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PTB chopper-stabilized amplifier

Used to improve measurements on thermocouples within the scope of the Avogadro Project

PTB-News 2.2015
Especially interesting for

metrology institutes, fundamental research

calibration laboratories

manufacturers of high-precision electronics

A chopper-stabilized amplifier with excellent properties has been developed at PTB and is used in a measurement set-up for the Avogadro Project to measure voltage on thermocouples. This has led to a reduction in noise amplitude by a factor of 4.

Measurement of the thermoelectric voltage difference across both junctions of a thermocouple, represented as the corresponding temperature difference (red curve). At a constant temperature difference, the result expected would be similar to the measurement of a short circuit (plotted here as a black curve for comparison purposes). Using the chopper-stabilized pre-amplifier (in the diagram after 8.5 hours) causes the noise amplitude to decrease by a factor of approx. 4, down to effectively 0.1 mK (peak to peak).

To measure small DC voltages, PTB has developed a chopper-stabilized amplifier with excellent properties. The amplifier exhibits very fast settling and an extreme-ly low voltage noise of only 0.73 nV/√Hz down to frequencies of a few millihertz. At the typical chopping frequency of 570  Hz, a current noise of 40  fA/√Hz with a 1/f corner frequency of 3  mHz is achieved. This amplifier had originally been designed as a null detector for the cryogenic current comparators developed by PTB, but the instrument is also very well suited for other demanding metrological applications such as high-precision voltage measurements on thermocouples.

For the redefinition of the Avo-gadro constant, the ratio of the volume of a silicon sphere to the volume of the unit cell of the sil-icon crystal must be determined. Special interferometers are used in vacuum to measure each of these two geometrical quantities with relative uncertainties lower than 10–9. Due to the low thermal expansion coefficient of approx. 2.5 · 10–6 K–1, the temperature of the silicon sphere must be measured accurately to a millikelvin or better. For this purpose, thermocouples – in combination with a platinum resistance thermometer – are used which, due to their small dimensions, can be mounted very flexibly on the test sample and, in addition, exhibit no self-heating. With approx. 40  µV/K, such thermocouples, however, have very poor sensitivity, which represents a considerable challenge when measuring the thermoelectric voltages. In this measurement set-up, the chopper-stabilized amplifier has now been used as a low-noise pre-amplifier – compared with the previously used commercial nano-voltmeter. The figure shows the results of a measurement series where the difference in the thermoelectric voltages has been measured in the two junctions of the thermocouple. According to these results, using the chopper-stabilized amplifier reduces the noise amplitude by a factor of 4. Aiming for the same measurement uncertainty, the chopper-stabilized amplifier thus enables the measuring time to be reduced by at a factor of 16, alternatively. At the same measuring time, the measurement uncertainty achieved is four times smaller.


Dietmar Drung
Department 7.2 Cryophysics and Spectrometry
Phone: +49 (0)30 3481-7342
E-mail: dietmar.drung(at)ptb.de

Scientific publication:

D. Drung, J.-H. Storm: Ultra-low noise chopper amplifier with low input charge injection. IEEE Trans. Instrum. Meas. 60, 2347-2352 (2011)