We develop and produce instruments optimized for very-high-sensitivity resistance measurements using lock-in amplifiers and DAQ devices. Our instruments are particularly dedicated for measurements in many critical applications, where reliable sensing of very small signals in the background of high disturbances is problematic.
The mission of ANMESYS s.r.o. is to offer a recently patented technology of new generation current sources with active common mode rejection for research and industrial applications with the aim to enhance performance of precision resistance measurements, and enable very-low resistance measurements to become a routine even in the most demanding conditions.
Original solutions for very-low resistance measurements
When micro-Ohm or nano-Ohm resistances have to be measured precisely using very low excitation currents, let's say currents of order of milliAmperes or less, then very small voltage signals, typically nano-Volts or less, have to be sensed with a reasonable resolution. Due to parasitic signals, such as thermoelectric or electromagnetic interference signals, which are superimposed with a signal of interest, and are usually greater or much greater than the signal of interest, precise DC measurements of very-low resistances become very difficult, or even impossible.
AC signals at the nano-Volt level can be properly sensed using advantageous AC techniques; however, AC measurements of very-low resistances are frequently associated with the following problems:
The problems described here are frequently believed to be a consequence of “ground loops” or “not proper shielding” of a measurement setup, but in reality, they are often a consequence of high common mode voltage, as illustrated below.
If resistance of current leads (say 20 Ohms) is very much higher than resistance of a tested resistor (let's say 100 micro-Ohms), then voltage across the tested resistor is very much less than the voltage drop on the current leads, thus a very small signal at presence of very high background (common mode voltage) has to be measured. This, however, creates optimal conditions for occurrence of common mode errors (see detailed explanation here) and observation of the typical problems described above. The utilization of the AMS220 Voltage Controlled Current Source with Active Common Mode Rejection in the AC resistance measurement setups for the current excitation of the tested resistor enables the exact solution of problems originating in common mode voltage.
Resistance measured by the AC-technique (e.g., using AC-resistance bridge or lock-in amplifier combined with AC-current source) differs from that obtained by the DC-technique. Typically, measured AC-resistance is greater or much greater than DC-resistance.
Resistance measurements of the same D.U.T. performed by different instruments (e.g., AC-resistance bridge, lock-in amplifier, AC-current source) yield different results.
The use of the same instrumentation (e.g., AC-resistance bridge or lock-in amplifier combined with AC-current source) to measure AC-resistance of the same D.U.T. by using different signal wires (e.g., because of different wiring in different cryostats) yields different results.