programmable gain opamp

Hello

I'm doing a programmable gain IC opamp. The gain is changeble through feedback resistors. I need 12 levels of gain. Normally it means 12 resistors turning on/off by 12 mos transistors. It consists resistance divider with one constant resistor, and changes opamp's gain. But i can't let it to be 12 digital inputs, so a demux 12/1 with 4 addressing bits can be used. How can you judge this solution?
I thought to make a resistors combination switch on by only 4 transistors to avoid using a digital interface, to save some power, but i can't find resistrors values.

I think these solutions can be wrong. I checked resistor's process and temperature dependence. It will be problem with gain accuracy when temperature changes. What can i do than?

regards

.

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