Study Of Lock-in Amplifiers Based On FPGA

The Lock-In Amplifier (LIA), with advantages of high detection capability, versatility and high reliability, and thus it has wide applications in weak signal detection, is equipment used to detect weak signals. With the development of digital technology, digital LIAs have been proposed to improve the performance, with benefits of lower drift and other non-ideal characteristics than analogy LIAs. So the digital LIAs have been developed greatly in many applications.On the basis of the survey of the existing LIAs in the market and in the laboratory, a digital LIA based on Field Programmable Gate Array (FPGA) has been developed. The improved design has been done according to the inadequacies of before design. The in-depth study, including noise and temperature characteristics, has also been researched as a good reference for further improvement design.The equivalent input noise is an important indicator of the LIA, and thus it is has been deeply analyzed from the four parts, including the preamplifier module, the AD module, the digital processing module and the DA module. Not only the amounts of noise for each part and the system have been tested, and the sources of noise for each part have also been analyzed, which is very important for further performance improvements. The tested system equivalent input noise can be as small as 3.5 nV/Hz when the gain of the preamplifier is 34 dB. Unfortunately the amount of the noise increases as the amplitude or the frequency of the input signal increase. This is due to the clock jitter of AD sampling, which is confirmed by the analysis and the measured results. The phenomenon appears at wave generation from DA in the same way. However, it can be eliminated by using high-quality dedicated clock system.At the same time the comprehensive indicators of the overall system have been tested, including the characteristics of the preamplifier, the system equivalent input noise, linearity, dynamic range and the temperature characteristics. The results show that the system equivalent input noise is stable on the time, the system has good demodulation linearity, and the dynamic range can be larger than 100 dB with small demodulation bandwidth. According to the tested results of the temperature characteristics of each module, it shows that the largest temperature drift is from the preamplifier, which is the main contribution of the system temperature drift. It has been tested that the system temperature drift in two cases with and without preamplifier can be-880 ppm/℃and 120 ppm/℃respectively. Meanwhile, the results have also been compared with SR844, a common digital LIA in the market, and the digital LIA based on FPGA is better than or equivalent to SR844 in many key features and it has advantages in power consumption, weight and volume.