## Selecting the Correct Nanovolt Amplifier
Selecting the Correct Nanovolt Amplifier. An important consideration when making a choice of low level measuring instrument is the range of source impedance with which it will be required to operate. All electrical conductors have resistance at normal
temperature and thus they generate thermal noise, proportional to the square
root of the resistance, which varies similarly with temperature. One of the important uses of ultra low level DC nanovolt amplifiers is the measurement of cryogenic sources. Much research is being undertaken into high temperature superconductivity. If a dc nanovolt amplifier is required to measure a source of, say, 100 ohms at 3 degrees K, it must cope with the source impedance of 100 ohms. However, if the source is at a temperature of 3 degrees K, then the noise of the source is equivalent to the noise of a 1 ohm resistor at room temperature. This makes the design of the amplifier very complicated. The measurement of this parameter is the ability of the nanovolt amplifier to cope with source resistance much higher than would normally be required for the measurement of low source resistance. EM has worked on this problem for many years and we have improved the cryogenic capability to a very large extent. The models N11 and A20 can operate with such a wide range of source resistance that their characteristics at room temperature can be extrapolated towards the source noise characteristic at helium temperature, and it can be seen that the instruments, while remaining at room temperature, can efficiently measure a source at helium temperature, over a small range of source resistance, and are useable over a wide range of source resistance. Care should be taken when selecting a DC Nanovolt Amplifier. The model A22 has an equivalent noise resistance of about 100 ohms, or the thermal noise generated by a perfect resistor of 100 ohms. As a general rule, if the resistance of the source to be measured, including the input leads is much greater than the enr of the amplifier, a lower specification amplifier may be the more economical choice. For example if the source to be measured is 30 ohms, then the DC Nanovolt Amplifier model A23, which has an enr of 30 ohms, will give a total noise of 60 ohms or 1nV per root Hz rms. If the source resistance is 100 ohms then the total noise from the source plus A23 would be 130 ohms or 1.5 nV per root Hz rms. If the A22 were used, which has an enr of 100 ohms, the total noise would be 200 ohms or 1.8 nV per root Hz. Once the source impedance increase beyond 100 ohms, the A22 would be the better choice. A similar case is between the choice of the A23, A10 and A30. Short circuit noise performance is by no means the only parameter to be considered. It is strongly recommended that, before selection, the graphs of noise against source resistance be studied carefully. These graphs are included in the data sheets for the instrument. EM manufacture a range of amplifier modules, some of which
are miniature units for mounting on to a printed circuit board. The range of
equivalent source resistance is from about 700 ohms to less than 1 ohm. Amplifiers can be made with lower noise still for special applications. |
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Queen's Award for Technological Achievement 1990. The DTI SMART Award. |