# Calculation of SFDR from DFT plot

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#### abhisek.dey007

##### Newbie level 4
Hi,
Can anyone give me some idea how I can calculate SFDR from the DFT plot obtained by using cadence ADE calculator. I want to know after getting the DFT plot, what function in the calculator do i need to choose to obtain SFDR?

Thanx.

#### JoannesPaulus

Its quite simple: it is the difference between the power on the fundamental and the highest tone. There is no calculator function but it is rather simple to write.

#### abhisek.dey007

##### Newbie level 4
Okay...can you tell how i can plot dBFS vs freq plot for FFT/DFT?

#### JoannesPaulus

What device are you talking about? Amplifier, ADC, DAC... in general, you just divide the output by the full-scale (normalize to the full-scale) before taking the DFT so that it corresponds to <output quantity>/<full scale quantity>=1->0dBFS

#### abhisek.dey007

##### Newbie level 4
thanx,,,I'm simulating a 12bit SAR ADC. Doing trans simulation for the DFT plot. But how to make the x-axis as freq?? I mean wat kind of simulation/calculator function is needed???

#### abhisek.dey007

##### Newbie level 4
guys plz help,,,how to plot DFT/FFT in a dB Vs. freq graph????

#### gururajb

##### Newbie level 3
After the transient simulation, open the calculator and u vl find 'vt' button. Press the vt button and select the net in the schematic.
Then select the 'dft' function from special functions and give the required data.
Now select 'dB20' button and press 'plot'.

#### lamoun

##### Member level 5
Before running the dft you have to use a DAC to make the 12bit output of the ADC an analog voltage.
You can easily do that using 12 VCCS with Ibias / 2Ibias / 4Ibias... all driving one single resistor.
Or you can use the calculator and do the same thing by math.

After the transient simulation, open the calculator and u vl find 'vt' button. Press the vt button and select the net in the schematic.
Then select the 'dft' function from special functions and give the required data.
Now select 'dB20' button and press 'plot'.
Before dB20 you can do JoannesPaulus in post #4

And use a sine input!

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