Selecting the Correct Nanovoltmeter

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 voltage, which is proportional to the square of the resistance.
This noise defines the limit of measurement. EM low level DC voltage measuring instruments approximate to the source thermal noise characteristic for about three decades of source resistance, and in the case of the N11, and N31, for more than four decades.

When making a selection, a knowledge of the range of source impedance to be measured would be very helpful.
All EM Nanovoltmeters and Picovoltmeters are able to operate down to zero source impedance and the noise they generate is then their equivalent noise resistance (enr).
As the source impedance is increased, the source noise power is added to that of the instrument, and begins to become significant when the source noise approaches the equivalent noise resistance of the instrument. As the source impedance is increased still further, the total noise is only slightly higher than the source noise until, at some source resistance, the total noise begins to increase more rapidly than the source alone, i.e. an increase of about 10dB per decade of source resistance.
The point at which the two characteristics begin to diverge is the maximum source resistance at which the instrument will operate without adding significantly to the source noise.

The models N11 and N31 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.

Extra care should be taken when selecting a DC Picovoltmeter. The model P12 generates such a low level of noise, that its equivalent noise resistance is only 30 milli-ohms, or the thermal noise generated by about 30 cm of 0.5mm copper wire. With this level of performance, it is essential to ensure that the instrument is not swamped by the noise from the source, or even the connecting leads.
For example, a 1 metre twisted pair of 1.5 mm copper connecting leads has a resistance of about 20 milli-ohms at 20 degrees C. This would add significantly to the noise of the P12 without any source resistance at all!
As a general rule, if the resistance of the source to be measured, including the input leads is greater than about 0.1 ohms, the model P13 may be the more practical choice.

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 small amplifier modules for mounting on to a printed circuit board or for including into small areas. The range of equivalent noise resistance is from about 150 ohms to less than 10 ohms.

Amplifiers can be made with lower noise still for special applications.