I am going to measure the IIP3 of an ADC. The supplier has recommended that ADC should work in -2dBFs input power. According to them I have to apply two tones at -8dBFs at the input so that the total power will be -2dBFs. I wonder why the tones should be 6dB lower than -2dBFs. Shouldn't they be 3dB lower so that the total power will be -2dBFs? The sum of two identical tones will be 3dB higher than each tone. Right?

 

I am going to measure the IIP3 of an ADC. The supplier has recommended that ADC should work in -2dBFs input power. According to them I have to apply two tones at -8dBFs at the input so that the total power will be -2dBFs. I wonder why the tones should be 6dB lower than -2dBFs. Shouldn't they be 3dB lower so that the total power will be -2dBFs? The sum of two identical tones will be 3dB higher than each tone. Right?

The criteria is the maximum amplitude of the composed signal. The two tones will reach the peak at the same time from time to time. If the two tones have the same amplitude (it is actually the case for inter-modulation measurement), the amplitude of each tone should be reduced by half to have the same peak amplitude as the case for single tone. So the power of each tone should be 6 dB less.