National Instruments BNC-2090A User Manual

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National Instruments BNC-2090A User Manual Manual pdf 28 page image

Chapter 3 Signal ConditioningBNC-2090A User Manual 3-6 ni.comFigure 3-3. Normalized Frequency Response of Lowpass FilterExampleWhen measuring low-frequency signals (about 4 Hz), if you have 400 Hznoise on your input signals, you can add a lowpass filter with a cutofffrequency of 4 Hz. The 400 Hz noise then attenuates by 40 dB. Notice thatyour 4 Hz signal also attenuates, by 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.If a capacitance of 1μF is available, the resistance is (by substitution intoEquation 3-1) about 39.8 kΩ. This resistance must be split evenly betweeneach input of a differential channel. Therefore, in this example, each inputchannel has a 19.89 kΩ resistor (or the closest standard value) in its seriespositions, C and D. NI recommends using 1% or better tolerance resistorsin this application because differences between the resistor values degradethe common-mode rejection ratio (CMRR).Figure 3-4 shows both the schematic and the component placement for a4 Hz lowpass filter placed on differential input channel 1. If the input signalsource is floating, you must place a bias return resistor in positions A and/orB (R28 and R27, respectively).–100–80–60–40–2000.01 0.1 1 10 100 1000 10000Normalized Frequency (fc)Output Amplitude (dB)