全差分放大器(一)

差分信号固有的外部噪声抑制特性使其多年来普遍应用于音频、数据传输和电话系统。如今，差分信号技术正在高速数据采集领域日益普及，这种应用场合中ADC带有差分输入，需要差分放大器来驱动。差分信号还可减少偶次谐波和增加动态范围。重点讨论集成全差分放大器的结构、电压定义及应用（包括与差分ADC输入的接口电路，抗混叠滤波器以及驱动传榆线）。     

Airflow distribution through perforated tiles in raised-floor data centers

Raised-floor data centers are the most commonly used facilities for housing computer and telecommunication equipment. To adequately cool this equipment, the cooling air through perforated tiles must be distributed properly. The airflow distribution depends on the pressure distribution or the flow field in the space under the raised floor (plenum); it is a complex function of a large number of variables, including the size of the plenum, the open area of the perforated tiles, the locations and flow rates of the computer room air conditioner (CRAC) units, and the size and location of the under-floor obstructions like cables and pipes. In this article, the effect of these parameters on the airflow distribution is studied using an idealized one-dimensional computational model. Within the one-dimensional framework, the airflow distribution is governed by two dimensionless parameters: one related to the pressure variation in the plenum and the other to the frictional resistance. Results, in terms of distributions of pressure in the plenum and flow rates through the perforated tiles, are presented over a range of values of these two parameters. These results provide an understanding of the fundamental fluid mechanical processes controlling the airflow distribution through the perforated tiles. The one-dimensional model is used to calculate flow rates for two possible arrangements of the CRAC units, and these results are compared with those given by a three-dimensional model.     

Temperature and Material Flow Prediction in Friction-Stir Spot Welding of Advanced High-Strength Steel

Friction-stir spot welding (FSSW) has been shown to be capable of joining advanced high-strength steel, with its flexibility in controlling the heat of welding and the resulting microstructure of the joint. This makes FSSW a potential alternative to resistance spot welding if tool life is sufficiently high, and if machine spindle loads are sufficiently low that the process can be implemented on an industrial robot. Robots for spot welding can typically sustain vertical loads of about 8 kN, but FSSW at tool speeds of less than 3000 rpm cause loads that are too high, in the range of 11–14 kN. Therefore, in the current work, tool speeds of 5000 rpm were employed to generate heat more quickly and to reduce welding loads to acceptable levels. Si₃N₄ tools were used for the welding experiments on 1.2-mm DP 980 steel. The FSSW process was modeled with a finite element approach using the Forge^® software. An updated Lagrangian scheme with explicit time integration was employed to predict the flow of the sheet material, subjected to boundary conditions of a rotating tool and a fixed backing plate. Material flow was calculated from a velocity field that is two-dimensional, but heat generated by friction was computed by a novel approach, where the rotational velocity component imparted to the sheet by the tool surface was included in the thermal boundary conditions. An isotropic, viscoplastic Norton-Hoff law was used to compute the material flow stress as a function of strain, strain rate, and temperature. The model predicted welding temperatures to within 4%, and the position of the joint interface to within 10%, of the experimental results.     

Capillary-Scale Hydrogel Microchannel Networks by Wire Templating

Microvascular networks are essential for the efficient transport of nutrients, waste products, and drugs throughout the body. Wire-templating is an accessible method for generating laboratory models of these blood vessel networks, but it has difficulty fabricating microchannels with diameters of ten microns and narrower, a requirement for modeling human capillaries. This study describes a suite of surface modification techniques to  selectively control the interactions amongst wires, hydrogels, and world-to-chip interfaces. This wire templating method enables the fabrication of perfusable hydrogel-based rounded cross-section capillary-scale networks whose diameters controllably narrow at bifurcations down to 6.1 ± 0.3 microns in diameter. Due to its low cost, accessibility, and compatibility with a wide range of common hydrogels of tunable stiffnesses such as collagen, this technique may increase the fidelity of experimental models of capillary networks for the study of human health and disease.