05-18-2011 06:54 AM - edited 05-18-2011 06:56 AM
I'm conducting shot noise measurements on single molecule junctions. In order to bias the sample with a very clean DC bias, we've purchased the NI PXI-4461 for our lab. According to the specifications, the output noise should be in a order of 23*10^-6V for a bandwidth of 80kHz. However, when testing the V output by connecting it to a spectrum analyzer (SR780) at V=0 DC bias and differential output mode (To generate the DC signal, I've simply used the test panels.), the output spectra was very noisy (x units are Hz y units are Vrms/sqrt(Hz)):
So we've discovered the output is actually very noisy and especially has a high peak at ~53kHz, probably from the AC power supply. Connecting a 10Hz low pass filter significantly improved the result:
Which shows that the output noise filters of the 4461 are probably not activated. Also the peak at 50kHz from the power supply still remains very substantial (later, we've managed to reduce it further by disconnecting the ground of the output). According the the specifications of the 4461, it should be possible to get a very clean DC signal from it:
"NI 4461 output channels have both analog and digital anti-imaging filters. These filters remove the unwanted out-of-band components generated when an analog signal is produced from digital data. The digital filters limit the bandwidth of the output signal to half the original conversion rate, thereby rejecting images caused by the 8-times oversampling process. The signals generated by the analog output circuitry are low-distortion, low-noise, flat-frequency analog signals."
So my questions are:
1. How can I make these anti imaging filters work using labview? (code examples will be great)
2. Are there is any other configuration parameters of the PXI-4461 which can be used to lower the output noise?
3. How can I reduce the power supply noise? (regarding this - we also have a computer PXIe-8106 running on the same PXI chassis)
05-19-2011 01:08 PM
It sounds like your main issue is filtering the output noise from your DSA card and you're trying to find a proprty to activate the anti-aliasing filters. There is a DAQmx channel property node that can filter that noise, and is described in the KnowledgeBase article here. The article covers the AI property, but you can use the class browser to find the AO version as in the attached picture. Hopefully this helps.
05-22-2011 06:06 PM
Thanks, I've manage to activate output filtering.
Actually, turns out much of the noise was caused because of a noisy ground and decoupling the ground solved most of the problem.
09-25-2012 06:45 PM
You will actually not need to apply the filtering to an output signal in this way. First, note that the 4461 will automatically condition any output signals, such that additional filtering is not required.
Second, the property you are trying to enable is "AO.EnhancedImageRejectionEnable," as opposed to "AI.EnhancedAliasRejectionEnable," as used in the below article:
The image property is one not associated with your device, and is the cause for your error.
Please let us know if you have any further questions, tokohanz!