Design and implementation of digital compensation method for the drooping sine response of Sigma Delta ADCs

Data Acquisition System (DAS) design is based on Sigma Delta Analog to Digital (A/D) Converters and Microcontrollers. These Sigma Delta converter uses inbuilt Sinc filters known as Cascaded Integrated Comb(CIC) to eliminate high frequency noise shaped by noise shaping. Sinc-in-frequency CIC filters, are almost universally used for decimation in delta-sigma ADCs, as they are easy to implement and nearly optimal for this use. However, these filters experience the problem of non-flat pass band (drooping response), and also from stop band ripples. This paper proposes the design and implementation of additional equiripple FIR filter to compensate the drooping response of Sinc filter. The advantage of this method is its design without using the multiplier unit and its implementation in any simple Microcontroller such as PIC Microcontroller. The FIR Filter has been designed using MATLAB and implemented using MPLAB Environment. The Sinc Filter and the FIR Filter are arranged in a cascaded form and the compensated response is the product of the Sinc and FIR Filter. The results of the implemented Compensator Filter in the PIC Microcontroller of the DAS are being discussed. This implementation is suitable for a present day high resolution aerospace telemetry system which uses embedded Sigma delta converters along with microcontroller or DSP processors.