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# What does a dB reference in voltage in LTSpice

Category: Software
Product Number: LTSpice
Software Version: XVII

I am used to the dB of a signal referencing the rms value of that signal, whether it be for power (50 ohm referenced) or voltage. However, it appears that LTSpice references the peak value.? For a transient analysis, I set the sinewave amplitude to 1V. When I plot this, I get a 1V peak waveform. If I switch to measure an AC analysis and set the AC amplitude to 1V, I see 0dB. That would seem to indicate that 0dB is referenced to a 1V peak signal instead of the rms or .707V. Is this correct for LTSpice?

• Hello there,

I think you are mixing up the definition of an SINE voltage source for a .tran transient analysis (Amplitude 1V = 1V pk) and the definition of AC Amplitude for a .ac Small signal AC analysis (1V = 1Vrms).

Agree that it is somewhat confusing but the amplitude for a PULSE waveform will be the peak, so it is consistent.

Best regards,

Martin.

• AC_1V.asc

This should make it clear, I hope.  To get a 1V rms sinewave for transient analysis (.tran), you would have to set the SINE voltage source amplitude to 1.414V.  The reason why the amplitude to define the voltage source is the peak voltage is that not everything is a sinewave.  RMS makes no sense for a PULSE waveform or PWL source.  For a small signal AC analysis (.ac), the AC=1 is just there to help you show gain and phase of Vout instead of having to type in Vout/Vin each time.  The two analyses are not connected at all.  You can prove this by changing the SINE voltage to an arbitrary value - it won't change the AC analysis.  Just comment out the .tran and uncomment the .ac to see the difference.

• Hi Martin,

Thanks for the replies. Yes, I understood the first answer as well as the second. Just didn't get a chance to respond in a timely manner. Now that you stated that in the second reply, I just noticed the text is different for the two. The functions for use with the transient analysis include the units, while the AC analysis does not. Thanks for clearing that up for me.

Bob

• Hi Bob,

You're most welcome.

Martin.