A new offset cancellation technique for temperature sensors & Design of 8-bit decimation filter for biomedical applications

Shakrani, Harsh and Dutta, Asudeb (2018) A new offset cancellation technique for temperature sensors & Design of 8-bit decimation filter for biomedical applications. Masters thesis, Indian Institute of Technology Hyderabad.

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Abstract

In our day to day life there are lot of things which we need to sense and then decide the course of action according to it. Many of these can be physically sensed easily, but the exact value of the sensed cannot be determined by human. There will be a lot of error in judged value and exact value. So instead of human sensing them and judging the exact value there are physical instruments which can provide lot more accurate value of sensed item than human, which are called SENSORS. There are lot of different sensors for sensing different things and one of prominent one is temperature sensor. Temperature sensor plays an important role in many applications. For example, maintaining a specific temperature is essential for equipment used to fabricate medical drugs, heat liquids or clean other equipment. For application like these, the accuracy of detection can be critical. The work done in this Thesis shows how to maintain the accuracy of temperature sensor. Temperature sensor used here is a Wheatstone bridge circuit consisting of two resistors and two thermistors. Mismatch between the resistors or thermistors will lead to incorrect detection of value, which is called OFFSET, therefore to maintain the accuracy the mismatch has to be minimized or removed. One of the Technique to minimize the offset and results pertaining to it has been displayed in this Thesis. Technique described in this Thesis consist of first sensing the difference between resistors value, one being the reference resistor and other the on-chip resistor used in temperature sensing, second amplifying the difference of resistor value using OPAMP, third sending the amplified signal to single ended SAR ADC, which gives digital bits as output. And according to the digital output changing resistor value using resistor switching method. Thus then this resistor will be used in wheat stone bridge temperature sensing. The work proposed here can increase or decrease on-chip resistor value depending on reference resistor. The wheat stone bridge Resistor can be changed by plus minus 5K ohms with respect to reference resistor. This is a onetime calibration technique used before start of sensing temperature. After the resistor have been calibrated, these resistors are used in wheat stone bridge along with thermistor to sense temperature and the differential output obtained through wheat stone is passed on to the dual ended SAR ADC, which gives digital representation of temperature sensed.

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