What amplitude on power spectrum if analog input is 7.3V,sine,80hz?

[Jimmy Duong]

I have an analog input of 7.3V, sine wave at 80 Hz feeding thru PXI-4475 (I have a PXI-1042 Chasis and PXI-8176 Controller and I am programming my application in Lab Windows-CVI). I see a sine wave with an amplitude of 7.3V in the time domain but I see an amplitude of 13.3V at 80Hz in the power spectrum. Should I see 7.3V in power spectrum display? My HP FFT Analyzer showed 7.3V at 80 Hz in power spectrum display. Please help. My email: duongjh@nswccd.navy.mil or my phone: 215-897-8766/7370. Thanks in advance.

[AVR]

There would appear to be a mathematical relationship between the 7.3 V sine wave and the 13.3 V amplitude as follows:

a) rms value of a sine wave at 7.3 V = 5.1619 V

b) square that result = 26.645

c) half the result = 13.32

The power spectrum should be expressed in terms of voltage squared. The power spectrum should produce a result that is a second order function of the initial amplitude. Are you sure the settings on the HP were for "power" when you read the output of 7.3V? The 13.3 V number appears to be the better of the two.