Enable onboard lowpass filter on PCI-6281

[JADaubs]

How do I enable the on-board lowpass analog filter on my PCI-6281 DAC from Labview 8.2? I can find many references to this being a property of the daq device, but none of the property dialogs available in MAX or in mxDAQ give me a chance to do this. Thanks for any advice.

ANdy

[devchander]

Take a look at this Link

The "2. Minimize Noise with Onboard Lowpass Filter" section illustrates how to programatically enable/disable the on board filter in this device, using a DAQmx Channel property Node in LabVIEW

[JADaubs]

---

@devchander wrote:

Take a look at this Link

The "2. Minimize Noise with Onboard Lowpass Filter" section illustrates how to programatically enable/disable the on board filter in this device, using a DAQmx Channel property Node in LabVIEW

---

Thank you so much! This is just what I could not find yesterday!
Best wishes,
Andy

[JADaubs]

I have a very perplexing situation. When I enable the onboard analog lowpass filter as prescribed, the data are noisier when the filter is enabled (input=T to enable property) than when it is disabled (input=F to enable property). In fact, when the filter is enebaled, the digitized signal is full of spikes. Am I missing something here?

Also, is the lowpass break frequency option operable on the 6281?

Thanks,

Andy

[devchander]

Andy,

What is the frequency range of your signal??

[JADaubs]

---

@devchander wrote:

Andy,

What is the frequency range of your signal??

---

I am driving the speaker with a 200-300 Hz signal.
Thanks,
Andy

[stilly32]

Hi Andy,

Can you post screenshots of your results with and without the filter enabled? Also, specifications like sample rate are nice. Just a question, when you say you are driving a speaker at 200-300hz, is that from an external device or from the AO on the 6281? The 6281 does have low pass filters available and if you followed the link referenced before you should be enabling them correctly. Another tool I like to use when working with filtering is looking at the acquisition in the frequency domain - the Spectral Measurements express vi is a great tool for this.

Hope this helps, please post back with screenshots and we'll see what we can do.

Cheers,

Andrew S.

National Instruments

[JADaubs]

---

@stilly32 wrote:

Hi Andy,

Can you post screenshots of your results with and without the filter enabled? Also, specifications like sample rate are nice. Just a question, when you say you are driving a speaker at 200-300hz, is that from an external device or from the AO on the 6281? The 6281 does have low pass filters available and if you followed the link referenced before you should be enabling them correctly. Another tool I like to use when working with filtering is looking at the acquisition in the frequency domain - the Spectral Measurements express vi is a great tool for this.

Hope this helps, please post back with screenshots and we'll see what we can do.

Cheers,

Andrew S.

National Instruments

---

I am driving the speaker with the AO on the 6281. Over the weekend, I discovered (by using a probe on the error out from the sampling task) that the 10 Khz sampling rate for the two-channel task I had set up was too fast for the settling time of the amplifier. Going to a single channel task eliminated the noise completely.
I am still puzzled by the analog break frequency. According to Gavin Gee, a very helpful NI engineer, the break frequency is fixed on the 6281 at 40 KHz, a value only useful from an anti-aliasing standpoint for sampling rates over 80 KHz. What is the use of the filter for sampling at lower rates (say, 10 KHz)? Is there any noise reduction advantage to enabling the filter at lower sampling rates? Thanks,
Andy

[stilly32]

Hi Andy,

"What is the use of the filter for sampling at lower rates (say, 10 KHz)? Is there any noise reduction advantage to enabling the filter at lower sampling rates?"

It's not intuitive at first, but there is definitely high frequency noise reduction when you use a filter, no matter what sampling rate you use. The filtering takes place before the ADC samples the signal, so it will sample the filtered signal. To illustrate this, I generated a 300Hz sine wave with gaussian noise with at a sample rate of 1MHz. You can see the signal on the top graph. I then passed it through a 40k lowpass filter - the bottom graph illustrates this. As you can see, the bottom one looks much cleaner, and that would be the that would be sampled.

There is still low frequency noise in the signal, but most high frequency component have been removed. So if you have high frequency noise, enabling the filter will help. If you just have low frequency (f< 40kHz), there will still be some attenuation of the signal from about 10khz and up. You can see this from the filter graph in the 6281 specifications.

Hope this helps, please post back if this doesn't clear things up.

Andrew S

National Instruments 

Message Edited by stilly32 on 01-30-2007 10:58 AM