External Analog Lowpass Filter (anti-aliasing filter) for NI-USB 6251

[pinguim]

Hello everybody!

-I´m trying to acquire a voltage analog signal (high frequency content) by using a BNC 2110 connected to a bord NI USB-6251. I was informed in this Labview Forum that NI USB-6251 has no programmable analog lowpass filter (or anti-aliasing filter), so that I cannot avoid Aliasing during the digitization of my signals. For my application, the cutoff frequency of the analogue lowpass filter should be equal to 100 kHz or higher (maximum of 500 kHz). A possible solution to solve my problem, would be to work with an external analog lowpass filter before digitizing the voltage signal. Based on this I would like to know:

1) Does National Instruments develop such external analog filters? I need a filter which also presents an analog output, so that I could also send the analog lowpass filtered signal to my NI USB-6251 in order to appropriately digitize it!

2) What model of external lowpass filter would be compatible with NI USB-6251?

Any help would be highly appreciated!

Best regards!

[johnsold]

I am unaware of any NI analog filters.

There are companies which make anti-aliasing filters.  (tte.com is one).

Depending on your requirements, you may be able to get by with something as simple as a resistor and a capacitor.  What is the signal source, in particular, the output impedance?  How many channels are you digitizing? Differential or single-ended? What is the noise or interference spectrum? What is your sampling rate?

Lynn

[HoussamK · Accepted solution]

Hi,

all of the high resolution M-Series cards (628x) are equipped with a low pass filter that can be programmatically enabled or disabled. For anti-aliasing filter feature, consider the National Instruments DSA (Dynamic Signal Acquisition) Boards Sound and Vibration Measurement Hardware
currently the NI 9221, 9225, 9227, 9229, 9233, 9234, 9235, 9236, 9237 and 9239 C Series modules feature anti-alias filters. These modules are designed for very high-accuracy measurements for which anti-aliasing filters are a necessity.

Houssam Kassri

NI Germany

[pinguim]

Hi Lynn

-Thank you very much for your reply and information! I have already checked the link you sent me! It could maybe be a more practical solution to my problem than constructing my resistor-capacitor filter!

-I have no idea what is the signal source´s output impedance, as it should be inform by the manufacture of the monitoring sytem which I am using! This information is not available to me! Maybe I could measure it! Do you know a simple way to measure it without demaging the monitoring devide? The signal source´s  flow is: Transducer-Monitoring System-NI block 6251

-I need to digitize at least 3 channels at the same time in differential mode!

-I don´t know what is the noise or interference spectrum! For knowing that, I should first to acquire my signals!

-The sampling rate to be used depends on the lowpass filter´ cuttoff frequency to be selected!

best regards!

[pinguim]

Hi Lynn

-Thank you very much for your reply and information! I have already checked the link you sent me! It could maybe be a more practical solution to my problem than constructing my resistor-capacitor filter!

-I have no idea what is the signal source´s output impedance, as it should be inform by the manufacture of the monitoring sytem which I am using! This information is not available to me! Maybe I could measure it! Do you know a simple way to measure it without demaging the monitoring devide? The signal source´s  flow is: Transducer-Monitoring System-NI block 6251

-I need to digitize at least 3 channels at the same time in differential mode!

-I don´t know what is the noise or interference spectrum! For knowing that, I should first to acquire my signals!

-The sampling rate to be used depends on the lowpass filter´ cuttoff frequency to be selected!

best regards!

[pinguim]

Hi Houssam Kassri,

-Thank you a lot for this information! Now I can realize the NI-USB 6251 is not appropriate for my application! I´m going to check the link you sent me, and the NI 92xx and C Series modules as well!

best regards!

[johnsold]

Measuring the output impedance of the transducer/monitoring system may not be easy if the manufacturer has not provided much information.  However, your description as "transducer/monitoring system" suggests that it consists of separate transducer and signal conditioner modules. If this is the case, it is likely that the output impedance of the signal conditioner (monitoring system) will be low (unless the output is a current like 4-20 mA).  The signal conditioner may also include bandwidth limiting so that an anti-aliasing filter would be unnecessary.

What are you measuring? What kind of transducer is used?

Lynn

[pinguim]

Hi Lynn,

-Thank you for the information and help! The signal flow of my application is featured as follow:

Acoustic Emission (AE) transducer - AE Monitoring System - BNC 2110 Block Connector - NI-USB 6251

- The AE Monitoring System has a RAW analog output (frequency range= 100 Hz up to 900 kHz), which is connected to the BNC 2110. The BNC 2110 Block is connected to the NI-USB 6251. Based on the AE Monitoring System manufacture´s information, the RAW analog output has no bandwidth limiting (or anti-aliasing filter). Because of that, I am looking for an analog lowpass filter (or anti-aliasing filter) to correctly digitize the analog singals!

best regards!

[pinguim]

Hi Lynn,

-Thank you for the information and help! The signal flow of my application is featured as follow:

Acoustic Emission (AE) transducer - AE Monitoring System - BNC 2110 Block Connector - NI-USB 6251

- The AE Monitoring System has a RAW analog output (frequency range= 100 Hz up to 900 kHz), which is connected to the BNC 2110. The BNC 2110 Block is connected to the NI-USB 6251. Based on the AE Monitoring System manufacture´s information, the RAW analog output has no bandwidth limiting (or anti-aliasing filter). Because of that, I am looking for an analog lowpass filter (or anti-aliasing filter) to correctly digitize the analog singals!

best regards!

[johnsold]

Since you are considering a bandwidth of 100 kHz to 500 kHz (from your first post), you should definitely use an anti-aliasing filter.

The USB-6251 has a specified input impedance of 10 Gohms in parallel with 100 pF from each input to ground. Assuming that the AE Monitoring system has a fairly low output impedance, the input capacitance of the U 6251 could be used as part of the filter. In the image below R1, R2, C1, and C2 represent the input impedance of the 6251. R3 and R4 are external resistors which form the balance of the filter. The values shown below provide a cutoff of approximately 100 kHz.  The DC error due to R3 and R4 is less than 2 parts per million which is well below the resolution of the 6251.

Lynn

[pinguim]

Hi Lynn,

-Thank you a lot for the information and help! It seems a very practical and interesting sollution! I will try to implemment it and let you know if it works!

best regards!

[pinguim]

Hi Lynn,

-I´m trying to implement the proposed solution as described in your last reply! So far, it does not seem clear to me, how can I carry out the physical connections between the board (NI USB 6251-Mass Termination) and the external resistors (described as R3 and R4 in your reply). There seems to be no available pinouts inside the “NI USB 6251-Mass Termination” device, as illustrated in Figure-1.

-My experimental rig consist of : a) AE Transducer; b) AE Coupler; c) BNC 2110 from National Instrumenst; d)NI USB 6251 Mass Terminator and e) Personal Computer, (Figure-2). The signal flow is as follows: a) to b) to c) to d) to e). If there is no possibility to implement the solution described earlier, how could I carry out a similar solution, by connecting the externals resistor (R3 and R4) to the BNC 2110, instead of to the board “NI USB 6251-Mass Termination”?

best regards!