Dietmar Schroeder

Technische Universität Hamburg-Harburg, Hamburg, Hamburg, Germany

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Publications (45)22.25 Total impact

  • 48th annual conference of the German Society for Biomedical Engineering; 10/2014
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    ABSTRACT: Memristive devices are promising candidates for future non-volatile memory applications and mixed-signal circuits. In the field of neuromorphic engineering these devices are especially interesting to emulate neuronal functionality. Therefore, new materials and material combinations are currently investigated, which are often not compatible with Si-technology processes. The underlying mechanisms of the device often remain unclear and are paired with low device endurance and yield. These facts define the current most challenging development tasks towards a reliable memristive device technology. In this respect, the MemFlash concept is of particular interest. A MemFlash device results from a diode configuration wiring scheme of a floating gate transistor, which enables the persistent device resistance to be varied according to the history of the charge flow through the device. In this study, we investigate the scaling conditions of the floating gate oxide thickness with respect to possible applications in the field of neuromorphic engineering. We show that MemFlash cells exhibit essential features with respect to neuromorphic applications. In particular, cells with thin floating gate oxides show a limited synaptic weight growth together with low energy dissipation. MemFlash cells present an attractive alternative for state-of-art memresitive devices. The emulation of associative learning is discussed by implementing a single MemFlash cell in an analogue circuit.
    Semiconductor Science and Technology 09/2014; 29(10):104011. · 1.92 Impact Factor
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    ABSTRACT: In this work a 130 nm CMOS 64 channel neural stimulator is presented, which is scalable by connecting it in a daisy chain configuration, for applications requiring larger number of stimulation sites, as it is of interest for retinal implants with improved resolution. Each channel is composed of a hybrid architecture current steering 8 bit DAC, enabling the low power consumption and high channel integration on a small chip area. Besides, the DAC allows stimulating with several waveforms in order to save stimulation energy. An on-chip module was implemented to control galvanostatic deposition of PEDOT on the electrodes. A schema is presented to avoid the residual charge due to cross electrode stimulation and process mismatch.
    IEEE MECBME 2014; 02/2014
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    ABSTRACT: Electrical Neurostimulation has been effective for treating and reducing symptoms of neurological diseases, for alleviating some types of chronic pain and for restoring sensory or neuromuscular deficits. Because of the energy and size limitation on fully implantable devices, it is important to keep devices small and at low power. It was already demonstrated that non-rectangular waveforms provide a more energy-efficient neural stimulation. In this article is shown how a hybrid architecture of current steering DAC is suitable to this application, because of its specifications: the ability to convert several waveforms directly from digital to analog current signals, low power consumption, small chip area requirement, the capability of sharing common stages and also for its simplicity. An ASIC has been developed composed of four stimulator channels, capable of driving several current waveforms. The design is implemented in 130 nm CMOS technology.
    IEEE BMEiCON 2012; 01/2014
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    ABSTRACT: Diodes in forward direction exhibit excellent ESD ruggedness and are thus widely used in both discrete and on-chip electro-static discharge (ESD) protection devices. Due to the conductivity modulation under an ESD stress, a transient voltage overshoot is observed at the beginning of a fast discharge event. Since the voltage overshoot can be harmful, understanding the origin of the overshoot is crucial to design optimized protection diodes. In this paper, it will be shown that existing models can result in much underestimated overshoot voltage, especially for diodes with a large lowly doped region. This can be attributed to the negligence of transient charge distribution in the lowly doped region. A new model that takes this effect into account as well as impact-ionization is presented.
    IEEE Transactions on Electron Devices 01/2014; 61(8):2682-2689. · 2.06 Impact Factor
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    ABSTRACT: This paper describes a single stage fully differential folded cascode operational transconductance amplifier (OTA) with fully differential gain boosting amplifiers designed in 1.2 VDC 130nm CMOS process technology. The amplifier is the driver for the analog to digital converter (ADC) of a blood pressure sensing medical implant [1] which uses wireless power transmission as its energy source [2]. The medical implant’s design specifications require an amplifier with wide output swing, high accuracy, and the bandwidth to drive a differential 5 pF load at 4 ksps with minimum power consumption. The gain boosting amplifiers are telescopic cascoded amplifiers, which, by their design, have an input and output common mode voltage relationship which is opposite to what is needed for a gain boosted cascode configuration. To correct for this, a method for DC shifting the amplifier’s input voltage using a switched capacitor network is presented.
    02/2013;
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    ABSTRACT: This paper presents intramuscular electromyogram (EMG) signals obtained with a fully implantable measurement system that were recorded during goal directed arm movements. In a first implantation thin film electrodes were epimysially implanted on the deltoideus of a rhesus macaque and the encapsulation process was monitored by impedance measurements. Increase of impedance reached a constant level after four weeks indicating a complete encapsulation of electrodes. EMG recorded with these electrodes yielded a signal-to-noise ratio of about 80 dB at 200 Hz. The EMG recorded during goal-directed arm movements showed a high similarity to movements in the same direction and at the same time presented clear differences between different movement directions in time domain. Six classifiers and seven time and frequency domain features were investigated with the aim of discriminating the direction of arm movement from EMG signals. Reliable recognition of arm movements was achieved for a subset of the movements under investigation only. A second implantation of the whole measurement system for nine weeks demonstrated simple handling during surgery and good biotolerance in the animals.
    IEEE Transactions on Instrumentation and Measurement 01/2013; · 1.71 Impact Factor
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    ABSTRACT: A memristive operation mode of a single floating gate transistor is presented. The device resistance varied accordingly to the charge flow through the device. Hysteretic current-voltages including a resistance storage capability were observed. These experimental findings are theoretically supported by a capacitive based model. The presented two-terminal MemFlash-cell can be considered as a potential substitute for any memristive device (especially for reconfigurable logic, cross-bar arrays, and neuromorphic circuits) and is basically compatible with current Si-fabrication technology. The obvious trade-off between a memristive device based on a state-of-the-art silicon process technology and power consumption concerns will be discussed.
    Applied Physics Letters 12/2012; 101(26). · 3.52 Impact Factor
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    ABSTRACT: Neuromuscular electrical stimulation is useful for restoring the movement of extremities in the treatment of some neurological disorders. A portable neuromuscular stimulator was developed for rehabilitation, Functional Electrical Stimulation (FNS) and for experimentation. The device is battery powered and can be controlled wirelessly. The adjustable current output is one of the advantages of this stimulator, because it can deliver several waveforms and it is possible to change every parameter on the stimulation signal. Power consumption, high selectivity, patient comfort and portability are important terms for medical electronic devices. Several setups were developed for the neuromuscular stimulator in order to perform different experiments.
    ICT_Open 2012; 10/2012
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    ABSTRACT: Electrical Neurostimulation is useful in several medical therapies, for the treatment of neuromuscular deficits and for restoring physiological and sensory functions. Fully implantable devices are preferable in order to reach high selectivity and to give comfort to the patient. However, smaller devices bring more challenges regarding fabrication techniques and technology compliances, and power consumption is an important issue. In this study we analyze the use of different waveforms for performing neuromuscular stimulation, and we show how different waveforms require less energy to produce the same muscle reaction.
    IEEE BMEI 2012; 10/2012
  • Biomedizinische Technik/Biomedical Engineering 09/2012; · 1.16 Impact Factor
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    ABSTRACT: Electrical Neurostimulation has been effective in several medical therapies and also for restoring physiological, sensory and neuromuscular deficits. The rectangular pulse waveform has been used as a standard shape for neural stimulation. However, it has been shown that non-rectangular waveforms provide a more energy-efficient neural stimulation. An ASIC has been developed composed of a stimulator, capable of driving several current waveforms, and an analog channel for biosignal acquisition. The design is implemented in 130 nm / 1.2 V CMOS technology, requiring a silicon area of 0.696 mm2. Experimental results show that the stimulator can generate analog signals from a digital input of 8 bits. The output stage can drive up to ±9.8 μA, with a DNL and INL of 0.47 and 1.05 LSB, respectively. Its SFDR is 50.2 dB. And it consumes a maximum of 128.12 μW. The analog input channel presents a power consumption of 140 μW, a gain of 52.2 dB, a bandwidth of 0.5 – 1130 Hz and 10 μVrms of noise.
    Biodevices 2012; 02/2012
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    ABSTRACT: An array test structure for highly parallelized stressing and measurements of ultrathin MOS gate dielectrics is presented. The array test structure consisting of thousands of NMOS devices under test (DUTs) provides a large and significant statistical base for analysis of dielectric breakdown and the stress induced degradation of transistor parameters. The test array has been fabricated in a standard mixed-mode 130 nm CMOS technology. As such technologies offer both thin and thick gate dielectrics for MOS transistors, different gate dielectric thicknesses have been chosen for DUTs and digital control logic which gives the possibility to stress the DUTs with high gate voltages and prevent the control logic from degradation.
    IEEE Transactions on Semiconductor Manufacturing 01/2012; 25(2):130-135. · 0.86 Impact Factor
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    ABSTRACT: A capacitive sensor system is integrated into an active capacitive ECG electrode. This system provides a signal, which is correlated with movement artifacts during ECG signal acquisition, and can be used as reference signal in an adaptive filter.
    Biomedical Engineering International Conference (BMEiCON), 2012; 01/2012
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    ABSTRACT: Introduction Recent development of prosthetic limbs has resulted in a high number of degrees of freedom. In contrast state of the art control of prosthetic limbs is still limited to one degree of freedom at a time. The presented work is part of the development of a fully implantable EMG recording system that aims at providing an intuitive way of simultaneous control of multiple degrees of freedom for upper limb prosthetic devices. Methods A fully implantable device, comprising four bipolar polyimide-based electrodes that are connected to an electric circuit filtering, amplifying and digitizing the recorded EMG-signals, was developed. Energy is inductively coupled into the device and data is transmitted by means of a RF link using the MICS-band. Electrodes were subepimysially implanted into the shoulder and upper arm muscles of a rhesus macaque. The animal performed voluntarily and repeatedly goal directed movements while the EMG of three involved muscles was recorded. This data was analysed offline to determine the features best suited to establishing a robust and reliable detection of the direction of movement. Results We were able to reliably record EMG from tree different muscles. The identified set of features allowed a precise detection of the onset and the direction of reaching movements. Even activation of neighbouring muscles could be precisely differentiated (cross talk). Compared to the use of surface EMG the control is less prone to errors stemming from external influences, such as sweat and movement, and can generate a higher number of independent control signals. Conclusion The developed system (implant & signal analysis) are a promising route towards an intuitive, multi-degree-of-freedom prostheses control system. The highest impact can be achieved for amputees who have lost a hand but maintained the muscles in the lower arm and for high level amputees that underwent targeted muscle reinnervation.
    BMT - Biomedizinische Technik 2011; 09/2011
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    ABSTRACT: An ASIC was developed to provide stimulation currents with digitally defined waveforms. The system is composed of a stimulator and a biosignal amplifier. The stimulator consists of a serial input interface, an 8 bit DAC, and an output stage capable to drive bipolar current signals. The amplifier is composed of an AC-coupled preamplifier, a lowpass filter and a postamplifier. With this circuit it is possible to stimulate neurons and, additionally, to detect and monitor bioelectrical signals.
    BMT 2011; 09/2011
  • BIODEVICES 2011 - Proceedings of the International Conference on Biomedical Electronics and Devices, Rome, Italy, 26-29 January, 2011; 01/2011
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    ABSTRACT: PURPOSE To evaluate a concept for non-invasive measurement of the pressure in the aneurysm sac following endovascular aortic aneurysm repair (EVAR) using a prototypic stent-graft with an integrated foldable network of pressure sensors covering the surface of the implant and capable of wireless digital data transmission. METHOD AND MATERIALS Based on a commercially available aortic stent-graft we initially constructed a prototype with 4 silicon-based pressure sensors arranged on the surface of the implant fitting into a standard 24 F delivery sheath. Digital data conversion is performed by a microcontroller integrated into the sensor array. Energy transfer is performed by inductive coupling, with a data transfer rate of 25 kbit / s (adequate for 80 samples / s / channel). The receiver puts out standardized data via a Universal Serial Bus (USB) port, which can be used for 4-dimensional visualization of the pressure distribution. Based on this prototype we designed a dedicated application specific integrated circuit (ASIC) capable of 256 single pressure sensors in order to provide coverage of the whole surface of a future implant. Due to the full integration of all microelectronic devices on the implant no further procedural steps are necessary during the intervention or later on. RESULTS The in vitro tests in a vessel model demonstrated a reliable 4-dimensional acquisition of a pressure profile on the surface of the implant and the precise wireless energy and digital data transmission over a distance of 50 cm. Ongoing in vivo experiments in a swine model are currently being performed to confirm these results. CONCLUSION The non-invasive acquisition of a pressure profile in the aneurysm sac after EVAR by integrating the detectors on the outer surface of the stent-graft allows detecting a regional pressure elevation due to early endoleak, and it is a practical and efficient option for continuous and direct pressure monitoring which reduces the necessity of follow-up imaging to rule out an endoleak. An implementation into a product and testing in a clinical study seems feasible. CLINICAL RELEVANCE/APPLICATION An integrated stent-graft, capable of multi-dimensional telemetric aneurysm sac pressure measurement after EVAR, can become a practical and efficient alternative in follow-up monitoring.
    Radiological Society of North America 2010 Scientific Assembly and Annual Meeting; 12/2010
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    ABSTRACT: Impact-ionization at low and high electric field as well as the temperature dependence has to be modeled well in order to improve the predictive capability of TCAD tools. The high field behavior is of particular interest for ESD protection devices with low breakdown voltages which are used to protect ICs made with modern technologies. In this paper, the model for estimating the impact-ionization proposed by Valdinoci with the parameters of Reggiani has been examined with diodes of various breakdown voltages. It was found that the experimental breakdown voltages of the diodes are underestimated using that model. The cause was traced back to the overestimation of the electron impact-ionization coefficient at high electric fields. By adjusting the model parameters to the experiments of Van Overstraeten and Grant, who measured the impact-ionization coefficient in silicon for fields up to 7.7 × 10<sup>5</sup> V/cm, we extend the model's validity to high fields. With the new parameter set, a much better agreement to the measured breakdown voltages is obtained. As a check for the temperature dependence of the impact-ionization, the diodes were further investigated under 100 ns transmission line pulses (TLP). The measured high-current I-V characteristic is well reproduced by simulations using the new model, as opposed to the well-established model based on Chynoweth's law. Both the failure level and the damage location are well predicted by the simulation.
    Simulation of Semiconductor Processes and Devices (SISPAD), 2010 International Conference on; 10/2010
  • Nashwa Abo Elneel, Erkan Aksoy, Dietmar Schroeder
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    ABSTRACT: A novel adaptable analog/digital converter (ADC) that combines analog/digital conversion and entropy-coding for integrated data compression and low-power operation is reported. The converter has high flexibility of operation in terms of adaptable resolution, conversion rate and input signal statistics. This feature allows to adaptively react to changes of the situation and to put the device in each case into the optimum configuration. The ADC has been realized in a 0.6μm CMOS technology with a peak resolution of 12 bit and 200kS/s maximum sampling rate. A comprehensive power model of the converter is presented that reflects precisely the power consumption determined from experiments. The model is very useful for optimizing the converter configuration in the node of a wireless sensor network for specific situations. A feasible real-life application is demonstrated. KeywordsEntropy-coding-Resolution-adaptive-A/D conversion-Successive approximation
    Analog Integrated Circuits and Signal Processing 09/2010; 64(3):249-259. · 0.55 Impact Factor