I don't know when it started, but I've become so impressed by the typical values of op amp parameters that I've paid less attention to the maximum and minimum values. The gold medalists are always the most popular among the winners. It's easy to get into trouble if you only pay attention to the typical values of the parameters when using op amps. In this case, it's difficult to detect the mistakes yourself.
Problem in the case: When using the amplifier to build the circuit, two of the 15 chips failed. In the design of the case, when the output voltage exceeds ±38.7mV, a "failure" problem will occur. The circuit in the case is a differential amplifier circuit built using ADA4851-1. The following is the schematic diagram of the case:
After analysis, the author of the book concluded that the "failure" problem occurred because the maximum value of the input offset voltage was ignored during use.
The input offset voltage of the ADA4851-1 is typically 0.6mV and has a maximum value of 3.4mV.
The gain in this case is about 54.4 times. When the input offset voltage is at its typical value, the output after amplification is 32.7mV, which is less than the threshold value of 38.7mV and there is no "failure" problem. However, when the input offset voltage is at its maximum value, the output after amplification is 185.5mV, which is much larger than the threshold value, resulting in a "failure" problem.
In addition, the author also checked the input offset voltage distribution diagram. It can be seen from the diagram that the proportion is more obvious when the input offset voltage is ±1mV. At this time, the corresponding output voltage is ±54.4mV, and there is still a risk of "failure".
Before reading this book, I had no idea about the existence of step-by-step diagrams, and I never thought of looking at them. I still need to read more books with case analyses in my daily studies.
The solution finally proposed by the author is to modify the judgment threshold and replace it with the low offset voltage amplifier ADA4528.
This kind of problem can be understood at a glance, but similar problems will easily occur in actual application if not enough attention is paid. Moreover, this case exists in listed companies. Looking at more of these cases can effectively avoid these pitfalls. Learning things from the experiences of predecessors is faster and more profound.
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