EM A20是一种可以对毫微伏进行测量的敏感的放大器。根据设计,它被用在校准、低温研究和仪器系统内。 A20的生产是为了满足EM零位探测器内仪器具备类似灵敏度的要求。它的短路输入很低,等同于低于10Ω的噪声阻抗,借助很短的响应时间给出了100μV的解决方案。 对于A20能够运行的电源电阻范围,我们给与了特别关注。对于电源电阻针对噪声电压波形,我们开展了研究。这一研究表明了它的重要性。“电流”和“电压”噪声线路从300K被外推至3K。当A20处于室温条件下时,它可以在氮点下对电源进行有效的测量,同时又不提高噪声水平。 从1×102至1×106之间,存在着4种增益设定。借助25条渠道的D型连接,施加各种等级的电压,以选择各种增益设定。这种D型连接可以对滤波器进行控制,并能连接输出,提供电源并发挥其他的作用。在有要求的情况下,让用户对增益设定进行定义是可能的。在这种情况下,在外部安装增益设定阻抗,用户也能对其实施控制。4个增益控制线路保持开路,增益阻抗也在连接器上的点10和11之间连接。 在机箱边缘可以接入控制器,以将补偿电压和电流清零。输入连接在铜制终端之上,螺帽和垫圈都由相同的高纯度铜支撑,因此热电势效应就被最小化了。 A20被安装在一个重型高导磁率合金内,输入单元被装在一个重型计量高导磁率合金双屏之中。
The EM model A20 is a very sensitive amplifier for making sub nanovolt measurements and is designed to be used in calibration, cryogenic research and instrumentation systems. The A20 has been produced in response to requests for an amplifier of similar sensitivity to those used in EM null detectors, having a very low short circuit input equivalent noise resistance of less than 10 ohms, giving resolution of a hundred picovolts with comparatively short response time. Particular care has been taken with respect to the range of source resistance with which the A20 can operate. This parameter is very important when measuring cryogenic sources. A study of the graph overleaf of source resistance against noise voltage will show the importance of this. The `voltage' and `current' noise lines have been extrapolated from 300 deg K to 3 deg K. and it can be seen that the A20, while remaining at room temperature can efficiently measure a source at helium temperature, without adding significantly to the overall noise. There are 4 gain settings from a thousand to a million and these are selected by voltage levels applied via the 25 way `D' type connector, which has a control for a filter, as well as carrying the output connection, the power supply and other functions. It is possible to have user defined gain settings if required, in which case the gain determining resistors are mounted externally and are under the user抯 control. The four gain control pins are left open circuit and the gain resistor is connected between pins 10 and 11 on the 慏&O5533; connector. Controls are accessible at the edge of the case for nulling offset voltage and current. The input connections are made on copper terminals with nuts and washers all made from the same high purity copper, so that thermal emf is minimised. The A20 is mounted in a heavy duty mumetal case, and the input unit is enclosed in double screens of heavy gauge mumetal.
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