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New current source for measurements of very small resistances

AMS220: current source for measurements of very small resistances

When measuring very small resistances, a common effect is the so-called common mode rejection  (CMR). The error that occurs from this effect can heavily distort the measured voltage. Fig. 1a shows this distortion with a determined resistance of 100 mΩ. A common current source provides a current of 10 mA and the voltage we measured is an incorrect 3 µV instead of 1 µV. The reason for this is an assumed but realistic value resistance of 20 Ω between the earth, sample and current source.

Current source
Fig. 1a: Four-wire resistance measurement with common current source connected to ground. The indicated measured voltage value deviates considerably from the voltage actually applied to the sample.
Fig. 1b: Four-wire resistance measurement with current source that actively suppresses CMR. The indicated measured voltage value corresponds to the actual voltage applied to the sample.

Our new partner Anmesys has developed a special current source that can suppress the CMR effect: AMS220. It comes with a lock-in amplifier and can also be used as regular current source. 

Fig. 1 shows the setup with AMS220: The CMR effect is suppressed and as a result, the correct voltage of 1 µV is detected. Contrary to regular sources, the patented AMS220 is free-floating. 

The current is directly controlled by input voltage. Fig. 2 shows a typical setup for an AC resistance measurement with lock-in amplifier and AMS220. 

Fig. 2
Fig. 2

The common mode rejection effect may occur

  • with samples with small resistances (and high contact resistances, where applicable)
  • during AC-resistance measurements, for example with lock-in technique
  • during resistance measurements in cryogenics, when the sample resistance decreases with low temperatures
  • with thermometry 
Frequency range DC – 200 Hz (“active CMR” switched on)
DC – 20 kHz (“active CMR” switched off)
Input control voltage range 3.6 VRMS / ±5 V (DC)
Current-voltage relationship 1 µA/V to 10 mA/VMax. AC current (RMS) 36 mA
Max. DC current ±50 mA 

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Simone Paziani
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