How to change cutoff frequency of SCC-LP module?

[Ken P.]

Can anyone tell me the resistors and capacitors to change in a SCC-LP02 module to change its frequency response.  The SCC-LP02 has a 4th order butterworth filter with a cutoff at 50Hz.  For my application, I need to have filters with cutoffs at 15 Hz and 10 Hz.  Looking at the board in the SCC-LP02, I assume it uses a Sallen-Key filter implementation around the dual op-amp LT1112 of U1 and U4 (one for each of the two channels).

Thank you

Ken

[Ed W]

Hi Ken,

Thank you for posting to the NI forums.  We do not publicize the components used in our products.  In addition to this, I would strongly recommend against tampering with the internal circuitry of your device, as this is not supported and it will void the warranty.

Instead, I would recommend using something from our SCXI Signal Conditioning product line.  The SCXI-1141/2/3 all have programmable cutoff frequencies that range from 10 to 25kHz.  The 1143 is the most similar to your current device in that it also uses a Butterworth filter.

Ed W.

Applications Engineer

National Instruments

[Hwy-Star]

Why doesn't N.I. publish schematics for their products?  It's not like there is anything unique or mystical about the LPxx series of filters.  I spent 30 years in flight test instrumentation and was responsible for purchasing millions of dollars worth of data acquisition systems and all of the manufacturers gave complete schematics with the hardware.  It's great to say "you should just buy so-and-so modules", but when you are talking to a company with a budget, that's not always possible.  National Instruments charges top dollar for their products so it would be nice to have some real documentation.  I would much rather change the filter frequencies myself and risk loosing the warranty than spend another $400 plus per channel or god knows how much on a whole new system.  The word is SUPPORT...