Performing FFT to a signal with a large DC offset would often result in a big impulse around frequency 0 Hz, thus masking out the signals of interests with relatively small amplitude.

Origin provides two methods to remove DC offset from the original signal before performing FFT:

- Using FFT High-Pass Filter
- Subtracting the Mean of Original Signal

Note that these two methods can get the same result. You can also not filter the input, but set zero to the zero frequency point for FFT result.

#### Using FFT High-Pass Filter

Any DC bias on the signal will show up in the frequency domain as amplitude at zero Hz, by setting the cutoff frequency to be zero DC offset can be filtered. Steps are as following:

- Highlight the source signal column
*Amplitude*, and select menu**Analysis: Signal Processing: FFT Filters**. -
In the pop-up dialog, choose

**High Pass**for**Filter Type**, uncheck**Auto**checkbox to set**Cutoff Frequency**to zero and clear the**Keep DC offset**check-box.

- Click
**OK**button to get the result without DC offset.

#### Subtracting the Mean of Original Signal

- Now we have the original signal stored in column B (
*Amplitude)*. Hold Ctrl + D to add a new column C for storing DC offset removed signal and input*DC Offset Removed*as**Long Name**. -
Highlight column C, and select

**Set Column Values**from the context menu to bring up the**Set Values**dialog. -
In

**Before Formula Scripts**panel of the**Set Values**dialog, put the script:

1 |
stats col(b); |

Then in the column formula text box, fill column C using the formula:

1 |
col(b)-stats.mean |

- Column C is the result without DC offset.

**Tips: **

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:

1 |
DCOffset = Mean(col(B)); |

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