[HSPICE] .ac simulation: The input is scaled down a thousand times

[ch hsiao]

Hello, I'm currently working with a project of an op amp.

And the small signal characteristic is "vout / vin = 562.4780k"
Then I conduct the ac simulation and encounter a problem:

When in the ac simulation, converting 562.478K into dBV, the result should be 115dB

But the AC conversion result is 55dB, and the conversion value of 55dB to v/v is 562

Obviously, hspice reduced the input by 1000 times and performed wrong calculations. How can I solve it?
Thank you so much.

 

[ch hsiao]

I had resolved the question that i reset the ac input from 0.001 to default 1, and i got the right answer, many thanks.

 

[KlausST]

Hi,

coud it be you misinterpreted "dBV"?
dB values are relative values where you calculate dB = 20 x log10(V1/V2), wher V1/V2 is a unitless ratio between two values.

But "dBV" gives an absolute value, it relates to "1V RMS".

example:
20dB is a voltage gain of 10 (no unit)
20dBV is an absolute voltage of 10V RMS

your case:
-60dBV means 1mV

Klaus

--- Updated Sep 13, 2023 ---

Hi,

coud it be you misinterpreted "dBV"?
dB values are relative values where you calculate dB = 20 x log10(V1/V2), wher V1/V2 is a unitless ratio between two values.

But "dBV" gives an absolute value, it relates to "1V RMS".

example:
20dB is a voltage gain of 10 (no unit)
20dBV is an absolute voltage of 10V RMS

your case:
-60dBV means 1mV

Klaus

 

[crutschow]

I had resolved the question that i reset the ac input from 0.001 to default 1, and i got the right answer, many thanks.

Yes, Spice calculates the dB value referenced to the input voltage, so If you want the reading to be in dB gain, then the input must be 1Vac.
And since Spice uses linear models for all the components when it does an AC simulation, the value of the input can be any value without otherwise affecting circuit operation.