A microcontroller-based lock-in amplifier for sub-milliohm resistance measurements

This paper presents a novel approach to the design of a digital ohmmeter with a resolution of <60 mu Omega based on a general-purpose microcontroller and a high-impedance instrumentation amplifier only. The design uses two digital I/O-pins to alternate the current through the sample resistor and combined with a proper firmware routine, the design is a lock-in detector that discriminates any signal that is out of phase/frequency with the reference signal. This makes it possible to selectively detect the mu V drop across sample resistors down to 55.6 mu Omega using only the current that can be supplied by the digital output pins of a microcontroller. This is achieved without the need for an external reference signal generator and does not rely on the computing processing power of a digital signal processor. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4731683]