# Small bump in the Road with SCA design using OP amp

Discussion in 'Homework Help' started by RyanKim, Feb 17, 2013.

1. ### RyanKim Thread Starter Member

Sep 18, 2011
37
1
Hello everyone,

I appreciate anyone looking at this at the moment. I need to design a multistage ampflifier circuit for a signal conditioning amplifier connected to a pressure transducer. Using op amps, my first stage has a gain of 5.556. Due to loading issues and ideal op amp properties this is as much gain as I can squeeze out of stage 1.

The problem is this. The whole system needs a variable gain from 44.444 to 88.888. (Due to transducer output span min and max datasheet values) My problem is for stage 2, using a POT and a fixed resistor, I cannot manage to accomplish this range.

For instance, if stage 1 is 5.556 to get a total gain of 40 stage 2 must be around 7 and to get a gain of 90ish Stage 2 would need a gain of around 18. The POT is limited from a fractional value of 0.4 to 0.6. And the minimum value of resistor to keep it "ideal" would be 10K and 10M. Due to the limitations on the POT I cannot find a fixed and variable resistor pair to get me a gain from 7-18 for stage 2.

I was hoping someone could lead me in the right direction in terms of fixing this problem or some considerations for me to take in.

Sincerely,

Ryan Kim.

2. ### MrChips Moderator

Oct 2, 2009
12,623
3,451
Well no one can help you until you post a circuit diagram.

3. ### RyanKim Thread Starter Member

Sep 18, 2011
37
1
ahh apologies...

So the first picture was my original design...but thinking now...it maybe better to use the second configuration to further reduce loading and zero drift (Instrumentation amp config). Problem is I always find it difficult to find a value for the variable resistor (plus a fixed resistor) that would get me 40-90 gain in each design.

My power supply is from 0-9V DC (battery)

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