# How to Choose my Sampling Frequency to Get a Proper Signal Shape?

Updated Mar 3, 2018

• NI-9242
• NI-9219

## Issue Details

• I am reading the correct frequency, however my amplitude is incorrect.  Sometimes it is too small, sometimes it is nearly correct.  How do I make the amplitude consistently correct?  I am sampling at the Nyquist Rate.
• I am working on a data acquisition system. The voltage and current signals are sinusoidal, however when plotting them using the below sampling frequency, I am receiving a different shape:
I/O Sampling Frequency= 2.5 * Frequency_Signal

What is the best sampling frequency  that will reproduce a correct shape of my signal?

## Solution

According to the Shannon Sampling Theorem, you can maintain the frequency content of the signal by sampling at the Nyquist rate; i.e. using a sampling frequency at least twice the maximum frequency component in the sampled signal. Maintaining the frequency content does not mean maintaining the signal shape.

If you want to receive a proper signal shape, as a rule of thumb, you should sample at 10 times the signal frequency; i.e. if the sinusoidal is at 50Hz, you should sample at 500Hz, which means a sampling period of 2ms.
This link demonstrates different cases on sampling, showing clearly the difference in shape when sampling at 10 times the frequency of the input signal.