Balaji Jayaraman

High Precision 16-bit Readout Gas Sensor Interface in 0.13μm CMOS

A high precision 16-bit readout circuit for gas sensor interface is realized in 0.13μm CMOS technology. The front end signal conditioning circuit comprises a low power resistance-to-time period converter with subthreshold operation of transistors. The time-period proportional to the transducer output is digitized using a counter based frequency-to-digital conversion (FDC) technique. In order to handle the sensor signal with a dynamic range of 160dB, subranging technique is employed to provide feedback to front end circuit interfacing the sensor. While the 3 MSBs of 16-bit word restrict the input frequency to FDC between 1MHz and 10MHz, the 13-bit counter operates on this signal to provide a worst case resolution of 0.4%. The power consumption of the readout circuit is 366μW from a 1.2V supply.

Thermo-mechanical characterization of surface-micromachined microheaters using in-line digital holography

This paper describes the application of lensless in-line digital holographic microscopy (DHM) to carry out thermo-mechanical characterization of microheaters fabricated through PolyMUMPs three-layer polysilicon surface micromachining process and subjected to a high thermal load. The mechanical deformation of the microheaters on the electrothermal excitation due to thermal stress is analyzed. The numerically reconstructed holographic images of the microheaters clearly indicate the regions under high stress. A double-exposure method has been used to obtain the quantitative measurements of the deformations, from the phase analysis of the hologram fringes. The measured deformations correlate well with the theoretical values predicted by a thermo-mechanical analytical model. The results show that lensless in-line DHM with Fourier analysis is an effective method for evaluating the thermo-mechanical characteristics of MEMS components.

Thermal characterization of microheaters from the dynamic response

Thermal characterization of surface-micromachined microheaters is carried out from their dynamic response to electrothermal excitations. An electrical equivalent circuit model is developed for the thermo-mechanical system. The mechanical parameters are extracted from the frequency response obtained using a laser Doppler vibrometer. The resonant frequencies of the microheaters are measured and compared with FEM simulations. The thermal time constants are obtained from the electrical equivalent model by fitting the model response to the measured frequency response. Microheaters with an active area of 140 µm × 140 µm have been realized on two different layers (poly-1 and poly-2) with two different air gaps (2 µm and 2.75 µm). The effective time constants, combining thermal and mechanical responses, are in the range of 0.13–0.22 ms for heaters on the poly-1 layer and 1.9 µs–0.15 ms for microheaters on the poly-2 layer. The thermal time constants of the microheaters are in the range of a few microseconds, thus making them suitable for sensor applications that need a faster thermal response.

Performance Analysis of Subthreshold Cascode Current Mirror in 130 nm CMOS Technology

This paper reports on the systematic analysis of impact of biasing the transistors in subthreshold versus saturation conditions on analog circuit performance. A cascode current mirror, designed in 130 nm CMOS technology, is biased in subthreshold and saturation regions and its performance has been analyzed through rigorous analytical modeling. The analytical results have been validated with SPICE simulations. We demonstrate that the subthreshold operation provides better performance in terms of linearity, power, area, output impedance and tolerance to temperature variation, making it a preferable option for applications such as signal conditioning circuitry for environmental sensors. On the other hand, biasing the circuit in saturation is preferable for applications like transceivers and data converters where high bandwidth, SNR and low sensitivity to process variations are the key requirements. Based on this analysis, we have prototyped a sensor interface circuit for resistance measurement on 130 nm CMOS technology, using subthreshold cascode current mirrors as the key building blocks. This current mirror results in 14x lower power compared to above-threshold operation. The interface circuit spans 5 orders of magnitude of resistance, and consumes an ultra low power of 326 μW.

Integrated CMOS gas sensors

We present a low power gas sensor system on CMOS platform consisting of micromachined polysilicon microheater, temperature controller circuit, resistance readout circuit and SnO2 transducer film. The design criteria for different building blocks of the system is elaborated. The microheaters are optimized for temperature uniformity as well as static and dynamic response. The electrical equivalent model for the microheater is derived by extracting thermal and mechanical poles through extensive laser doppler vibrometer measurements. The temperature controller and readout circuit are realized on 130nm CMOS technology. The temperature controller re-uses the heater as a temperature sensor and controls the duty cycle of the waveform driving the gate of the power MOSFET which supplies heater current. The readout circuit, with subthreshold operation of the MOSFETs, is based on resistance to time period conversion followed by frequency to digital converter. Subthreshold operatin of MOSFETs coupled with sub-ranging technique, achieves ultra low power consumption with more than five orders of magnitude dynamic range. RF sputtered SnO2 film is optimized for its microstructure to achive high sensitivity to sense LPG gas.