Above is a screen-shot from an LTSpice simulation. It is a freq. response of an LC tank circuit with a 5V sinusoidal AC source.
When I plot it, the y-axis is in dB.
When I look at the voltage it is 13.979dB which corresponds to 5V from the formula 20*log(V1/1).(1 is the reference voltage?)
But the current is plotted in negative dB. Why is that so? What does -dB mean here for the current? is that something to do with reference current in 20*log(I1/Iref)?
Answer
What does -dB mean here for the current?
0 dBI is exactly 1 amp (not to be confused with dBi which is the gain of an antenna compared to the benchmark (i)sotropic antenna)
A current of 100 mA is -20 dBI etc..
So when you have decibels for current as the Y axis then it is implicit that they the Y axis is dB relative to 1 amp (or dBI).
One further thing to consider is that the graph may be plotting peak current and peak current is \$\sqrt2\$ times bigger (for a sinewave). Read the LTSpice help advice to know exactly.
I've just checked on google about usage of the term "dBI" as representing current and nothing comes up. You can find dBuA for dB relative to 1 uA but nothing seems to mention dBI - I'm interested if anyone can provide some evidence to substantiate the usage of the term "dBI".
Sure, dBV are used and this is quite common and one website I visited compared decibels for current and voltage by calling them dB(volts) and dB(amps) - maybe "dBI" just isn't used so as not to confuse folk with dBi?
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