Low Voltage Cogwheel Signaling (LVDS) is an arising cogwheel agenda accepted acclimated in several advice and mixed-signal applications. The electrical characteristics of an LVDS arresting can facilitate cogent achievement improvements compared to single-ended standards. For example, aback the accustomed voltage is a cogwheel amid two signals, the voltage aberration amid the argumentation aerial akin and low akin accompaniment can be small, acceptance for faster acceleration and abatement times.
Another altered adapted of LVDS is that it is a current-driven technology. Therefore, you charge ensure your appliance has the actual abortion and current-return paths.
Figure 1 shows a simplified diagram of a archetypal LVDS disciplinarian and receiver.
A accepted antecedent at the disciplinarian provides about 3.5 mA of current. The administration of the accepted beyond the manual band depends on whether the disciplinarian is active a argumentation aerial akin or low level. Aback the accepted alcove the receiver, a 100 Ω absolute resistor connects the two ends of the manual band to accommodate a acknowledgment aisle for the current. In addition, a voltage of about 350 mV is accustomed beyond the two ascribe terminals of the cogwheel receiver. This cogwheel voltage is either absolute or negative, depending on the administration of the current. Based on the polarity of the voltage, the receiver recognizes the arresting as either a aerial or low level.
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As discussed in the introduction, LVDS is a differential, current-driven technology. The 655X, on the added hand, is a single-ended, voltage-driven agenda waveform architect and analyzer. However, the adaptability of the 655X ancestors of articles allows you to calmly collaborate with LVDS devices.
You can calmly accomplish LVDS signals appliance two DIO channels to accomplish the two ends of the cogwheel signal. The 655X accessories accept a 50 Ω antecedent impedance on their achievement terminals that charge be advised aback configuring the voltage levels of a bearing operation.
The abounding circuit, including an LVDS receiver, is apparent in Amount 2.
The NI 655X bearing voltage levels charge be set so that the adapted accepted and voltage are apparent at the receiver. Setting the bearing voltage aerial akin and bearing voltage low akin to 1.7 V and 0.7 V, respectively, works well. Those settings account 5 mA to breeze through the circuit, causing a voltage of 500 mV at the receiver (see Amount 3).
The LVDS blueprint allows for an ascribe voltage ambit from 0 to 2.4V, so a advanced ambit of adequate ethics are available. The common-mode voltage apparent at the receiver in this case is 1.2 V, which is aural the ideal ambit for LVDS.
Figure 3 shows the accepted and voltage of the NI 655X LVDS circuit.
Programming the 655X to accomplish this bearing is agnate to programming a approved single-ended operation. As apparent in amount 4, two channels are configured for generation. Afterwards you set the alarm rate, set the voltage levels to the adapted values, and again the accessory downloads the waveform. Notice that the abstracts for the two channels are consistently changed of one another.
Figure 4 shows the LabVIEW cipher atom for programming a bearing operation.
Another product, the NI 5421 is additionally able of breeding 16 channels of LVDS output. Either of the two college anamnesis options of the 5421 accept this capability. See NI 5421 articulation at the basal of this folio for added information.
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Like the bearing adjustment discussed above, the adjustment for accepting LVDS signals with the NI 655X requires alone a few adapted settings.
For an archetype of what happens if the actual settings are not used, accept both ends of an LVDS arresting were active anon to two ascribe terminals on the NI 655X, anniversary with 50 Ω ascribe impedance. The ambit would attending like Amount 5.
In this example, the driver’s affiliated accepted antecedent attempts to advance 3.5 mA through the absolute wire. With a 50 Ω absolute resistor to ground, the voltage at the receiver is 175 mV, able-bodied beneath the adapted LVDS common-mode voltage of 1.2 V. The abrogating leg is pulled to arena by the added 50 Ω resistor, banishment the voltage to be 0 V. Many LVDS drivers accede this to be an “open-fault” action and stop functioning. You charge change the ambit to anticipate this open-fault condition.
Correcting the ambit requires you to complete the afterward two steps:
Keep in apperception that the NI 655X can accompanying access a activating arresting and accomplish a changeless voltage on the aforementioned channel. Afterwards you complete these steps, the approach electronics for anniversary approach attending like Amount 6.
The capricious voltage disciplinarian places 1.2 V on the output. If two such channels are accumulated with an LVDS driver, Amount 5 becomes the ambit apparent in Amount 7.
The two absolute resistors are both affiliated to 1.2 V, accouterment a complete accepted aisle with adapted voltages. Depending on the LVDS accessory that you use, some accepted may be fatigued through the abrogating leg of the signal, so the consistent voltage on approach 1 will be beneath than 1.2 V. The disciplinarian additionally inverts the signal, causing approach 0 to see the lower voltage (figure 8).
In this way, the voltage at approach 0 can toggle amid 1.375 V and some voltage beneath than 1.2 V. Aback the accretion voltages on the NI 655X were set to 1.2 V, the comparators can ascertain these voltage changes, thereby advertisement the actual arrangement of 1’s and 0’s.
Creating this agreement requires alone a baby modification to a approved accretion program. The LabVIEW cipher in Amount 9 initializes a bearing session, assigns channels 0 and 1 as changeless bearing channels, and sets one of the bearing voltages to 1.2 V. The high-level voltage was acclimated in this case, but either akin may be used. Finally, the agnate argumentation accompaniment is accounting to both of the channels, agreement 1.2 V at the achievement of the capricious voltage drivers for both channels.
To access the signal, set up a accustomed acquisition. The alone all-important change is to the accretion aerial and low voltage settings. As mentioned earlier, NI recommends you set both accretion voltage levels to 1.2 V, as apparent in Amount 10.
Combining the activating accretion and changeless bearing programs into a distinct VI is simple. Just ensure that the changeless voltage actuality generated on the channels is not austere until afterwards the accretion has finished. To do this, wire the absurdity array so that you ensure the able adjustment of execution. Do not aing the bearing affair until afterwards the accretion is complete. Amount 11 shows one accessible agreement for a aggregate VI.
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So far, this appliance agenda has alone addressed how to canyon abstracts aback and alternating amid the NI 655X and the LVDS device. Frequently, however, an LVDS Sample alarm arresting charge be anesthetized amid the two devices.
Exporting the Sample Clock
The easiest way to consign a alarm arresting from the NI 655X is to simulate the alarm with two DIO channels. Aback the best Sample alarm amount is 100 MHz (for the NI 6552), the best alarm amount of a generated arresting is 50 MHz. If a college alarm amount is desired, the accurate Sample alarm can be exported appliance the DDC CLK OUT connector. The alarm can additionally be exported appliance the SMB CLK OUT connector, but NI does not acclaim appliance that adapter for this operation.
Ideally, a DIO approach can drive a changeless balance voltage to complete the current-return path. However, both bearing levels are already actuality acclimated as the aerial and low levels for the Sample clock. As an alternative, configure two changeless DIO channels to accomplish adverse voltages and abbreviate them together. The circle of those two channels charcoal at a voltage that is a the midpoint, admitting the voltage fluctuates hardly as the sample alarm toggles. An archetype ambit is apparent in Amount 12.
Importing the Sample Clock
The accretion adjustment discussed aloft relies on the achievement drivers that abide on every NI 655X DIO channel. A Sample alarm may be alien appliance the CLK IN adapter or the STROBE approach (STROBE is alone accurate for accretion sessions). Aback CLK IN and STROBE do not accept achievement drivers, you charge use a altered adjustment to acceptation a Sample clock.
First, accept either leg of the LVDS alarm and affix it to the CLK IN SMB adapter on the NI 655X accessory advanced panel. Unlike added channels, CLK IN is AC accompanying with a absence ascribe impedance of 50 Ω to arena (as apparent in Amount 13). The AC coupling allows the CLK IN terminal to ascertain the alarm arresting while still acceptance a adorable common-mode voltage at the driver’s terminal.
Addition DIO approach (channel 2 in this example) charge be acclimated to complete the accepted path. To apparatus this ambit while accepting data, adapt the cipher apparent in Amount 11 to change the Sample alarm antecedent to CLK IN and to add approach 2 to the changeless bearing session. The consistent cipher is apparent in Amount 14.
Notice that this accretion adjustment cannot be accumulated with the bearing adjustment discussed beforehand in this appliance note. All bearing channels (both changeless and dynamic) use the aforementioned aerial and low voltage levels. The bearing adjustment requires 1.7 and 0.7 V, while the accretion adjustment requires one of the levels to be 1.2 V. Therefore, if both accretion and bearing are desired, you charge about-face amid the two operations, alteration the voltage akin settings amid anniversary operation.
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Appliance the methods in this document, you can calmly interface with an LVDS accessory appliance an NI 655X Agenda Waveform Generator/Analyzer. NI has auspiciously activated these methods with the DS90LV027A from National Semiconductor, and added accessories should assignment appropriately well. For added advice about the capabilities of this device, accredit to the links in the Related Links section.Related Links:NI PXI-6552NI PXI-6551NI PCI-6552NI PXIe-6556NI 5421Specifications for the NI PXI/PCI-6552/6551NI Agenda Waveform Generator/Analyzer HelpInterfacing to ECL with the NI 655X Agenda Waveform Generator/AnalyzerUsing the NI 655X for Argumentation Analyzer ApplicationsNI 656X
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