A FPGA-based 7-order 1-bit sigma-delta modulator for high-precision signal generation

This paper presents a design of a FPGA-based 7-order 1-bit sigma-delta modulator with zero points residing in noise band. The modulator, incorporating with a 24-bit sine wave source, a 1-bit DAC and a subsequent reconstruction filter, generates a signal to test a high-precision sigma-delta ADC. The test signal, which contains both sine wave and high frequency noise, is sampled by the ADC under test. Usually, the noise in the test signal can not be fully suppressed by subsequent decimation filters of the ADC. So decimation filters should be taken into account for designing the modulator. To compensate for the noise suppression limitation of the decimation filters, zero points are introduced to the noise band of the modulator noise transfer function (NTF). With the adding of these noise band zero points, the SNR of ADC output data reaches 115 dB in the range of 0~500 Hz, which is about 10 dB higher than that of a system without these zero points.