Chapter 4 Signal ConditioningAMUX-64T User Manual4-8 ©National Instruments CorporationBuilding Lowpass FiltersYou can easily install simple resistance-capacitance (RC) lowpass filtersin the AMUX-64T board on any differential input channel. The filters areuseful for accurate measurement and noise rejection. By substitutingresistance and capacitance values into the following equation (hereafterreferred to as equation 4-1), you can calculate a simple, one-pole RC filterto have a –3 dB point (fc or cutoff frequency):(4-1)The frequency response rolls off at a rate of –20 dB per decade of increasethereafter. A Bode plot of the amplitude versus normalized frequency isshown in Figure 4-3.Figure 4-3. Normalized Frequency Response of Lowpass FilterWhen measuring low-frequency signals (about 4 Hz), if you have 400 Hznoise on your inputs, you can add a lowpass filter with a cutoff frequencyof 4 Hz. The 400 Hz noise then attenuates by 40 dB. Notice that your 4 Hzsignal also attenuates, but by only 3 dB. Do not neglect any potentialattenuation of signals of interest by this low-order filter.You must also choose the filter component values. You can select theresistance or the capacitance arbitrarily; one value determines the other.Picking the capacitor first and letting its value determine the resistancerequired is preferable because more standard resistor values are available.fc12πRC( )-------------------=AmplitudeNormalized Frequency1dB| | | | | |0.1 1 10 100 1,000 10,000(f )0.10.010.0010.00010–20–40–60–80c
