Why Does Signal Level Attenuation Happens When the Frequency of Measured Signal is High ?

Updated Jul 13, 2020

Reported In

Hardware

  • Multifunction I/O Device

Issue Details

I am using multifunction I/O DAQ device. When I measure sine wave whose frequency is 500 kHz and the peak is 1 V, I found that the measured value has lower peak though I could measure 1 V level signal when the frequency of sine wave is much lower. Why does this happen ?

Solution

DAQ device has frequency characteristic specification. This is referred to as bandwidth in the specification sheet.
For example, if you see the specification sheet of PXIe-6363 , you will find the following description.

The image above indicates that PXIe-6363 has characteristic frequency 1.7 MHz where measured signal is attenuated -3 dB. So if the frequency of measured signal is 1.7 MHz, the signal level is attenuated -3 dB.
This is also explained in this document .

Not only PXIe-6363 but other DAQ devices have their own bandwidth specification. At this frequency the signal is attenuated -3 dB. Frequency characteristic graphs should be similar to the graph above.
 

Additional Information

The attenuation is originated from the low pass filter characteristic. The filter is implemented on the front end of the hardware. This filter is there to help reduce noise at higher frequencies.