CMOS A/D converter implementation for IMU applications

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2013
Nuzumlalı, Ömer Lütfi
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Today’s digital output accelerometers do not reach navigation grade performance; therefore they cannot be used in navigation grade inertial measurement units (IMU). Although analog output accelerometers meet the navigation requirements, their outputs have to be digitized in order to be used in navigation processors to run navigation algorithms. However, the performance of the digitizer circuit has to be higher than the accelerometers so as not to degrade the overall performance. This work presents a design of a high resolution CMOS based current to frequency converter (CFC), also known as integrating analog to digital converter (IADC), especially for current output navigation grade accelerometers. Contrary to the traditional designs, positive and negative input currents are separated from each other to eliminate the error due to both comparator offset voltages and mismatch in the value of integrating capacitor. Also, the output current of the accelerometer is directly integrated to get rid of any error originated from resistive current divider circuitry by the help of a superior folded cascode operational amplifier with Monticelli output stage. This operational amplifier can drive 150 mA by dissipating only 600 µA quiescent current at the output stage. In addition, a current equalization mechanism is added to the circuit in order to reduce scale factor asymmetry errors. This mechanism balances positive and negative feedback currents at each temperature; as a result scale factor asymmetry error is reduced to 20 ppm without any calibration in Cadence simulations. Design and fabrication of the integrating analog to digital converter was done in a standard 0.35 µm CMOS technology. The fabricated chip size with pads is 8.38 mm2. With the designed and fabricated converter, single-axis accelerometers were tested for acceleration levels in the range of ±25 g. The implemented system achieves 0.54 µg/ velocity random walk coefficient, 0.9 µg bias instability and 0.0002 m/sec quantization error with the current output navigation grade QA2000 accelerometer from Honeywell Inc. at stationary zero g condition. Compared to the digitization mechanism using ADS1281 32 bit Sigma Delta (ΣΔ) converter from Texas Instrument, this system improves the performance of velocity random walk coefficient two times and bias instability one and half times. Furthermore, quantization error is approximately ten times better than the commercial navigation grade IMUs.
Analog-to-digital converters., Accelerometers., Inertial navigation., Metal oxide semiconductors, Complementary.
http://etd.lib.metu.edu.tr/upload/12616449/index.pdf
https://hdl.handle.net/11511/23042
Graduate School of Natural and Applied Sciences, Thesis

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Citation Formats
Ö. L. Nuzumlalı, “CMOS A/D converter implementation for IMU applications,” M.S. - Master of Science, Middle East Technical University, 2013.
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