Low voltage and low power are two key requirements for on-chip realization of wireless power and data telemetry for applications in biomedical sensor instrumentation. Batteryless operation and wireless telemetry facilitate robust, reliable, and longer lifetime of the implant unit. As an ongoing research work, this paper demonstrates a low-power low-voltage sensor readout circuit which could be easily powered up with an inductive link. This paper presents two versions of readout circuits that have been designed and fabricated in bulk complementary metal-oxide semiconductor (CMOS) processes. Either version can detect a sensor current in the range of 0.2 μA to 2 μA and generate square-wave data signal whose frequency is proportional to the sensor current. The first version of the circuit is fabricated in a 0.35-μ m CMOS process and it can generate an amplitude-shift-keying (ASK) signal while consuming 400 μ W of power with a 1.5-V power supply. Measurement results indicate that the ASK chip generates 76 Hz to 500 Hz frequency of a square-wave data signal for the specified sensor current range. The second version of the readout circuit is fabricated in a 0.5-μ m CMOS process and produces a frequency-shift-keying (FSK) signal while consuming 1.675 mW of power with a 2.5-V power supply. The generated data frequency from the FSK chip is 1 kHz and 9 kHz for the lowest and the highest sensor currents, respectively. Measurement results confirm the functionalities of both prototype schemes. The prototype circuit has potential applications in the monitoring of blood glucose level, lactate in the bloodstream, and pH or oxygen in a physiological system/environment.
A low power sensor read-out circuit has been implemented in
0.35 µm CMOS technology that
consumes only 400 µW of power
and occupies an area of 0.66 mm2. The circuit is capable of converting the current signal from any generic biosensor into an
amplitude shift keying (ASK) signal. The on-chip potentiostat biases the chemical sensor
electrodes to create the sensor current which is then integrated and buffered to generate a
square wave with a frequency proportional to the sensor current level. A programmable
frequency divider is incorporated to fix the ASK envelope frequency to be inbetween
20 Hz and 20 kHz, which is within the audible range of human hearing. The entire
transmitter block operates with a supply voltage as low as 1.5 V, and it can be
easily powered up by an external RF source. Test results emulate the simulation
results with good agreement and corroborate the efficacy of the designed system.
We used an unobtrusive approach, keystroke logging, to examine students’ cognitive states during essay writing. Based on data contained in the logs, we classified writing process data into three states: text production, long pause, and editing. We used semi-Markov processes to model the sequences of writing states and compared the state transition time and probability for demographic subgroups that were matched on writing proficiency. Results suggested that the subgroups employed different processes in essay writing.
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