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Unformatted text preview: 4) Choose a capacitor that has the following suggested characteristics: AXL or RDL package Tolerance of 20% Maximum voltage of at least 25 V Adding Components
Using the circuit shown in Figure 5-11, you can use a two-component circuit to build a simple RC filter with analog input, analog output, or digital input. You can build a single-ended analog input RC filter with pads F and B for one channel and pads G and D for the next channel. You can build a differential analog input RC filter with pads F and E. For TRIG1, you can use pads R1 and RC1. For AO, you can use R2 and RC2 for DAC1OUT, and you can use R3 and RC3 for DAC0OUT. National Instruments Corporation 5-11 SCB-68 Shielded Connector Block User Manual Chapter 5 Adding Components for Special Functions For any type of lowpass filter, use Equation 5-5 to determine the cut-off frequency (fc). 1 fc = -------------2RC (5-5) Single-Ended Lowpass Filter
To build a single-ended lowpass filter, refer to Figure 5-12. Add the resistor to position B or D, depending on the AI channel you are using. Add the capacitor to position F or G, depending on the AI channel you are using. ACH<i> + Vin AIGND CF,G + RB,D Vm Figure 5-12. SCB-68 Circuit Diagram for a Single-Ended Lowpass Filter Differential Lowpass Filter
To build a differential lowpass filter, refer to Figure 5-13. Add the resistor to position E and the capacitor to position F.
CF + RE Vm ACH<i> + Vin ACH<i+8> Figure 5-13. SCB-68 Circuit Diagram for a Differential Lowpass Filter Analog Output and Digital Input Lowpass Filtering
For DAC0OUT, add the resistor to position RC3 and the capacitor to position R3. For DAC1OUT, add the resistor to position RC2 and the capacitor to position R2. For TRIG1, add the resistor to position RC1 and the capacitor to position R1. SCB-68 Shielded Connector Block User Manual 5-12 ni.com Chapter 5 Adding Components for Special Functions Lowpass Filtering Applications
Noise filtering and antialiasing are two applications that use lowpass filters. Noise Filtering
You can use a lowpass filter to highly attenuate the noise frequency on a measured signal. For example, power lines commonly add a noise frequency of 60 Hz. Adding a filter with fc< 60 Hz at the input of the measurement system causes the noise frequency to fall into the stopband. Referring to Equation 5-4, fix the resistor value at 10 k to calculate the capacitor value and choose a commercial capacitor value that satisfies the following relationship: 1 C > ---------------------------------------2 ( 10, 000 ) ( 60 ) (5-6) Antialiasing Filtering
Aliasing causes high-frequency signal components to appear as a low-frequency signal, as Figure 5-14 shows. 1 1 0 2 4 6 8 10 Input Signal Sampled Points Reconstructed Signal Figure 5-14. Aliasing of a High-Frequency Signal The solid line depicts a high-frequency signal being sampled at the indicated points. When these points are connected to reconstruct the waveform, as shown by the dotted line, the signal appears to have a lower frequency. Any signal with a frequency greater than one-half of its sample rate is aliased and incorrectly analyzed as having a frequency below one-half the sample rate. This limiting frequency of one-half the sample rate is known as the Nyquist frequency. National Instruments Corporation 5-13 SCB-68 Shielded Connector Block User Manual Chapter 5 Adding Components for Special Functions To prevent aliasing, remove all signal components with frequencies greater than the Nyquist frequency from input signals before those signals are sampled. Once a data sample is aliased, it is impossible to accurately reconstruct the original signal. To design a lowpass filter that attenuates signal components with a frequency higher than half of the Nyquist frequency, substitute the half Nyquist value for the fc value in Equation 5-6. The following devices provide antialiasing filters and do not need to have the filters implemented at the SCB-68 terminal block: NI PCI/PXI-61XX (not including the NI PCI-6110/6111) NI PCI-445X NI PCI-455X Special Consideration for Analog Input Channels
Filtering increases the settling time of the instrumentation amplifier to the time constant of the filter used. Adding RC filters to scanning channels greatly reduces the practical scanning rate, since the instrumentation amplifier settling time can be increased to 10T or longer, where T = (R)(C). You can use RC filters with single-ended or differential inputs. Special Consideration for Analog Output Signals
Lowpass filters can smooth stairstep-like curves on AO signals. If the curves are not smoothed, the AO signals can be a hazard for some external circuitry connected to it. Figure 5-15 shows the output of a lowpass filter when a stairstep-like signal is the input. SCB-68 Shielded Connector Block User Manual 5-14 ni.com Chapter 5 Adding Components for Special Functions Volts (V) Time (t) Figure 5-15. Lowpass Filtering of AO Signals Special Consideration for Digital Trigger Input Signals
Lowpass filters can...
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