This paper introduces the matching problems of engineers in accuracy, stability and temperature drift in the process of resistance selection, which provides a method for the basic selection of precision resistance.
It can provide you with suggestions when selecting resistance accuracy and temperature drift.
A the accuracy requirement is too high and the stability is ignored
If the 0603 photoresist precision resistor with accuracy of ± 0.01% is select, the change of 0603 photoresist alloy resistance value may be close to 1000ppm (± 0.1%), which is the result of overload operation for 1000 hours. Therefore, 0603 photoresist precision alloy resistor is likely to quickly exceed the nominal accuracy requirement of ± 0.01% in its life cycle. In general, stability should be at least equal to the accuracy index. The Engineer may refer to the indicators of "load life" and "durability" in the reliability test column of this specification.
Test method for service life of standard load: load within 1000 hours or more, turn on and off for 90 minutes and 30 minutes at rated power and 70 ℃ ambient temperature, and observe the resistance change of resistance before and after the test. The load life is related to time, power and temperature. The higher the load power, the higher the ambient temperature and the longer the time, the greater the change of resistance, but this change is not linear. With the extension of loading time, this change will slow down unless the properties of resistive materials decrease. Load life is an important basis for predicting whether the irreversible change of resistance value at the end of product life is within the allowable range, because the change of resistance value after load is irreversible.
According to the empirical formula, for applications that depend on initial accuracy and end-of-life accuracy, we meet the following low load life requirements. Engineers can flexibly select the high requirements in practical application according to the actual design requirements. If both typical and maximum values of load life are indicated in the specification, the reference value shall be used.
B. the accuracy requirement is too high and the temperature drift is ignored
If an 0805 size resistor with an accuracy of ± 0.1% is select and used under the environmental conditions of 0-60 ℃, the temperature drift is ± 25ppm, and its resistance value shifts 1000ppm (0.1%) relative to the room temperature of 20 ℃ at 60 ℃. At 60 ℃, the deviation of resistance value may be ± 0.2%, which is the initial accuracy error of resistor. However, in practical application, the temperature of resistance loading may be higher, and even exceed the resistance value will be more serious.
You can see this index in the temperature coefficient (TCR) column in the specification. It should be noted that the TCR index needs to correspond to the temperature range. This temperature range is not equal to the operating temperature range. The temperature drift index only indicates the index in the range of 20 ~ 60 ℃, but a large part of the current detection resistance can work in the range of - 55 - + 170 ℃. Generally, working above 60 ℃ is for the current detection resistance with serious heating. If the temperature drift beyond this temperature range is not marked in the specification, it means that the temperature coefficient exceeding 20 ~ 60 ℃ may be much greater than the nominal value.
If the temperature range of the temperature drift is defined as equal to the working temperature range of the resistance, and if the initial accuracy is very important, the matching relationship between the temperature drift and the accuracy we propose is:
Many other factors should be considered when selecting the accuracy of influencing factors such as stress and power coefficient, voltage coefficient and thermoelectric potential caused by humid environment and pulse circuit and installation. The specific application model is very complex.
Scheme recommendation: 06303 photoresist alloy resistor
