CMOS A/D converter implementation for IMU applications

Nuzumlalı, Ömer Lütfi
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.


Capacitive cmos readouts for high performance mems accelerometers
Sönmez, Uğur; Külah, Haluk; Akın, Tayfun; Department of Electrical and Electronics Engineering (2011)
MEMS accelerometers are quickly approaching navigation grade performance and navigation market for MEMS accelerometer systems are expected to grow in the recent years. Compared to conventional accelerometers, these micromachined sensors are smaller and more durable but are generally worse in terms of noise and dynamic range performance. Since MEMS accelerometers are already dominant in the tactical and consumer electronics market, as they are in all modern smart phones today, there is significant demand for...
CMOS readout electronics for mis-matched and mode-matched MEMS gyroscopes
Yeşil, Ferhat; Akın, Tayfun; Department of Electrical and Electronics Engineering (2015)
This thesis presents the CMOS readout electronics for both mismatched and mode-matched MEMS gyroscopes. A systematic design of MEMS gyroscope's control loop parameters, which is insensitive to sensor parameters and environmental conditions, is necessary for robust and high performance operation. Extra to the systematic design for high performance operation, some special techniques should be used to further increase the performance of the sensor. In this thesis, as a performance increasing technique, mode-ma...
Phase sensitive detection of motor fault signatures in the presence of noise
Akin, Bilal; Toliyat, Hamid; Orguner, Umut; Rayner, Mark (2007-05-05)
In this paper, digital signal processor (DSP)-based phase-sensitive motor fault signature detection is presented. The implemented method has a powerful line current noise suppression capability while detecting the fault signatures. Because the line current of inverter driven motors involve low order harmonies, high frequency switching disturbances, and the noise generated by harsh industrial environment; the real-time fault analyses yield erroneous or fluctuating fault signatures. This situation becomes a s...
Noise analysis and characterization of a sigma-delta/capacitive microaccelerometer
Külah, Haluk; Yazdi, N; Najafi, K (2006-02-01)
This paper reports a high-sensitivity low-noise capacitive accelerometer system with one micro-g/root Hz resolution. The accelerometer and interface electronics together operate as a second-order electromechanical sigma-delta modulator. A detailed noise analysis of electromechanical sigma-delta capacitive accelerometers with a final goal of achieving sub-jig resolution is also presented. The analysis and test results have shown that amplifier thermal and sensor charging reference voltage noises are dominant...
Identification of inertial sensor error parameters
Altınöz, Bağış; Leblebicioğlu, Mehmet Kemal; Department of Electrical and Electronics Engineering (2015)
Inertial sensors (gyroscopes and accelerometers) that are used in navigation systems have distinct error characteristics such as bias, scale factor, random walk, etc. Calibration and characterization tests are done with 2 or 3 axes rate tables in order to identify these errors. It is possible to utilize error characteristics of these devices, and the navigation accuracy is directly dependent on the accuracy of this identification process. In this thesis, inertial sensor error parameters are identified by a ...
Citation Formats
Ö. L. Nuzumlalı, “CMOS A/D converter implementation for IMU applications,” M.S. - Master of Science, Middle East Technical University, 2013.