Lock-in amplification based on sigma-delta oversampling

Abstract

Synchronous detection is used to detect and measure very low-level signals in the presence of significant noise. A defining characteristic of this measurement approach is the use of a periodic probe signal to excite the system under test. This is followed by mixing of the reference signal and its phase-quadrature with the measured signal. Standard analog to digital converters are employed, usually with the mixing and filtering performed digitally. Most practical high-resolution analog to digital converters employ oversampled sigma-delta modulation and are incorporated as a separate functional block. This paper derives a processing algorithm that combines the oversampled analog to digital conversion with signal mixing into one functional block. There are several important advantages of this approach. The computational complexity of the lock-in amplifier is substantially reduced, with no loss of accuracy. Moreover, the requirement for high-resolution analog-to-digital conversion is relaxed; it is replaced with low-resolution high-rate sampling, which is typically much easier to realize in practice. Experimental results are presented to demonstrate the correctness of the technique as determined via theory and simulation.