ECG Demonstration Board

Transcription

1 ECG Demonstration Board Fall 2012 Sponsored By: Texas Instruments Design Team : Matt Affeldt, Alex Volinski, Derek Brower, Phil Jaworski, Jung-Chun Lu Michigan State University

2 Introduction: ECG boards and machines are being used all over the world in hospitals and other health care facilities. A biopotential is the electric potential measured between living cells, and the ECG boards are designed to take in the biopotential from cells and run them through electrodes. Measuring these allows for the monitoring of the signals being given off by the heart, which has beneficial information for people in the medical field. Since depending on these machines is a daily necessity for medical staff globally, having the machines work as well as possible is critical. Texas Instruments has decided they wanted to design and create a new ECG board. The design of the board is going to use the standard Texas Instruments design for the basic functionality. However, the specs after that are very vague and open for change. The idea is to implement any design additions to increase accuracy, reliability, and lower cost of the board, as seen fit by the design team. Background: Boards designed to measure electrocardiography signals, or electrical activity of the heart, have been in use for quite some time. Recently, Texas Instruments (TI) has had many clients interested in using ECG board technology for various applications. TI has an outdated ECG demonstration board using their chipset, and many clients have decided they can save money by creating their own board instead of purchasing much of the technology from TI. The focal problems in the current TI board include overcomplexity and the use of outdated chipsets. Texas Instruments is in need of a redesigned ECG demonstration board based on current technology to impress prospective customers. Rise in client demand, their desire to save money, competitors technology, and TI s outdated board created the need for this redesign. While ECG boards themselves are not new technology, this project revolves around redesigning, updating, and streamlining it using current chipsets to impress customers and meet their needs. The final design should be capable of reading ECG analog signals with a driven-right leg. Board functionality can be improved upon from the base design by including the Stellaris microcontroller or using live signals instead of simulation.

3 Objectives: Customer Requirements: The final requirements for our TI-based ECG Demonstration board are to design and test based on pre-designed TI schematics for signal processing. Our test inputs will initially be generated by an ECG simulator and the output will be displayed on a Stellaris microcontroller evaluation module pre-programmed as a 2-channel oscilloscope. Our PCB must have analog circuitry that interfaces an ECG simulator with the microcontroller evaluation module. In our design, we will be using the INA333 lower power, precision instrumentation amplifier and 9V battery and buck converter/ldo for power. We will then integrate AFE with the Stellaris o-scope demo kit and use electrodes for live signals instead of the ECG simulator, and eventually, use large PCB pads to measure blood flow from the thumb and forefinger. Noise reduction is key for accuracy, so we will run circuit simulations from the requested simulation program PCB Artist and keep all output signals as close to the ideal as possible. The cost of the board should be within range of our budget, and we will follow the step-by-step time constraints given by the customer strictly to ensure our final product arrives on the requested date. Proposed Solution: Initially, we will be testing and simulating common amplifiers using the requested parts with the program PCB Artist. Our RL Drive Amplifier Serves as noise cancellation and a common mode bias. It reduces common-mode interference and increases stability. It provides a common electrode bias at the reference voltage, and feeds back the inverter common-mode noise signal to reduce the overall noise seen at the inputs of the instrumentation amplifier s gain stage. A parallel RC combination for the left arm, right arm, and the right leg represents the added passive-electrode connection impedances. We will use this to reduce the overall noise signal at each input, which will filter the residual noise, or reject common-mode noise. The final AFE (analog front end) will be an analog to digital converter that starts with an input clamp. The common-mode level of the image sensor s output signal could range from 0V to 9V, so the signal must be ac-coupled to the AFE. The input clamp restores the dc level of the signal to an optimal point with the supply range of the AFE. The tested inputs will be electrodes, and eventually, PCB pads. Our goal is for the AFE to create the clearest possible signal for the Stellaris O-scope to read.

4 Requested Parts for Use: -INA333 Low power precision amplifier -Integrate AFE with Stellaris o-scope demo kit -Stellaris microcontroller -Electrodes and PCB pads for test signals -9V battery and buck converter/ldo for power Figure 1. Stability design using PCB Artist Figure 2. RL Drive Amplifier

5 Figure 3. Ideal INA Front End Risk Analysis Design of PCB Artist Avoid bias between bypass caps and active device, routing bias into the bypass caps and into the active component. Ensure bypass caps are on same layer as active component. Minimize loops that arise the EMI issue Using common-mode transformers, reducing CM current For both logic and analog, design the differential signal environment Well connection of zero impedance ground Requires good floor planning first for layout ECG Demonstration Board The patient safety, providing large impedance between patient and input filtering Ensure to alleviate input current and voltage noise Electrode resistance varies with contact and moisture and need to presents problem for Right Leg Drive stability Alert to leakage to inputs, improving EMI/EFI with shield drive Project Management:

6 In order to have a successful project, planning is of utmost importance. A schedule of important dates as well as reasonable estimates for all tasks related to the project is necessary to stay focused. Each individual has a non-technical role in order to ensure that the project continues to move forward and that the project is done correctly. All team members will be contributing to the project in a technical manner as seen fit by the team to take advantage of each team member s strengths. It is important for all members of the group to be familiar with the PCB design process as well as how to build the circuit board once it is fabricated to reduce the project s downtime if a single member becomes incapacitated for a brief period. Non-Technical Roles: Matt Affeldt Manager Alex Volinski Webmaster Phil Jaworski Document Preparation Nick Lu Lab Coordinator Derek Brower Presentation Preparation Customer Schedule This project is very customer oriented, and in this case, the customer has a set schedule that the team must follow to ensure appropriate progress throughout the duration of the project. The following table, Table 1, shows the important dates determined by the customer of this project. Task Deadline Complete Two Test Circuits October 4, 2012 Design, Build, Test ECG Board November 8, 2012 Final Result December 7, 2012 Table 1. Customer Schedule Team Schedule and Task In order to ensure that all customer deadlines are met, the team created a Gantt chart to organize all subtasks for customer deadlines as well as all subtasks related to the project management. This includes a proposal, technical review and

7 final presentation. Below, in Figure 4, is a reduced version of the team s Gantt chart showing high-level tasks, as well as their estimated time to completion. Figure 4. Top Level Gantt Chart Finally, the project is working under constraints from supervisors that include more deadlines. These deadlines are to show supervisors that the project is making acceptable progress and eventually, the project must be presented to justify the time and cost of the project as well as showcase the abilities of the five team members. Below is a table, Table 2, which outlines the supervisors required deadlines. Task Deadline Written Pre-proposal Week of September 16, 2012 Final Written Proposal Week of September 30, 2012 Proposal Presentation October 5, 2012

8 Technical Presentation November 9, 2012 Final Presentation December 7, 2012 Table 2. Supervisor Deadlines for the Project As presented in the two deadline tables, the project is on a tight schedule and many parts of the project will have to be done concurrently. In order to keep the project focused, the team will be meeting every Thursday at 12:00pm for the sole purpose of discussing progress and determine any large design choices. The team will also meet Monday, Wednesday and Friday briefly around 11:00am to discuss any smaller issues that may have arisen in the past few days. Dr. Hoole is assigned as the project facilitator who will be meeting with the team on Thursdays around 12:30pm- 1:00pm to discuss any logistics that the team may need assistance with. Cost Each design team has a budget of $500, aiming to achieve the lowest possible cost of the project. Fortunately, TI provides all the expensive components involved with the ECG Demonstration Board. The TINA-TI, SPICE simulation, the PCB Artist, analog circuit design using op-amps, instrumentation amplifiers, and the power devices have been given to the team. The library of various parts can be obtained from the free online website, with restrictions of quantity. Since TI has these units, the project meeting the design specifications is already at minimum cost. The excess in TI available components is not anticipated and does not delimit design process. If the team wants to improve the ECG Board with better design to meet the specifications from the sponsor, the team is required to order parts from the manufacturers with DigiKey. Also, the printed circuit boards and some operational amplifiers are not covered. Each PCB is approximately $33 for 2-layer and $66 for 4-layer, so the estimated cost by pessimistic approach is $130, a cost below the targeting budget. Hence, the team has no doubt that additional funds will not need to be requested.