EM Electronics.


The EM DC Nanovoltmeter model N51 is the latest and most advanced instrument in our range and is the result of years of development in the measurement of very low level DC potential. All the experience gained from the EM range of Nanovoltmeters has gone into the design of the N51 and some new and important facilities have been added. The new N51 has better cryogenic performance in that the voltage and ‘current’ noise curves have been pushed further apart, thus giving efficient measurement over a wider range of source impedance at cryogenic temperature. This is a very important characteristic when making measurements at cryogenic temperature.

The new N51 has an optically isolated output as well as range and filter control, making it suitable for use with automatic systems.

Special care has been taken to ensure that the input stage causes the least possible disturbance to the subject being measured, and the N51 is designed to operate in sensitive circuits at very low levels. With a little care, it is possible to resolve 100 picovolts, with input impedance which increases to over 1000 Mohm. If the N51 is used with a digital voltmeter, 100 picovolt bits may be displayed with a minimum of flicker. The instrument is also designed to have a high degree of linearity.

One of the very important features of the N51 is its ability to measure sources with a wide range of impedance, by following the source thermal noise characteristic closely from 6 ohms to over 100 Kohm. This means that the N51, while remaining at room temperature, may be used efficiently to measure a source at helium temperature, and even at helium three temperatures for some measurements. (See the graph of noise voltage against source resistance)

The excellent stability of this instrument makes it ideal for long term drift measurements, and it may be connected to a recorder and left unattended for long periods.

One new feature of the N51 is the ability to control the range and filter functions remotely via a connector at the rear of the instrument. The gain of the N51 may be set remotely with the values of 1K, 10K, 100K, 1M and 10M, by applying a potential of +5v to the relevant pin on the 9 way D connector, at the rear of the instrument, with reference to isolated 0v, which is the case and screen of the plug. Filter time constants from 220mS to 10S may be set remotely.

The optically coupled output has a good linearity and a very high isolation resistance.

The six position low pass filter gives the operator control over the response time and reduction of noise from the circuit being measured.

A calibrated offset voltage control is provided, with an 11 position multiplier switch, giving a maximum back off capability of plus and minus one millivolt. This allows measurement of small changes in comparatively large signals. The level of input which can be offset on any range will be limited only by the resolution of the calibrated potentiometer, i.e., better than 0.01%.

The pure copper input terminals ensure that the very high thermal stability of the input amplifier is maintained, and also that connection to the instrument may be made easily.

There are two outputs at the rear of the instrument, one direct and the other isolated. When a high linearity is required, the direct output should be used, if possible, although the isolator also has a good linearity.

The N51 is battery operated from internal chargeable batteries and so has the advantage of complete isolation.

The batteries give operating time of 100 hours per charge. There is an input where an isolated low current charger may be connected to give continuous operation.

This instrument is engineered to a very high standard and comes with a plug on terminal shield to reduce the effects of draughts.




Click here for graphs : Noise and Impedance Characteristics


Input 1. 12 ranges. +/- 6nV fsd to +/- 2mV fsd
Input 2. +/- 6mV fsd to +/- 200V fsd


Direct Output. +/- 0.2%
Isolated Output. +/- 0.5%
Meter. +/- 1.5%


Direct output better than 100 ppm


Zero Drift
Better than 500pV per degree C.


Equivalent noise resistance approximately 6 ohms. Noise voltage depends on filter setting, e.g., filter time constant ten seconds gives peak to peak noise voltage of approximately 200 picovolts with the input short circuited.


Source Resistance
With a source resistance of less than 6 ohms, the instrument noise is substantially constant down to zero ohms. With source resistance from 6 ohms to over 100Kohm, the instrument adds insignificantly to the source noise. Above about 200Kohm the instrument noise increases at about 20dB per decade of source resistance.


100 picovolts.


Input Impedance
Input 1. Greater than 10Mohm, two seconds after a step input, increasing with time to over 1Gohm.
Input 2. 10Mohm


Offset Current
Adjustable to zero at the front panel. Temperature coefficient better than 1pA per degree C.


Response Time Constant
Depends on setting of filter and range, Minimum. 250 milliseconds. Maximum. 50 seconds.


Six position low pass filter operated by a switch on the front panel giving time constants of 250 milliseconds to 50 seconds, or the remotely controlled filter time constant 250 milliseconds to 10 seconds, depending on range selected.


Ten turn potentiometer calibrated 0-1000 with 11 position switch giving maximum offsets of plus and minus 100nV, 1uV, 10uV, 100uV, 1mV and a '0' position. Resolution. Better than 0.01%. Accuracy. Better than 0.5%.



Series Mode Rejection
Greater than 80dB at 50Hz.


Direct and isolated outputs give plus and minus 2V for full scale deflection. Amplifier gain equals the reciprocal of the range setting in volts.


Input to case. Greater than 100Mohm.

Input to Output: Greater than 1Gohm


Inputs. EM low thermal emf terminals for 'Input 1'.

Screw posts with 4mm sockets for 'Input 2' and 'Case’.

Plug on terminal cover provided.

Outputs. Direct: 4mm sockets at rear.

Isolated: 4mm sockets at rear and on 9 way plug.

9 way D Plug for Isolated Range and Filter Control.

Pin Number. Function.

1. 1K Gain + 5v Input

2. 10K Gain + 5v Input

3. 100K Gain + 5v Input

4. 1M Gain + 5v Input

5. Filter 1 + 5v Input

6. Filter 2 + 5v Input

7. Isolated Supply +5v

8. Diode Output

9. DC Output

Case. 0V

If all gain inputs are left at 0v, the gain will be 10M

If both filter inputs are left at 0v, the filter will have time constant of 100mS


105mm mirror scale calibrated 20-0-20, and 6.32-0-6.32.


Power Supply
Internal chargeable batteries, giving 100 hours operation from a full charge. Mains powered charger fully charges the batteries in 16 hours with the instrument switched off or 20 hours when in use. Power required 240V or 110V. 50Hz or 60Hz. 5VA. There is an alternative input to charge the batteries from an external, low noise, current source. Minimum voltage. 20V. Battery capacity. 2AH.


Width. 314mm (12.4")
Depth. 322mm (12.7")
Height. 179mm (7") Including folded feet.


18Kg (40lbs)


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Low Level Voltage Measuring Instruments


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