Introduction to electronic calibration and methods for correcting manufacturing tolerances in medical equipment designs - AN4703

This tutorial discusses how the proper design of trim, adjustment, and calibration circuits can correct system tolerances, making medical equipment safer, more accurate, and more affordable. Calibration subjects addressed include compensating for component tolerances, using final-test calibration, improving reliability through power-on self-test and continuous/periodic calibration, enabling accurate automated adjustments, replacing mechanical trims with all-electronic equivalents, and leveraging precision voltage references for digital calibration. Making medical equipment accurate, safe, and affordable with electronic calibration Medical equipment is an area where everyone expects accuracy and safety. At the same time, equipment must be affordable. How can manufacturers deliver "perfect" equipment at a reasonable price? In a word, calibration. All practical components, both mechanical and electronic, have manufacturing tolerances. The more relaxed the tolerance, the more affordable the component. When components are assembled into a system, the individual tolerances sum to create a total system error tolerance. Through the proper design of trim, adjustment, and calibration circuits, it is possible to correct these system errors, thereby making equipment safe, accurate, and affordable. Calibration can reduce cost in many areas. It can be used to remove manufacturing tolerances, specify less-expensive components, reduce test time, improve reliability, increase customer satisfaction, reduce customer returns, lower warrantee costs, and speed product delivery. Digitally controlled calibration devices and potentiometers (pots) are replacing mechanical pots in many medical systems. This digital approach results in better reliability and improved patient safety. This increased dependability can reduce product liability concerns. Another advantage is reduced test time and expense by removing human error. Automatic test equipment (ATE) can perform the test functions quickly and precisely, time after time. In addition, digital devices are insensitive to dust, dirt, and moisture, which can cause failure in mechanical pots. Testing and calibration fall into three broad areas: production-line final testing, periodic self-testing, and continuous monitoring and readjustment. Practical products may use some or all of the above test methods. Compensating for component tolerances using final-test calibration Final-test calibration corrects for errors caused by the combined tolerances of many components. One or more adjustments may be required to calibrate the device under test (DUT) to meet a manufacturer's specifications. To provide a simple example, we will say that this equipment uses resistors with five percent tolerance in several circuits. In design, we simulate the circuits and perform Monte Carlo testing. That is, we randomly change the resistor values within the tolerance limits to explore their effects on the output signal. The simulation results in a family of curves that show the worst-case errors that the resistor tolerances cause. With this knowledge, the designer decides to use the circuits as-is and to simply adjust the offset and gain during final test to meet system specifications. So, we make measurements in the final production test and have a human set the gain and offset using two mechanical pots. Calibration is complete, but have we solved the problem, masked the problem, or added a bigger unknown? Experienced production engineers know human error is a real issue. Unintentional slips can ruin the best of plans. Asking a human to perform a boring, repetitive task is asking for problems. A better way is to automate such a task. Electrically adjustable calibration devices enable quick automatic testing, which improves repeatability, reduces cost, and enhances safety by removing the human-error factor. Improving reliability and long-term stability by poweron self-test and continuous/periodic calibration Manufacturing tolerances are compensated for by calibration during the final production test, and that data is utilized when a system is powered up. Environmental parameters in the field also create a need for test and calibration. Such environmental factors include circuit component aging, temperature, humidity, and signal level and bias. Some circuits contain control or average information, which can be periodically memorized. These factors are accounted for with a combination of self-test at powerup and periodic or continuous testing. The field testing may be as simple as sensing temperature and compensating accordingly, or they may be more complex.