EM Electronics.



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 userís control. The four gain control pins are left open circuit and the gain resistor is connected between pins 10 and 11 on the ĎDí 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.





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.


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.


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.



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.



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


Input Impedance.

Greater than 30 Megohms, 3 seconds after an input step

increasing with time to 1 Gohm, see input impedance graph.


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.


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.


Output Level.

The A20 will provide an output of plus or minus 3 volts and can

drive loads of up to 5mA.



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 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.

All unused pins should be left open circuit.


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



4 holes required M4 centres 8.4 inches by 5.6.inches.


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)

Click here for graph : Noise and Impedance Characteristics


About EM Electronics

Selecting the Correct Nanovolt Amplifier


Low Level Voltage Measuring Instruments


Ultra Low Level DC Voltage Amplifiers

Low Level DC Voltage Amplifier Modules

PC Boards for Modules 


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