What's the difference between the two LVDS receivers? - Page 1
Forum section: electronics / beginners

author: LoveLaika
author_rank: Frequent Contributor
timestamp: October 28, 2024, 05:31:47 pm
content: I'm looking up ways to convert a digital LVDS signal into a single-ended signal. Doing some research, I found a basic image for a receiver, but I'm a bit confused by it.Looking at the first image of the basic LVDS receiver, you can see that it's a simple receiver with a termination resistor connecting the terminals. I just want to be clear in my understanding. Is that receiver a specialized like the IC like as theMAX3280, or could it also be an op-amp configured to convert the LVDS to a single-ended signal? Since you're (more or less) taking the difference between the two signals and rejecting the common-mode portion, could any single (appropriate) op-amp work as an LVDS receiver and convert it to a single-ended signal?On the other hand, as shown in my second image, I found this image from anAD articlethat shows how to use two op-amps to convert the LVDS to a single-ended signal with gain. This one looks like a method that uses op-amps, and it looks like it can work for both digital and/or analog LVDS. Is this just another way to convert the signal?

author: PGPG
author_rank: Frequent Contributor
timestamp: October 28, 2024, 07:46:31 pm
content: Quote from: LoveLaika on October 28, 2024, 05:31:47 pmOp-amp used without feedback converts differential signal into single-ended two state signal.But there are the reasons why except OpAms there also exists comparators that at first look you can think - it is the same.And there exists also LVDS receivers.Each of these devices are designed to fulfil different requirements.Op-amp working with +,- supply should allow to make stable amplifier of gain set with resistors but need not to have output voltages at specified digital stages (so it is possible that its maximum negative output is not close to its negative supply (it can be for example 1.5V from it).Comparator need not to allow to build stable amplifier and in most cases uses its negative supply as GND reference when its output is 0 state it should be rather close to 0V and not 1.5V like it can be for Op-amp.Op-amps and comparators have to have as small as possible input offset.Comparators are much faster then op-amps.I think that LVDS receivers (I have never used, and never read such IC datasheet) are much faster than comparators which was achieved at the cost of significantly decreasing the accuracy of the input voltage comparison.

author: langwadt
author_rank: Super Contributor
timestamp: October 28, 2024, 08:48:31 pm
content: LVDS is a digital standard usually high speed, use the appropriate reciever, it is cheap and works

author: LoveLaika
author_rank: Frequent Contributor
timestamp: October 28, 2024, 09:18:44 pm
content: Thanks for your insight. After doing some mock-up simulations with mock-up digital signals (at least, what I believe to be a correct LVDS signal), I found that the second option with 2 op-amps is unsuitable for my needs. Results for that one seemed to work best with a dual-power supply whereas I'm working with a single-power supply, so an LVDS receiver would be the best.

author: LoveLaika
author_rank: Frequent Contributor
timestamp: October 28, 2024, 09:26:17 pm
content: Thanks for replying. I came to that same conclusion myself as well. I got a mock-up using theAM26LS32receiver on a breadboard to see if this sort of IC will work. Is this the sort of thing that you're talking about? Also, by chance, is it typical to set it to active low assuming that there's no incoming differential signal? Would it be safe to set input G to GND to make it low when not in use?EDIT: Never mind. I get it now. It's meant to work with active-high or active-low systems via G or NOT G. Never mind. Looks like I have to set one or the other appropriately.With that, if the inputs are left floating, it should go to high (+5V), but the output is reading +3.6 volts. Seems kind of odd.

author: langwadt
author_rank: Super Contributor
timestamp: October 28, 2024, 11:56:40 pm
content: Quote from: LoveLaika on October 28, 2024, 09:26:17 pmthose a generic differential receivers, LVDS is a specific differential standard, 2.5V supply ~1.25V common mode, ~100-350mV differential swing

author: LoveLaika
author_rank: Frequent Contributor
timestamp: October 29, 2024, 03:48:10 am
content: Looking at Digikey, it's meant for RS422/423, so clearly not LVDS. But at least I had the right idea. With the protocol having a 2V voltage swing, would that be the cause for the wonky output voltage?In any case, hypothetically, what would occur if we tried to input an LVDS signal? Voltage wise, the chip should be able to tolerate it, but because it's not the right protocol, what would we see at the output? If this (kinda) works, then we can see about getting a proper chip. Pity it's all SMD.

author: Chalcogenide
author_rank: Regular Contributor
timestamp: October 29, 2024, 07:37:42 am
content: There are dedicated IC receivers that convert LVDS to LVCMOS/TTL levels (DS90LV048A, SN65LVDS series, etc...), but if you want to avoid them the simplest way is to just use a fast comparator with 100 ohm differential termination resistor. Make sure that it is fast enough and that the low-to-high and high-to-low propagation times are somewhat similar (TLV3601 for example). You can add a weak pull-up and pull-down either side of the 100 ohm termination to bias the comparator towards a positive or negative value if you want to add fail-safe output in case the input is disconnected.

author: LoveLaika
author_rank: Frequent Contributor
timestamp: October 29, 2024, 01:30:56 pm
content: Thank you for the idea. I'm not really trying to avoid them at this rate; I just want to make sure that what I'm doing is the right idea with regards to the type of IC that I should get. I thought for something like this, breadboarding it would help.

author: LoveLaika
author_rank: Frequent Contributor
timestamp: October 31, 2024, 02:33:41 pm
content: Quote from: Chalcogenide on October 29, 2024, 07:37:42 amFor curiosity's sake, I saw what would happen if I fed the LVDS signal into the AM26LS32 receiver on a breadboard. Turns out, it worked well for our needs. The differential input signal doesn't seem to be a true LVDS signal at high frequencies; rather, it seems to output at an LVDS voltage level. It's not a data signal but it's more of a trigger for some experiment. I had to modify it a bit by removing the input termination resistor and adding a small output capacitor, but it successfully converted the differential signal into a 3.3V Logic pulse on the scale of a few microseconds.

author: langwadt
author_rank: Super Contributor
timestamp: October 31, 2024, 03:09:38 pm
content: Quote from: LoveLaika on October 31, 2024, 02:33:41 pmLVDS is not short hand for differential it is a specific standard, there are other differential standards, figure out which one it is...removing termination, adding capacitor, are you just doing random thing in the hope that it will accidently do what you think it should?

author: LoveLaika
author_rank: Frequent Contributor
timestamp: November 01, 2024, 12:50:35 am
content: Well, it might not be the right or appropriate chip for the job, but it was just out of curiosity. As a proof of concept, it worked. Just gotta switch over to the right one. Once I saw that the thing did what I wanted, I just was curious to see how to improve on it.