A 0.028-mm² 32 -MHz <i>RC</i> Frequency Reference With an Inaccuracy of ±900 ppm From -40 ^∘C to 125 ^∘C and ±1600 ppm After Accelerated Aging

This article describes the design and implementation of a compact CMOS RC frequency reference based on N-type diffusion (N-diff) resistors and metal-insulator–metal (MIM) capacitors. It consists of a frequency-locked loop (FLL) that locks the period of a voltage-controlled oscillator (VCO) to the time it takes a current source to charge a capacitor to a reference voltage. Conventionally, the temperature compensation of such FLLs involves the use of resistors with different temperature dependencies. In this work, however, this is done by using two bipolar junction transistor (BJT)-based current sources with different temperature dependencies to charge a MIM capacitor and generate a reference voltage across an N-diff resistor, respectively. Implemented in a standard 180-nm technology, the resulting frequency reference achieves small size (0.028 mm2), moderate inaccuracy (±900 ppm) from <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${-} 40~{^{\circ }}$ </tex-math></inline-formula>C to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$125~{^{\circ }}$ </tex-math></inline-formula>C, and low drift (±1600 ppm) after accelerated aging. The versatility of the proposed temperature compensation scheme is validated by replacing the N-diff resistor with a P-poly resistor. However, the latter exhibits greater inaccuracy (+2000/−2500 ppm) and more drift (−2600/−8100 ppm) after accelerated aging.