismatches between I and Q paths is reduced by dynamic matching. As a result, the modulator is divided into two separate parts without crossing signal lines between I and Q paths and its layout design can be greatly simplified compared with conventional modulators. We have conducted MATLAB simulations to confirm the effectiveness of the proposed structure.