Performance Analysis of a Comparator Based Mixed-Signal Control Loop in 28 nm CMOS

In differential signaling systems using copper wires common mode signals are the cause of emission of electromagnetic energy. Especially in Automotive Ethernet systems this is a challenging problem. Beside classical passive components like common mode chokes active circuits can help to reduce the emission. This allows inexpensive and resource-conserving unshielded twisted pair cables to be used. This paper shows the approach of using a mixed-signal control loop based on a comparator and a 8 bit DAC for regulating the common mode voltage of an Automotive Ethernet DAC in 28 nm CMOS. An attenuation for interferers with frequencies up to 500 kHz is achieved and reaches up to 15 dB at maximum. The control loop utilizes the successive approximation algorithm commonly used for delay locked loops and DC trimming in mixed-signal circuits. In contrast to known applications the performance and usability at higher frequencies is considered in this paper. Being a nonlinear, time-variant system an analytical design of the control loop is very difficult. Therefore parametrical measurements show the dependency of frequency, amplitude and signal form of an applied common mode interferer source.