This paper proposes the design of ASIC for weather monitoring application and to implement it using FPAA (Field Programmable Analog Array) and FPGA (Field Programmable Gate Array). Three atmospheric parameters such as temperature, humidity and CO2 are considered for experimentation. Sensor signals are ported to FPAA through input ports for analog signal processing. The serial digital output data from FPAA is fed to custom designed digital processor in FPGA and the real time atmospheric parameters are displayed in LCD display. The simulation results are obtained for the custom designed processor using Xilinx Spartan-3AN tool and design was optimized for better performance. Hardware implementation of the proposed ASIC mixed signal processer uses Anadigm AN231E04 for analog processing and Xilinx Spartan-3AN FPGA for digital processing. The experiment result shows the good circuit integrity and functionality, and also faster computational response.