6.5 MHz Resonant signal in 5752

[avantibob]

Hi,

I am using a No 5752 with a PXIe 7962R for the acquisition of some ultrasonic signals. The problem I am having is that I get 6.5 MHz resonance when I measure the signal through my PXI system.  When I measure the signal with an oscilloscope I do not see the resonance and get the signal I expect. I have attached an example of the signal in the scope (InScope.jpg) and the one from the PXI (Ring.jpg). I tested the PXI acquisition with a function generator to insure that it was working properly. It is when I test the system with an ultrasonic (piezoelectric) transducer through a preamp do I see the problem. My first thought was that it may be an impedance matching problem, but both the function generator and the preamp are 50 Ohm sources and the amplitudes are within the limits of the 5752. I hoping that someone will have some insight to help me out.

Thanks in advance for your help

[JefeL]

Hi Avantibob.

Your post indicates that we are testing with your own code. Have we tried using an example for the 5752 such as the NI 5752 Finite Acquisition Multiple Channels? We would like to see if the resonance occurs when we use a shipping example. This will help us narrow down the issue to code or hardware. 

Additionaly, when testing with the function generator, were we tesitng at the resonant frequency or performing a frequency sweep? Can we elaborate more how how the function generator tests were performed?

Thank you.

[avantibob]

Jeff,

Yes I am testing with my own code, but as per your suggestion I tested using NI 5752 Finite Acquisition Multiple Channels. Unfortunately, the resonance was still present, so it appears to be hardware problem. I believe it maybe some kind of feedback, for the lack a better word, from the preamp I am using. Perhaps the draw down resistor I am using is incorrect, right now it is 100 Ohm.

As for the testing with the function generator, I did test the software with a sweep of frequency ranging from 500 kHz to 10 MHz which covers the bandwidth I expect to experience in this project. The function generator testing was performed on the AI 0 channel with a peak to peak voltage between 0.25 Volts and 2 Volts.

Any help is greatly appreciated.

Bob

[Nathan-P]

Can you provide a schematic of your setup?

[avantibob]

Its not to fancy, but I hope it helps.

[avantibob]

Also here is the data sheet for the preamp

[Nathan-P]

What is the resonant frequency of your ultrasonic array elements?

[avantibob]

2.25 MHz

[Nathan-P]

Also, could you provide more details on the "Terminal Interfaces" to the 5752?  You may have an impedance mismatch issue, since the 5752 is a differential board, which means that between the + and - connectors, the impedance is actually 100 ohms...

[Nathan-P]

2.25MHz, that means that 6.5MHz is pretty close to the 3rd order harmonic, I don't have much analog background, but that seems suspect...