A monolithic, constant-fraction discriminator using distributed R-C delay line shaping

A monolithic, CMOS, constant-fraction discriminator (CFD) was designed and fabricated in a 1.2-μ N-well process. This circuit uses an on-chip, distributed R-C delay line to realize the constant-fraction shaping. The delay line is constructed of a 4.8-μ wide, 500-μ long serpentine layer of polysilicon above a grounded second layer of polysilicon. This line generates about 1.1 ns of delay for a 5-ns risetime signal with a slope degradation of only 15%. The CFD also features DC feedback for both the arming and zero-crossing discriminators to eliminate timing errors caused by offsets. The entire circuit, including the delay line, requires an area of 200 μ×950 μ. The timing walk for 5-ns risetime signals over the dynamic range from -20 mV to -2 V was less than ±150 ps. Each channel of the CFD consumed ~15 mW from a single 5-V supply