A Master of Science Thesis in Electrical Engineering submitted by Nazanin Neshatvar entitled, "Designing Low Frequency I.C Filter Using Pseudo Resistor for Biopotential Measurements," submitted in December 2010. Available are both soft and hard copies of the thesis.
Standard medical diagnosis requires attaching sensors or electrodes to the patient and then connecting these electrodes to bulky equipment powered by mains supply. This is usually an acceptable procedure for short term patient monitoring. However, for long term monitoring, this approach is neither comfortable nor durable. Therefore considering fully implantable electronic systems which have many useful applications including monitoring cardiac irregularities in pacemaker systems, are the preferred. This thesis intends to show the design of low frequency IC filters. Given that large time constants are required in low cutoff frequency filter designs, various approaches can be utilized to provide large values of resistors or capacitors. To demonstrate the proposed method, third order low-pass and high-pass filters for ECG signals with cutoff frequencies in range of 0.1-100 Hz are implemented. Butterworth topology is used in both filter designs to preserve the morphological information of the ECG signals. Here, we focused on increasing the resistor values via using MOS pseudo resistor where large values of resistors can be accomplished with small die area. A pseudo resistor is simply a MOSFET that is biased in weak inversion region where the gate voltage is below threshold voltage. Weak inversion is used in ultra low power designs. When transistor is biased in weak inversion Vg < Vth , for fixed gate voltage, different approaches can be utilized to provide large values of resistors. This can be summarized as follow: If Vgs<Vth and VDS<UT , where UT is the thermal voltage =26 mV , then the transistor has a fixed resistance and it is of the order 109 ohms. This resistor can be used in designing filters if VDS is less than 3UT to 5UT and this resistance is called the pseudo resistor. If Vgs<Vth and VDS>UT, then the transistor has a fixed resistance and it is independent of VDS. However, since λ is not zero, ro exists and it is of the order 109 ohms and the transistor acts as a current source. However, VDS should be greater than 100 mV which means biasing is required. This resistor can be used in designing filters if VDS is more than 100mV, this concept has been used in OTA designs. If transistor is in saturation and Vgs= VDS<UT , then the transistor has a fixed resistance and it is of the order 1010 ohms. This resistor can be used in designing filters if VDS is less than 25mv. If transistor is in saturation and Vth>Vgs= VDS>UT, then the transistor has an exponential relationship between ID and VDS. This prevents from using the transistor as a resistor.