Using PCI-6111 to get worse SNR than using USB-6221 for the same signal source

[li8866]

The problem could be due to the input configuration. For PCI-6111, there is only one option: pseudodifferential. But for USB-6111, there are three: RSE, NRSE, DIFFERENTIAL. When i choose RSE, I get good SNR. But i prefer PCI-6111 because of higher speed. The input signal is anolog voltage in the time domain, then i do the FFT to get the frequency domain information. The SNR i mentioned above refers to the signal in frequency domain. Could anyone help me to sovle this problem?

[Ben_L]

Hi,

 Your application sounds very interesting, can I ask what frequency range your signal is coming in at?

For DC voltage application and low frequencies, Pseudo Differential works perfect. For higher frequencies, you should take into consideration that this capacitor is going to look like a short circuit. This may be the reason you are having issues with your PCI-6111.

Best Regards

[li8866]

Thank you for your instant reply. Currently, the measured frequency range is from several Hz to tens of kHz. In the next stage, the frequency could be as high as several MHz. what i am interested is AC component of the voltage signal. That is the reason that i do the FFT. Thanks again!

[Ben_L]

Hello again,

Unfortunately the PCI-6111 is limited to Pseudodifferential mode and only has a 12-bit resolution, the USB-6211 is not fast enough to acquire data in the MHz domain. You should be fine using the USB-6211 for initial measurements but when you want to measure higher frequency components the DAQ device will have trouble keeping up with the input and you will loose information.

In order to satisfy the requirements of your application you should look into getting some alternative hardware, look through the products at http://www.ni.com/dataacquisition/.

Contact your local National Instruments branch to your discus options with an Inside Sales Engineer.

Best Regards