Signal Conditioning

[B_Lind]

Looking for a low pass filter(~5kHz) module that would interface to the NI9205 when measuring a differential signal.

The signal is a squarewave signal (500Hz) that has 25kHz of noise riding on it..

Is it nessary to add a RC to both the  plus(+) and minus(-) inputs minimize the degradation of the amplifier’s CMRR ?

See attached..

Thanks.

[James-D]

Hi B_Lind,

We do not provide any modules that act solely as low pass filters to interface with an analog input module such as the 9205. We do offer some other modules (high-speed digitizers) that have more sophisticated internal circuitry and would include some more filtering built in to the device. 

As for the 9205, the article listed below is a good article to reference for wiring considerations that we recommend to deal with noise. 

http://www.ni.com/white-paper/3344/en

I hope this helps.

Regards,

[B_Lind]

Going to add a 100uf capactor across the input of each differential input to filter the differential mode noise..

The source impedance is less than one ohm. We will be scanning multiple channels with each channel having a similar squarewave input signal. 

 What should my concerns be as far as settling time and ghosting ?

Thanks

[James-D]

Hi B_Lind,

Since the source impedance is so low you shouldn't have any concerns as far as settling time and ghosting. The low source impedance will make the time constant of the low-pass filter very small and will help prevent ghosting. Please see the article shown below for more information on ghosting and how/why it occurs. 

http://digital.ni.com/public.nsf/allkb/73CB0FB296814E2286256FFD00028DDF?OpenDocument

As a side note, please keep in mind when implementing the capacitors for a low-pass filter that the 9205 has an internal 100pF capacitor connected in parallel before the ADC as shown in the diagram in the article above.

Regards,