A20模块是EM公司为测量亚纳伏量级信号而设计的超灵敏放大器，可用于标定、低温研究和仪器系统。A20具有极低的短路输入噪声，等效输入电压噪声低于10Ω电阻所产生的热噪声，它在相对短的响应时间内实现100pV量级的分辨率。
需要注意A20所适用的源电阻范围，特别是在测量低温源时。在源电阻与电压噪声的关系图（图1）中，将温度为300K时的电压噪声线和电流噪声线抽离，得到温度为3K时的热噪声曲线。可以看到，A20在室温下依然有效地测量在氦温度下的源，而不显著增加整体噪声。
A20共有1,000至1,000,000的4档增益，可通过25针D型连接器设置。D型连接器具有控制滤波器、供电、输出连接及其它功能。当增益控制电阻安装在外部时，还可以自定义增益设置。有4个增益控制管脚被预留，增益控制电阻连接在D型连接器的10脚和11脚之间。
通过A20壳体侧边的旋钮可以容易地调节偏置电压和偏置电流。信号输入端为带有螺母和垫圈的高纯度铜柱，便于将热量最小化。
A20安装在高磁导率金属盒中，输入单元则密封在双层高磁导率屏蔽盒中。
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.
直流亚纳伏放大器模块A20
噪声
等效噪声电阻低于10Ω，电压噪声取决于带宽，在时间常数为10s时，电压噪声峰峰值为250pV。
Noise.
Equivalent noise resistance less than 10 ohms. Noise voltage depends on bandwidth e.g. rise time constant 10 seconds gives peak to peak noise voltage of 250 picovolts.
偏置电压
可通过侧边的十旋电位器将偏置电压调节至零，电位器温度系数优于300pV/℃。
Offset Voltage.
Adjustable to zero by a ten turn potentiometer accessible at the the edge of the case. Temperature coefficient better than 300pV per degree C.
偏置电流
可通过侧边的十旋电位器将偏置电流调节至零，电位器温度系数优于10 pA/℃。
Offset Current.
Adjustable to zero by a ten turn potentiometer accessible at the edge of the case. Temperature coefficient better than 10pA per degree C.
增益
增益
在内部将增益设置为四档：1K、10K、100K、1M。增益精度为0.02%，温度系数为5 ppm/℃。
Gain.
There are 4 gain settings 1K, 10K, 100K, and 1M. These gains are set internally.
Accuracy 0.02%, subject to noise floor.
Temperature coefficient 5ppm per degree C.
滤波器
可选择滤波器所增加的时间常数如下：
范围时间常数
1K 不增加
10K 100ms
100K 1s
1M 10s
Filter.
A filter may be selected which gives the following rise time constants.
Range Time Constant
1K: No Increase
10K: 100 milliseconds
100K: 1 second
1M: 10 seconds
输入阻抗
开机3s后输入阻抗高于30MΩ，稳定后增大至GΩ，如图2所示。
Input Impedance.
Greater than 30 Megohms, 3 seconds after an input step increasing with time to 1 Gohm, see input impedance graph
源阻抗
源阻抗在10Ω至100kΩ范围时，A20遵循源电阻热噪声特性。在源电阻低于10Ω时，噪声曲线基本平坦；源电阻高于100kΩ时，源电阻每增大10倍，噪声电压增加10dB。输入噪声特性图（图1）表明，如果在氦温度下将电压噪声和电流噪声线从热噪声线中抽离出，那么在1K源电阻下，A20几乎不会增加总的噪声。
Source Resistance.
The A20 closely follows the source thermal noise characteristic
with sources resistances of 10 ohms up to around 100 Kohms.
With source resistances below 10 ohms, the noise is substantially
constant down to zero ohms. Above 100 Kohms, the noise voltage
increases over the thermal noise characteristic by about 10 dB per
decade of source resistance. The input noise characteristic graph
shows that, if the `voltage' and `current' noise lines are
extrapolated as far as the thermal noise line for helium
temperature, the A20 still adds insignificantly to the total noise at around 1K source impedance.
输入信号量级
输入信号应在±2mV范围内。当输入电压超过2V时虽不会造成放大器损坏，但如果输入持续饱和，可能造成热偏置，这导致输入噪声需要一定时间降低至初始水平。
Input Level.
The A20 will accept input voltage up to 2mV. Input voltage up to 2
volts will not cause damage, but if the input is saturated for any
length of time, there may be a thermal offset, which could take
time to reduce to sub nanovolt levels.
输出信号量级
A20输出范围为±3V，驱动负载电流可达5mA。
Output Level.
The A20 will provide an output of plus or minus 3 volts and can
drive loads of up to 5mA.
输入连接
A20输入端为带有螺母和垫圈的纯铜柱。铜柱螺纹为2BA，直径4.8mm。
输入连接线应使用不镀锡的低热噪声铜线。在实际应用中，将新的变压器绕线剥去外部绝缘层，即满足要求。
Connectors.
The input connectors for the A20 are pure copper terminals with
nuts and washers made from the same material. The threads are
2BA (4.8mm diameter)
Input connections should be made using low thermal emf untinned
copper wire. In practice, new, unbent transformer wire,
carefully stripped of insulation, is satisfactory.
D25连接器
管脚功能
1 滤波器
2 增益1M
3 增益100K
4 增益10K
5 增益1K
6 0V
7 负供电端
8 正供电端
10 直接输出
11 反馈
15 滤波器输出
18 信号0V
19 屏蔽
外部屏蔽驱动连接至A20金属壳体。19(屏蔽)脚和6(0V)脚之间电阻不低于10GΩ。为控制增益和滤波功能，应将合适的管脚连接至+5V。
所有未使用的管脚应保持开路状态。
D25 Plug Connection.
Pin Function
1 Filter
2 Gain 1M
3 Gain 100K
4 Gain 10K
5 Gain 1K
6 0v
7 Supply Negative
8 Supply Positive
10 Direct Output
11 Feedback
15 Filtered Outpu
18 Signal 0v
19 Case
The plug outer shield is connected to the A20 case.
Isolation between case and 0v. 10 Gohms minimum.
For control of gain and filter functions, the appropriate pin should
be connected to + 5 volts.
电源
供电
最小供电为±5.5V，最大供电为±8V，推荐供电为±6V。
静态供电电流为1.5mA。当放大器增益为1K、输入至最大许可值时，供电电流增大至4mA。
Power Supply.
Minimum +/- 5.5 volts. Maximum +/- 8 volts.
Nominal supply voltage +/- 6 volts
Current requirement 1.5 milliamps quiescent.
This may increase up to 4 milliamps when the amplifier sees a full
scale input on the 1K range
固定
4个固定孔规格为M4，中心距为8.4*5.6英寸。
Fixing.
4 holes required M4 centres 8.4 inches by 5.6.inches.
尺寸和重量
长：8.75英寸（222mm）
宽：6英寸（152.4mm）
厚：3.7英寸（95mm）
重量：9磅（4kg）
Dimensions and Weight.
Length 8.75 inches (222mm)
Width 6 inches (152.4mm)
Depth including terminals 3.7 inches (95mm)
Weight 9 lbs (4 Kg)
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