Frequency Programmable Lock-in Amplification Circuit for the Measurement of Liquid Low Electrical Conductivity

Based on Direct Digital Synthesis (DDS) technique, a lock-in amplification circuit was designed with programmable working frequency between 0 and 1.25 MHz and automatic phase tracing ability. It was used for the measurement of liquid electrical conductivity. Experimental results show its resolution is better than 0.001 μS at the measurement range of 1-20 μS. As a general lock-in amplifier design, this circuit can also be used for other applications where weak signals are to be measured.