I am in the middle of a project and found myself in need of measuring the amplitude of a sine wave fixed at 455KHz. Is there a simple and efficient method for this that I am stupid enough not to see or is it a complicated measurement to do without any external circuitry more then the arduino itself?
Depends on magnitude, as you probably know, arduino (UNO - don’t know your board) has max. resolution 1 mV using internal reference.
To have adequate results, I think at least 50 mV of waveform should be present at input. Than you may try accelerate ADC, up to 4 MHz clock or so, running undersampling about 300 kHz , if bandwidth of the input lower than 150 k you may get some data.
Other things accuracy, for better than 5-10 % external circuitry is not an option.
Also keep in mind that a true sine wave is an AC voltage with both positive and negative sections and that arduino analog input pins must not be wired directly to negative voltage at any time or pin damage can result. So some external signal conditioning circuitry would be required.