把DAC的输出从单端模式转换到差分模式的电路

高速DAC,比如模拟器件(AnalogDevices)公司的AD9776/78/79TxDAC系列,能提供差分输出,但对于低端交流电应用或高精度电平设置应用,配备差分转换电路的单端电流输出DAC提供了一种新颖的方法来生成差分波形控制功能。图1中的基本电路组合了电流输出DAC(即IC1,如8位AD5424D A C)和一个单端至差分运算放大级IC2、IC3A、IC3B——来产生要求的输出。对于双电源应用,可选择DAC的单极工作模式来达到DAC的最优性能。DAC利用单一运算放大器提供了双象限倍增或单极输出电压摆动。DAC的输出需要缓冲器,这是因为对施加到DAC输入端的代…     

The lactulose breath hydrogen test and small intestinal bacterial overgrowth

OBJECTIVES: To i) document the sensitivity and specificity of a combined scintigraphic/lactulose breath hydrogen test for small intestinal bacterial overgrowth and ii) investigate the validity of currently accepted definitions of an abnormal lactulose breath hydrogen test based on "double peaks" in breath hydrogen concentrations. METHODS: Twenty-eight subjects were investigated with culture of proximal small intestinal aspirate and a 10-g lactulose breath hydrogen test combined with scintigraphy. Gastroduodenal pH, the presence or absence of gastric bacterial overgrowth, and the in vitro capability of overgrowth flora to ferment lactulose were determined. RESULTS: Sensitivity (16.7%) and specificity (70.0%) of the lactulose breath hydrogen test alone for small intestinal bacterial overgrowth were poor. Combination with scintigraphy resulted in 100% specificity, because double peaks in serial breath hydrogen concentrations may occur as a result of lactulose fermentation by cecal bacteria. Sensitivity increased to 38.9% with scintigraphy, because a single rise in breath hydrogen concentrations, commencing before the test meal reaches the cecum, may occur in this disorder. Sensitivity remained suboptimal irrespective of the definition of small intestinal bacterial overgrowth used, the nature of the overgrowth flora, favorable luminal pH, the presence of concurrent gastric bacterial overgrowth, or the in vitro ability of the overgrowth flora to ferment lactulose. CONCLUSIONS: Definitions of an abnormal lactulose breath hydrogen test based on the occurrence of double peaks in breath hydrogen concentrations are inappropriate. Not even the addition of scintigraphy renders this test a clinically useful alternative to culture of aspirate for diagnosing small intestinal bacterial overgrowth.     

Temperature and pump power dependent photoluminescence characterization of MBE grown GaAsBi on GaAs

Bulk and quantum well GaAs_1?xBi_x/GaAs layers with Bi mole fractions from 0.02 to 0.10 are grown by molecular-beam epitaxy at temperatures ranging from 280 to 320?°C. The samples are characterized using temperature and pump-power dependent photoluminescence measurements covering 8–300?K and 1–250?mW (7–1,800?W/cm²), respectively. The results indicate that there is strong reduction in bandgap energy with the incorporation of small amounts of Bi and that GaAsBi most likely forms a weak type-I band alignment with GaAs.     

The Difficulties of Designing Future Coastlines in the Face of Climate Change

This paper adopts two perspectives. The first is a framing process aimed at defining and examining the conditions for adopting adaptive coastal governance. The second applies to relevant themes of changing coastal policy, central to the testing of adaptive coastal governance, namely cooperative science, risk-sensitive planning, socially fair insurance cover and effective ways to design, finance and engage with local communities over actual coastal change. We illuminate both missions through case studies in North Norfolk (England) and Portugal, all notably affected by coastal change. In England and Portugal, there is a broad understanding and acceptance of the likely effects of climate change. This recognition encourages debates over risk-averse planning, the design of proactive insurance cover, creative relocation of endangered property and new ways of predicting and paying for coastal adjustment. Yet, moving from a basic willingness to engage with coastal change to actual practices of landscape adjustment through such policy shifts is proving very difficult. In this research, we find that coastal landscapes are lived experiences, resigned acceptances of inevitable change and hopeful imaginings. Coastal management institutions are not geared to resolving this incompatibility and this paper explains why.     

Determination of the effect of dairy powders on adherence of Streptococcus sobrinus and Streptococcus salivarius to hydroxylapatite and growth of these bacteria

Dental caries is a highly prevalent disease caused by colonisation of tooth surfaces by cariogenic bacteria, such as Streptococcus sobrinus and Streptococcus salivarius. Reducing initial adherence of such bacteria to teeth may delay onset of caries. Many foods, such as milk, can inhibit microbial adherence. In this investigation, the effect of untreated (UT) and enzyme-treated (ET) dairy powders on adherence of S. sobrinus and S. salivarius to hydroxylapatite (HA), an analogue of tooth enamel, was examined. Untreated (UT) acid whey protein concentrate (AWPC) 80 inhibited streptococcal adherence to phosphate-buffered saline-coated HA (PBS-HA) and saliva-coated HA (S-HA) by >80% at ?31.25 μg mL⁻¹. UT sweet WPC80, buttermilk powder and cream powder also significantly reduced adherence (P < 0.05). Enzyme-treatment of all dairy powders reduced their anti-adhesion activity. However, ET sweet WPC80 significantly inhibited growth of these streptococci (P < 0.05) at ?0.6 mg mL⁻¹. Therefore, dairy powders may reduce progression of dental caries by their anti-adhesion and/or antibacterial activity.     

Effects of emulsifying salts reduction on imitation cheese manufacture and functional properties

Imitation cheese (48%, 50% and 52% moisture) was manufactured using a Farinograph. The standard emulsifying salts (ES) concentration was 1.4%, giving a casein:ES ratio of 19:12. The effect of ES reduction on cheese manufacture and functionality, assessed by texture profile analysis, heat-induced flowability and dynamic rheology, was studied. Microstructure was investigated by light and cryo-scanning electron microscopy. Reducing ES increased processing time and hardness and decreased flowability and fat globule diameter. In comparison to standard ES, a reduction of up to 20% produced cheeses in reasonable processing times with slightly altered functionality. On further ES reduction, processing times greatly increased giving much harder and less meltable cheeses. Reducing ES by 40% increased processing times ∼3-fold, halved fat globule diameter and flowability and doubled hardness, compared to standard cheeses. At ES reduction above 40% the product obtained, after prolonged processing time, bore little resemblance to cheese.     

Effect of pH on survival, thermotolerance, and verotoxin production of Escherichia coli O157:H7 during simulated fermentation and storage

Heat treatment is increasingly being introduced to fermented meat processing, since the acid tolerance properties of Escherichia coli O157:H7 can permit this organism to survive traditional processing procedures. This study investigated the effect of growth pH and fermentation on the thermotolerance at 55 degrees C of E. coli O157:H7 in a model fermented meat system. E. coli O157:H7 (strain 380-94) was grown at pH 5.6 or 7.4 (18 h at 37 degrees C), fermented to pH 4.8 or 4.4 in brain heart infusion broth, and stored for 96 h. Cells grown at pH 5.6 had higher D values at 55 degrees C (D55 degrees C) than cells grown at pH 7.4 (P < 0.001). Cells fermented to pH 4.8 had higher D55 degrees C than those fermented to pH 4.4 (P < 0.001). Cells fermented to pH 4.8 demonstrated an increase in D55 degrees C during storage (P < 0.001), whereas cells fermented to pH 4.4 showed a decrease in D55 degrees C during the same period (P < 0.001). The effect of growth pH on verotoxin production by E. coli O157:H7 was assessed using the verotoxin assay. Cells grown at pH 5.6 had lower verotoxin production then cells grown at pH 7.4. This effect was not sustained over storage. These results indicate that a lower growth pH can confer cross-protection against heat. This has implications for the production of acidic foods, such as fermented meat, during which a heating step may be used to improve product safety.     

Effect of water activity on the crispiness of a biscuit (Crackerbread): Mechanical and acoustic evaluation

The objective of this study was to use mechanical and acoustic methods developed in our laboratory to monitor the changes in crispiness of a Crackerbread biscuit following equilibration at different water activities (a_w). Crackerbread samples were equilibrated at a_w of 0.113–0.98. The equilibrated Crackerbread samples were punctured with a cylindrical probe (5 mm) using an Instron Universal Testing Machine. The force and acoustic signals were simultaneously captured using Labview software. Changes in parameters associated with crispiness were modelled using Fermi’s equation. The number of sound peaks, force and sound curve lengths and area under sound amplitude–time curve decreased with increasing a_w. The sound frequency decreased linearly (R² = 0.926) with increasing a_w. The critical water activity (a_wc) of Crackerbread ranged between 0.51 and 0.59. Acoustic parameters were more sensitive to a_w-induced changes in Crackerbread occurring at a_wc than mechanical parameters.