This paper presents a 0.01mm2 10MHz FLL-based RC frequency reference that does not require the use of resistors with complementary temperature coefficients (TCs), thus facilitating its implementation in a wide range of standard CMOS processes. Fabricated in a 0.18μm CMOS process, it achieves a frequency inaccuracy of ±0.28% from −45°C to 125°C after a 1-point trim, which is currently the best reported performance for a compact (<0.02mm2) frequency reference.

CMOS frequency references based on RC oscillators are usually preferred over bulky crystals in loT applications [1-5]. However, due to the process spread and finite temperature coefficient (TC) of most on-chip resistors, RC oscillators require trimming and temperature compensation to achieve decent accuracy. Enabled by high-resolution trimming techniques such as DeltaSigma [1], [2] or pulse-density [3] modulation, recent designs can obtain good accuracy (<0.1 %) at the expense of large chip area. However, existing compact (<0.02mm2) designs suffer from frequency errors in the order of 1% or more [4], [5]. Moreover, their temperature compensation schemes usually require the use of resistors with complementary TCs, which are not available in all CMOS technologies.@en