Hi everyone.
Welcome to SDM, PCB Academy.
My name is Avital.
In the previous video we talked about differential signals its advantage over single ended signals with
two factors that affects differential impedance.
Now in this video I am going to discuss very popular differential signaling scheme which is LVDS, also
known as low voltage differential signaling.
I will talk about LVDS differential voltage, frequency data rate, etc. and how it is better than differential
signals, along with couple of simulations on Sigrity topology explorer.
So in short, I am going to talk about everything a hardware designer should know about LVDS.
So let's get started.
Let's start with very first question what is LVDS or low voltage differential signaling?
So LVDS is high speed long distance digital interface for serial communication over two wires.
That means we send or receive data one bit at a time.
Now, to understand LVDS signals electrically, we need to recall the working of differential signals.
From our last video.
So differential signals contrast to traditional single ended signals in two transmission lines which
are complementary to each other anyways.
So we send serial bits on these transmission lines and at the RX side comparator will generate the output
voltage.
So this is the general differential signaling scheme which provide noise immunity and common mode rejection.
And we have already seen this using SPI simulations on last video.
Now in case of LVDS transmitter, it consists of a current mode driver which provides a certain amount
of current.
So as per Ansi standards, it is 3.5 milliamps through the transmission lines of differential pairs.
Now at receiver, there is a 100 ohm termination resistor which is used to used to match the differential
impedance of transmission line which connects driver and receiver.
Now, as you can see, this termination resistor also provides a path between complementary signals
and high input impedance of receiver.
Cause the 3.5mA current coming from driver to flow through this 100 ohm termination resistor, and resulting
in 350 millivolt voltage difference between input terminal of receivers.
Let's see these three cases to understand the working of LVDS model.
So case one is, which is a default case when we are not sending any bit on differential pair signals.
So by default we'll have 1.2V common voltage or common mode voltage at the differential line.
As you can see in the waveform, let's see the second case when positive MOSFETs are closed.
So as soon as positive MOSFETs are closed Old 3.5 milliamps of current will start flowing on differential
pair and create 350 millivolt voltage difference between terminals of R x and this sort of waveform
will be formed.
So where you will see a clearly rise on non-inverting terminal, and it will rise up to 350 divided
by two volt.
And the inverting terminal will go low with the same voltage.
Let's see.
Case three when all the negative MOSFETs are closed and positive MOSFETs are open.
Current 3.5 milliamps will flow in opposite direction, and the waveform on differential pair signals
will be opposite.
So as you have seen on case two, inverting or complementary signal was going low and non-inverting
or true signal was going high.
In case of case three the exact opposite will happen.
The true signal will go low to 350 divided by two millivolt and the complementary signal will go high
for same voltage.
Now in this, if we say true signal is V1 and complimentary signal is V2, then v one equal to v plus
one by two of v differential and v two equal to v common minus one by two of v differential.
So in this case v common is 1.2V, which is the average of this waveform and v differential is 3.5 millivolt.
All right.
So two more things.
Then we will go for the simulation part.
Now what will be the output of receiver.
To derive that we need to separate differential and common part from the signal.
So as you can see the common voltage will be at 1.2V.
And differential signal will swing between 0.17 five volt to -0.175V.
That means the total swing will be 3.5 millivolt.
Now here what receiver will see?
We'll see what?
We'll see.
The difference of incoming differential waveforms.
That means the output voltage will be equal to V1 differential minus v2 differential.
So in first half cycle it will see 0.175 minus -0.175.
Now it will add this it will be 0.35V.
Right now let's see the other half cycle.
So on other half cycle we'll see minus.
So v1 differential will be -0.175.
And V2 differential will be 0.175.
And if we subtract that we'll get -0.35V.
All right.
So this will be the output of receiver side.
And we'll see total 700 millivolts of swing.
All right.
So now let's see the quick demonstration of LVDS.
For that I am using Sigrity Topology Explorer and Ibis models of LVDS drivers and receiver.
So I'm using two chips from TI, ds LVDS 1047 and 1048.
All right.
For that open topology explorer first.
And this is the topology.
And as you can see we have a transmitter.
This is differential signal of 100.9Ω differential impedance.
We'll also have 100 ohm termination resistors.
And we have discussed on whiteboard.
And then at last we have receiver.
Now where we need to enter all the information for these driver and receiver and trace properties.
So you can enter all this information in this property section.
As you can see here I have assigned the Ibis file.
If you click over this Ibis file you can just add all the Pin assignment, all the V differential voltage
time delay, etc..
All right I'll talk about this in very detail in next video.
And similarly for trace impedance you can add your stack of information here.
And as you can see as per this stack up I'm getting differential impedance of 100.90.
All right.
And where we enter all the data rate and bit pattern for that you have to click over Set analysis option.
Here you will find this option of adding data rate.
So as per data sheet of driver.
This particular driver which is DS LVDS 1047 supports up to 0.4GB.
All right.
And here I am sending 32 random bit patterns to get the simulation results.
Here you can set the corner frequency.
So either you can go for fast mode slow or fast slow slow fast I'm going for typical as of now.
And another thing which is important to check connectivity.
So before running the simulation you have to check the connectivity between driver and receiver, right?
It is properly connected or not.
To do that, you have to just run this tool and click over check button.
So from here it will verify.
Okay now it is ready.
Otherwise you will get a cross sign here.
That driver or receiver's pin two comma one is not connected to transmitter ten nine right.
But in our case it is connected.
Now let's run the simulation.
So first let's close this.
To run the transient analysis we have to just click over this start transient analysis.
So as you can see this is the waveform of R in one plus minus r in one minus okay.
So this is the same waveform that we have discussed for output of receiver.
All right.
And if you wanted to do the measurement you have just right click and set up some marker.
I just want to measure it vertically.
And just set this marker at the edge of this one.
If you want you can you can just zoom in and set those markers precisely.
But as you can see, the difference is very much close to 0.67V.
All right.
Similarly, if you want the these are the differential waveform which is coming at the input of receiver.
All right.
So similarly you can measure these as well.
And now if I wanted to compare.
So we have a transmission line between driver and receiver.
I wanted to compare what I am sending and what I am receiving at the receiver side.
All right.
To do that, firstly I'm just going to disable these and to zoom in just click over here and drag it
like this.
Okay.
Now I'm just going to compare R in one positive with.
Driver positive which is D out for positive.
All right.
And as you can see we have a very clear Time delay because of lengthy transmission line.
All right.
And if you want, you can just measure it.
To do that you have just again go to the measure difference.
And this time disable the vertical okay.
And you can just drag it here I'm just setting it at peak here.
And we can see the difference.
So it is around 780 1.4 picosecond time delay between driver signal and receiver signal.
All right.
And similarly if you want you can analyze the AI waveform for R in one plus and R in one minus.
To do that you have to just select the waveform and just uncheck all other waveforms and click over
this I button.
Once the AI waveform is there you have to add the mask.
So this mask information you will get from either from the standard or sometimes it is given on the
data sheet.
You have to fill all these details okay.
And you will simply apply the mask and see your AI waveform is within the limit or it is exceeding the
limit.
So then you can do other changes on your design and you know, make it working in your case.
All right.
So I'll explain that as well.
How to apply the mask and how to correct the waveform if you are getting some some kind of over limits.
So that's it for this, uh, simulation part.
Now in the next step, I'm just going to talk about some standards for LVDS, which is very important
to understand if we need to apply those for different applications.
So this is LVDS owners manual I'll attach this document in the description.
And here you'll find all the Ansi standards and the minimum and maximum limits for voltage frequency
and time delay etc..
All right.
So you can go through with this document.
It's like 5 or 6 page and it will give you enough brief about this standard.
Another one is IEEE standard.
And on page number 12 and page number 13 you will get a detailed table of voltage, current and frequency
and time delay specifications.
So you can go through with this document as well.
And based on these two documents we basically decide which chip we need to use for our application.
Like which standard is perfect fit for our application.
Either we should go for Ansi or IEEE.
I'll attach the project document as well.
So let me know in the comment section if you want me to discuss how I have created this topology and
how I have assigned the Ibis model, or edited the Ibis models.
So that's it for this video.
I hope you got the overview of LVDS, how to do different measurements and what are different terminologies
we use for LVDS, right?
Let me know in the comment section in case of any doubt and see you in the next video.