Benefits of the patented common mode voltage rejection technology
Outstanding features of the AMS220 can be illustrated using the following example. (More detailed discussion of this issue can be found here.)
If you are attempting to measure a test resistor of 100 µΩ resistance by the use of the test current of 10 mARMS, then the voltage across the test resistor is 100 µΩ x 10 mARMS = 1 µVRMS (see Fig. 1). However, if one of the current leads in your experimental setup is connected to a signal ground of an experimental setup and resistance of the corresponding current path is 20 Ω, then the signals at the voltage sense inputs of a (grounded) voltage-sensing instrument will contain an AC component (20 Ω × 10 mA = 200 mVRMS), which is very much greater thanthe voltage difference of these signals (1 µVRMS); the corresponding common mode voltage (defined as the average value of voltage potential at sense inputs with respect to the signal ground) is 200.0005 mVRMS. Considering an industry standard lock-in amplifier with Common Mode Rejection Ratio (CMRR) of ~100 dB (i.e. with suppression of common mode voltage ~105 times), an estimate for common mode error (resulting from common mode voltage) is 200 mVRMS/105 = 2 µVRMS. Because the result provided by the lock-in amplifier is a sum of the voltage difference of the signals applied to its voltage-sensing inputs (1 µVRMS) and common mode error (2 µVRMS), the result provided by the lock-in amplifier will be 3 µVRMS instead of 1 µVRMS! Of course, this is an artificial result, which in this example is even 3-times greater than the real value!
If the AMS220 is used to excite the test resistance with enabled active common mode rejection circuit (see Fig. 2), this circuit suppresses the common mode voltage typically to the level of few microVolts, or less. Based on this fact, it can be estimated that for instruments having CMRR=100 dB (or greater) corresponding common mode error will not exceed tens of picoVolts. Taking into account sensitivity limitations of industry standard lock-in amplifiers, it can be concluded that resistance measurements utilizing AMS220 are not affected by common mode errors. Note that the AMS220 is the only current source on the market possessing (patented) active common mode rejection capability.
Fig. 1 Four-wire resistance measurement setup with a current source with one of the current outputs connected to the signal ground. The indicated measured value (affected by common-mode error) is illustrative with regard to the discussed example.
Fig. 2 Four-wire resistance measurement setup utlizing current source with active common mode rejection ( U.S. Patent #9,285,809).
Descriptions provided in this article are subject to change without notice.
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