10MHz RC oscillator errs ±0.28% over -45 to 125°C
RC oscillators could be small or correct, and now that may be each after Delft College of Know-how*, Silicon Built-in and Tsinghua College revealed a 10MHz RC oscillator with ±0.28% accuracy over -45 to 125°V after a one-point trim, fabricated with a 0.01mm2 footprint in normal 0.18μm CMOS.
The scheme avoids needing the necessity for resistors with opposing temperature coefficients, which aren’t out there on all processes.
As an alternative it makes use of a frequency-locked loop the place the RC and a change act as a frequency to voltage converter for an oscillator (see diagram).
The ensuing voltage is in contrast with one other one, this time from a potentiometer constructed from R1, a p-poly (-0.02%/°C) resistor and R2 a trimmable mixture of p-poly and n-poly (-0.15%/°C).
The oscillator is voltage-controlled, and pushed by the distinction (through an integrator) between the RC-derived voltage and the potentiometer voltage – making the frequency-locked loop.
C in RC is a metal-insulator-metal capacitor with a coefficient of ~30ppm/°C, and R is one other -0.02%/°C p-poly resistor.
With crafty design, R2 could be made to maintain the identical resistance worth however have its coefficient swept throughout -40ppm/°C to 40ppm/°C in 16 steps by a digital trimming code – making temperature coefficient of the entire oscillator trimmable, permitting general drift to be minimised.
As C is applied as a capacitive DAC, frequency will also be trimmed digitally.
Consumption of the 100 x 100μm block is 27.5μA analogue plus 29.2μA digital (most for the output buffer) from 1.5V. Over 1.5 to 1.8V, frequency shifts 2,700ppm.
The temperature response is a hill, with 9.987MHz (on common) at -40 and +125°C, and 10.01MHz at 40°C with rising temperature, or 10.02MHz 40°C with falling temperature – the completely different values a results of hysteresis (1,500ppm worst-case).
Ageing at at 150°C for every week revealed drift, which is work-in-progress.
In one other RC oscillator paper at ISSCC 2023, the College of Illinois, Urbana regarded deep into the ageing of RC oscillators, pointed the finger at drifty p-poly resistors, and revealed a 100MHz oscillator with ±1,030ppm inaccuracy throughout -40 to 85°C after soaking at 125°C for 500 hours.
Its compensation scheme achieved a 1,000 hour 125°C drift of 500ppm, in contrast with 5,000ppm of getting older drift with out compensation.
The design can also be a frequency-locked loop, however with out the temperature-response-adjusted voltage reference.
It is a much more complicated oscillator that the one above, lastly occupying 0.22mm2 on a 65nm CMOS die, packaged in a plastic QFN.
Many issues have been found that may be exploited to enhance accuracy over time.
One discovering was that p-poly resistors drift so much with age, whereas n-poly and through resistors are higher.
One other was that present in a single path via some on-die parts makes them age in a single path, typically via electro-migration, so that they drift much less if the structure makes present stream in each instructions because the oscillator cycles.
The third vital discovery was that merely working makes an RC oscillator drift as time goes by, whereas an un-powered oscillator on the identical substrate drifts far much less.
Armed with the final piece of data, the ultimate design has a constantly operating oscillator, plus one other oscillator that’s solely run briefly (as soon as an hour) at 0.1% responsibility cycle. This output of this ‘younger’ oscillator is used to replace the calibration of the continual oscillator.
Consumption is 1.4μW/MHz.
ISSCC 2023 paper 3.4
A 0.01mm2 10MHz RC frequency reference with a 1-point on-chip-trimmed inaccuracy of ±0.28% from -45°C to 125°C in 0.18μm CMOS
ISSCC 2023 paper 3.5
A 1.4μW/MHz 100MHz RC oscillator with ±1030ppm inaccuracy from -40°C to 85°C after accelerated getting older for 500 hours at 125°C
*see additionally Delft College of Know-how in Electronics Weekly’s ISSCC 2023 Class-D amplifier article right here
The IEEE Worldwide Stable-State Circuits Convention, held yearly in San Francisco, is a world showcase for IC-based analogue, digital and RF circuity. It gives a chance for IC and circuit design engineers to keep up technical foreign money, and to community with consultants.
