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So I have a amplifier of which I want to set the gain to very precisely 2 times. I have a feedback network that controls this.
Because temperature coefficient reasons, I want to use two fixed resistors (0.1 %, 15 ppm/K) to set the gain and then use a small trimmer to just nudge it the right way.
My reasoning was that by making the trimmer very small, it's temperature coefficient will have less impact on the set value. If I have two 0.1 % resistors, then I know that my trimmer needs to only trim over a fraction (0.1 %) of the value of the two other resistors.
By my problem is the way I should wire the resistors up. The simple example would just be as follows:
However, I was told it was bad practice to use this technique, as during the movement of the wiper the trimmer might go open, and your circuit becomes open loop. This causes the op amp to potentially rail and damage the circuit that follows (which isn't really an issue in this case though).
Alternatively you can do the following:
However, in this case you can only tweak in a certain direction, so you would need to make the two resistors have different sizes so you know your ratio will start out too low and then use the trimmer to trim it into the right ratio. This isn't possible in my case because the size of the trimmer is then dictated also by the difference between the two fixed resistors, which will be a few percent because of the nature of E12 and E24 series. And since most of these low tempco resistors are only available in E24 series resistors, I can't get the size of the trimmer as low as I would like it. In addition, it's annoying on the BOM because you now need two types of resistors instead of one.
So what is the "right" way to do this?