Problem connecting analog input to PCI-6221 through external anti-aliasing filter

[miko77]

Hello,

We have designed an external analog antialiasing filter (4th order Bessel in a 2-stage Sallen Key topology) for conditioning  the signal from a current sensor. A preliminary version of the filter, employing the LM272 opamp (powered by a 12V single supply) has been implemented in breadboard. Using a frequency generator and an oscilloscope to monitor the input / output signals, the filter works as expected, with no noise present in the output.

However, as soon as the output of the filter is connected to  PCI-6221 (in RSE mode), there is a large amount of high-frequency noise superimposed on the output signal (e.g., a constant 2.5V output varies between about 2.2V and 2.8V). This noise can be observed in the oscilloscope, and is also present in the acquired data with the oscilloscope disconnected from the measurement setup.

When connecting the output of the filter to either USB-6009 or PCI-6023E the level of the added noise is significantly diminished (e.g. ranging from about 2.48V to 2.52V for a nominal 2.5V signal).

Any explanations and/or suggestions to resolve this issue would be greatly appreciated!

[johnsold]

It would help to see the data with the noise.

My first thought is that you have a grounding problem. Is the power supply for the filter circuit connected to the DAQ ground? Is the power supply linear or switching? What happens if you connect the current sensor directly to the DAQ device without the filter?

Lynn

[FatBoyMonchi]

Another thing to try, to understand where's the problem, may be to try to connect the output of the filter in differential mode.

Hope it helps,

bye.

[miko77]

Thanks for your response,

I have attached two example screenshots, with the results obtained for sampling the filter's output with 6221-37p and 6023E. Both of these cases are in RSE mode, while we have also tried differential mode with similar results. As I mentioned in my previous post, the noise is absent also when employing USB-6009 to read the signal.

Regarding your other questions, the (linear) power supply for the filter circuit shares a common ground with the DAQ, while the noise does not appear when connecting the current sensor directly to the DAQ device(s), without the filter.

Miko

[johnsold]

What is the bandwidth of the filter? Have you verified the performance of the filter with a signal generator and oscilloscope?

The input impedance of the USB-6009 is much lower (~144000 ohms) than the input impedances of most other devices. Try loading the output of the filter with 100000 to 150000 ohms to ground. 

Lynn

[miko77]

The filter bandwidth is 30 Hz, and its correct operation has been verified with a signal generator and oscilloscope.

I will try your suggestion with the loading resistor, although the puzzling thing is that the input impedance of the 6221-37p is quoted to be 10 GOhms, i.e., inbetween the 140 kOhms of the 6009 and the 100 GOhms of the 6023E, both of which yield noise-free results.

Thank you for your time once again.

[johnsold]

All of those devices use multiplexed input circuits to share one A/D converter across channels. Even though the input impedances are specified as resistance and capacitance, the real behavior can be more complicated when the multiplexer switches. It may be the some strange interaction between the filter circuit and the multiplexer on the 6221 is causing the problem. The load resistor will have an impedance much lower than any transient impedance changes from the multiplexer and may eliminate the problem if that was the source. The load resistor is much higher impedance than the output impedance of the filter circuit so it will not affect the performance of the filter.

Lynn

[umeshmnnit]

internal fault i had faced. no analog input no analog output. only digital working.

looked inside. IC 4816p was damaged which is a register array before the main IC . tow 4816P for total 16 analog inputs were inside. out of which one is damaged. this may be due to any overvoltage to analog input. main IC was also damaged in tbe form of a pinpoint in IC.  so never provide any voltage which can send current more than 20 m A max rating. a fuse of 10 mA before input may be used when we are using CB68 LP connector.