Can someone tell me the purpose of the resistors? I am thinking it must be something to do with setting a gain but I havent seen that configuration before.
If it is exactly as the schematic says, i.e. R9 is not installed and R63 is 0ohm, then the amplifier is simply in a buffer configuration.
Is this what you needed?
I apologize if I misunderstood your question. You were asking about how one might use the resistor between the inputs?
This can be used as a technique to stabilize the amplifier by manipulating its noise gain. By adding the resistor, the noise gain is increased while maintaining the signal gain. The increase in noise gain effectively increases the stability of your circuit. The tradeoff to this is that the noise of the circuit also effectively increases. You can read about this technique in this technical article, see the noise-gain manipulation section.
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