Hi,
If you do a quick research on OPAMP amplifier circuits ... you will immediately see that all circuits use resistors.
What you need to know:
* amplifier power supply
* amplifier input voltage peak-peak (already given)
* amplifier output voltage, min and max
--> output_voltage_range / input_voltage_range = gain
* output offset voltage
* frequency range of interest
* maybe other requirements, like distortion, (erroneous) overvoltage, precision
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For beginners I recommend to use a standard circuit "OPAMP differential amplifier circuit" using 2x resistor pairs (4 resistors in total).
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When I do an eletronics design, I first decide the application´s requirements. Without looking for a solution.
Btw: by writing this, I don´t meant that you need to do it the same way as I. You are free to go your own way.
So I´m just guessing what your "basic requirements are". Correct me if I´m wrong.
AC input --> unknown analog processing --> ADC --> unknown digital processing --> result.
The most important:
* define the AC input
* define the expected result
You already come with a transformer. I guess the transformer is not a requirement, it is a solution. It could be a good solution or not.
I personally did some measurements involving transformers (many years ago), but since then I avoided them and used a resistive measurement solution..
Reasons for avoiding: used high_impedance measurement instead of true isolaton (no safety risk). I did not like the phase shift and the distortion caused by transformers.
And as a benefit the resistive solution was cheaper and is able to process DC.
Klaus