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Title: Analog Building Blocks for Organic Smart Sensor Systems on Foil (Analoge bouwblokken voor organische slimme sensorsystemen op folie)
Other Titles: Analog Building Blocks for Organic Smart Sensor Systems on Foil
Authors: Marien, Hagen; M0317618
Issue Date: 1-Mar-2012
Abstract: Today's trend in mobile electronics towards smaller and lighter devices conflicts with the other market-driven interest for larger displays. Large displays can improve the user-friendliness of the devices. By using flexible and rollable displays made in organic electronics technology, small devices with large displays are enabled. In this technology a flexible plastic foil is used as the substrate for the electronic circuits. Asa result, the production of devices with flexible or even rollable displays is enabled. Besides the opportunities for flexible displays, there is also an interest for other appli-cations, such as RFID, printed solar cells, flexible lighting and organic smart sensor systems. For the realisation of these applications mostly digital building blocks are required. However, most of them also need analog building blocks for sensors, sensor read-out and analog-to-digital conversion. The design of analog organic circuits is the subject of this thesis. The main advantages of organic electronics technology are the low production temperatures, the low-cost and flexible plastic substrate and the printability. The production temperature for organic transistors stays below 150 degrees C during the process. This is very low compared to temperatures up to 1000 degrees C in standard Si CMOS technology. The low temperatures enable the use of low-cost flexible materials, such as a plastic foil, for substrates in the technology. Those materials would melt or burn at 1000 degrees C. Furthermore in recent years the solution-based deposition and the printing of organic electronics circuits have been reported. The combination of all these advantages creates a niche domain of large-area large-scale flexible applications. Despite all the advantageous properties, at the moment organic electronics technology suffers from some adverse properties that hamper the design of both digital andanalog organic circuits. The intrinsic gain of organic transistors is very low (typically 5). Only a limited component library is available at present. As active elements, onlyp-type transistors and, as passive elements, only capacitors are available. Resistive behavior is found through transistors biased in the linear regime. Furthermore, organic technology suffers from the variation of behavioral transistor parameters which result in mismatch and reliability issues for analog circuits. In this thesis the properties of organic electronics technology have been observed first.Then the design strategy for analog circuits has been adapted in such a way that adverse properties become harmless and that advantageous properties are fully exploited. One property of the organic transistors is the availability of a backgate pin. The use of this backgate pin is investigated and multiple techniques have been proposed to fully exploit this pin, e.g. for tuning the threshold voltage of the transistor.In this dissertation the design of several analog building blocks for organic smart sensor systems are presented. The presented building blocks are amplifiers, analog-to-digital converters, sensors and DC-DC converters. A single-stage differential amplifier is presented. Several analog techniques are applied to optimize the reliability and the gain of the single-stage amplifier. Bootstrapped gain enhancement is applied to the p-type load transistor. It combines both a low mismatch senstivity in DC and a high gain in AC for the load. Common-mode feedback is applied to reduce the mismatch sensitivity of the input transistors. Furthermore gate-backgate steering is used to increase the performance of all transistors. The amplifier has a gain of 15 dB . It is used in an AC-coupled 3-stage opamp. Subsequently, the design of a DC-connected 2-stage opamp is presented that applies the threshold voltage tuning technique to the input transistors in order to enable the DC connection of the stages. This 2-stage opamp has a gain of 20 dB . The design of a 1st -order DeltaSigma ADC is presented. It is built with an analog integrator, based on a 3-stage opamp. Furthermore, a high-gain comparator and a level shifter are used in the ADCs. The measured accuracy of the converter amounts to 26.5 dB in a 15.6 Hz signal bandwidth and at a clock speed of 500 Hz . The implementation of a 1D and a 2D flexible capacitive touch sensor is discussed in this work. The 1D sensor has a triangular capacitive touch pad with a position-dependent capacitance. In the sensor read-out, the charging current is copied and integrated on a fixed output capacitor. The sensor read-out has a sample rate of 1.5 k S / s . The simulated accuracy of the 1D sensor comes to 2 m m . The 2D flexible touch pad is based on the same working principle. An array of 4 by 4 pixels is measured in turns and the output signal shows the measured capacitance of the 16 pixels in a serial way. This output signal contains all the information about the position of the finger. According to simulations, the accuracy of the 2D sensor after interpolation is 0.3 m m . The design of an organic Dickson DC-DC converter is presented. It is built with a ring oscillator, with buffer stages and with a Dickson converter core. The switches in the forward path have been implemented with diode-connected transistors. The presented converter reaches high and low output voltages of 48 V and -33 V respectively at a power supply of 20 V . DC-DC converters are useful for biasing gates and back-gates with high or low voltages using the threshold voltage tuning technique. The proof of concept of this technique is given by the DC-connected 2-stage opamp which is biased with the high bias voltage provided by the Dickson DC-DC converter. The power consumption of the converter is negligible compared to the power consumption of the building blocks which are or can be biased by this converter. As a result, the contribution of this building block is validated.
Publication status: published
KU Leuven publication type: TH
Appears in Collections:ESAT - MICAS, Microelectronics and Sensors

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