Monitoring Voltage Differential with ADC for up to +-30V

[LMorr.3]

Hello,

I need to build an STM32 based solution which  will monitor the voltage differential between a .47Ohm resistor on a separate power amp circuit.  ( measure the amp's bias )   Both circuits will share a ground.

The voltage read across the resistor using a multimeter has a potential max range between +-30V ( the rails are +24V but I am using +-30V to be safe ).  When the bias is adjusted correctly, the potential between the resistor is 500mV, and the STM32 needs to use its ADC to trigger an interrupt or or set a flag when the voltage range is off by more than 100mV ( higher or lower ).  

How do I ensure I monitor the voltage differential without affecting the source considering it is an audio amp?

Also, I would like to do the same to monitor if the speaker output wires have more than 10mV differential, but the voltage can potentially be +-30V.  ( only for testing when amp's input signal is at zero )

[MasterT]

What you need is https://en.wikipedia.org/wiki/Instrumentation_amplifier

It does differential voltage amplification And down translation to defined level.

[LMorr.3]

Ok thanks.  To monitor speaker output for DC, could I use a zener to block any voltage above 3V into the ADC?  I could then watch for anything under 3 volts.  ( I could use the negative speaker wire as the ground reference for ADC conversion )

[MasterT]

You better using common resistive divider. To make an offeset 1.65V put 3 resistors, 

https://electronics.stackexchange.com/questions/22742/combined-adc-scaling-and-offset-with-variable-input-voltage-ranges.