How to sense current of amplifier output

Hello everyone,
I need to design a circuit to sense the output voltage and output current of an amp, the power is around 400W 4 ohm.
For the output voltage, I try to divide the voltage by 10 simply by potential divider and use peak detector to provide the information to the ADC
For the output current, I try to add a 0.02 ohm sense resistor along the output and get the voltage before and after the sense resistor. Divide both voltage by 10 and use op-amp subtractor to get the difference, then peak detector again to provide information to the ADC
However, problem here. For 0.02 ohm, I only got 0.2V difference even at full power. If divide by 10, difference will become 0.02V only. I can't distinguish the difference from the ADC for smaller output. Any suggestion to solve this?

You don't divide the output from the sense resistor, you multiple it to make it a reasonable value - why are you dividing it?.

Nigel Goodwin said:

You don't divide the output from the sense resistor, you multiple it to make it a reasonable value - why are you dividing it?.

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The problem is at full power, it's output is about 56V peak, it already over the input limit of the subtractor op-amp with +/-5V supply

Ricky Lam said:

The problem is at full power, it's output is about 56V peak, it already over the input limit of the subtractor op-amp with +/-5V supply

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It never occurred to me you were doing it in such a silly way

Place the sense resistor in the GROUND connection to the speaker.

Thanks for your suggestion. The problem is it won't work in bridge mode if the sense resistor in place in the ground connection path

Ricky Lam said:

Thanks for your suggestion. The problem is it won't work in bridge mode if the sense resistor in place in the ground connection path

Click to expand...

Right, so you've now introduced a completely different element to the original question

Why do you want to measure the current anyway?, you know the nominal speaker impedance, so the voltage is enough.

But assuming you do?, feed the differential opamp from an independent floating supply, then you only have the tiny voltage dropped across the sense resistor.

The reason is I want to know the output voltage and output current so we know the speaker impedance.
I purpose to feed the voltage to ADC but we need to reduce the voltage so it will not overload the ADC. However, it also result in poor resoultion.
Also, the DC offset of the op-amp also affect my result to ADC. If I multiply the result from the subtractor, although resolution will improve, but the DC offset will be multiplied as well.
Do yo have any suggested schematic for this?