# 3.70 FAQ-307 How do I remove DC offset before performing FFT?

Last Update: 2/4/2015

Two methods to remove DC offset from the original signal before performing FFT:

• Use FFT high-pass filter
1. Highlight the source signal column, and select menu Analysis: Signal Processing: FFT Filters.
2. In the pop-up dialog, choose High Pass for Filter Type, set Cutoff Frequency to zero and clear the Keep DC offset check-box.
3. Click OK button to get the result without DC offset.
• Subtract the mean of the original signal
1. Suppose the original signal is stored in column B. create a new column, say column C.
2. Highlight column C, and select Set Column Values from the context menu to bring up the Set Values dialog.
3. In Before Formula Scripts panel of the Set Values dialog, put the script:
stats col(b);
Then in the column formula text box, fill column C using the formula:
col(b)-stats.mean
4. Column C is the result without DC offset.

If you need to get the DC offset, open the dialog mentioned in method one, then use the low-pass filter, and set Cutoff Frequency to zero, or use the Mean function to calculate the mean of the signal:

DCOffset = Mean(col(B));

Keywords:DC, FFT, cutoff frequency, low-pass