Fully differential current-input CMOS amplifier front-end suppressing mixed signal substrate noise for optoelectronic applications

In recent optoelectronic communication systems, microprocessors tend to be imbedded on-chip with analog interface circuitry. This results in a critical substrate noise issues for mixed-signal chip designers because switching transients in digital MOS circuits can interfere with analog circuits integrated on the same die by means of coupling through the substrate. In order to optimize the dynamic range of the system and to minimize the sensitivity to substrate noise, many noise-reduction techniques, such as a P+ guard ring, a N-well guard ring, trench oxide isolation, and MOSCAP have been developed and employed to suppress substrate noise generated by clocking of the digital circuitry in microprocessor. In this paper, a fully differential method is described, which is used to reduce the substrate noise effect caused by the microprocessor. This approach has been implemented in a communications data processing application, in which the microprocessor is located next to the analog current-input optical data receiver and quantization circuits which have a sensitivity of -28 dBm and variable gain characteristic for power efficiency. Both simulated and experimental results of this design approach are presented herein