Field programmable gate arrays (FPGAs) are now considered as a real alternative for digital signal processing (DSP) applications. But, new methodologies are still needed to automatically map a DSP application into an FPGA with respect to design constraints such as area, power consumption, execution time and time-to-market. Moreover DSP applications are frequently specified using floating-point arithmetic whereas fixed-point arithmetic should be used on FPGA. In this paper, a high-level synthesis methodology under constraints is presented. The originality is to consider a computation accuracy constraint. The methodology is based on a fixed-point operator library which characterizes the operators cost according to their wordlength. An error noise propagation model is used to compute an analytical expression of the accuracy in function of the signals wordlength. To obtain an efficient hardware implementation, the data wordlength optimization process is coupled with the high-level synthesis. In addition, the accuracy evaluation is done through an analytical method, which drastically reduces the optimization time.