A Thermistor-Based Temperature Sensor for a Real-Time Clock With <formula formulatype="inline"><tex Notation="TeX">$\pm$</tex> </formula>2 ppm Frequency Stability

Abstract

This paper describes the design of a temperature sensor based on integrated poly-silicon thermistors. The thermistors are incorporated in a Wien-bridge RC filter, which, in turn, is embedded in a frequency-locked loop. The loop's output frequency is then determined by the filter's temperature-dependent phase shift, thus realizing an energy-efficient and high resolution temperature sensor. After a 3-point calibration, the sensor achieves an inaccuracy of less than 0.12 C (min-max) from 40 C to 85 C. This translates into a frequency stability of better than 2 ppm from 40 C to 85 C when the sensor is used to temperature compensate the quartz-crystal oscillator of a 32 kHz real-time clock. The 0.09 mm sensor also achieves 2.8 mK (rms) resolution in a 32 ms conversion time while dissipating only 31 W.

A Resistor-Based Temperature Sensor With a 0.13 pJ $\cdot$ K2 Resolution FoM

“…Integrated temperature sensors are widely used for the temperature compensation of frequency references [1][2][3][4][5][6][7]. This is a demanding application, as it requires sensors that can simultaneously achieve high resolution, high energy-efficiency and high stability.…”

A 5800-$\mu$ m2 Resistor-Based Temperature Sensor With a One-Point Trimmed Inaccuracy of ±1.2 °C ($3\sigma$ ) From −50 °C to 105 °C in 65-nm CMOS

“…A single-ended output buffer employs a level shifter for rail-to-rail operation and an inverter-based latch for 50 % duty cycle. The loop filter uses a large gm (100 μS) and small CINT (1.6 pF) to achieve a loop bandwidth of 160 kHz, which is wide enough to effectively reduce the phase noise of the CCO [5]. Fig.…”

A 0.25 mm2-Resistor-Based Temperature Sensor With an Inaccuracy of 0.12 °C (3<inline-formula> <tex-math notation="LaTeX">$\sigma$ </tex-math> </inline-formula>) From −55 °C to 125 °C

“…Despite the reduction of bridge sensitivity due to process spread, the residual error agrees well with simulations made in the TT corner (maximum error < 0.3°C). As in [4], [6], [12], this error can then be removed by a fixed polynomial.…”

“…Depending on their choice of reference, two classes of resistor-based temperature sensors can be identified: RC-based Spacekeeper for footnotes sensors [2]- [4], [6], [14], which use a frequency reference to digitize a temperature-dependent RC time constant; and dualresistor-based sensors [5], [12], [15], which digitize the resistance of a sensing resistor with respect to another resistor. As discussed in [20], RC-based sensors can achieve better stability and accuracy, because on-chip MIM capacitors are more stable and spread less, than on-chip resistors.…”