WEBVTT 00:00.480 --> 00:01.320 Hi everyone. 00:01.480 --> 00:03.080 Welcome to SDM, PCB Academy. 00:03.120 --> 00:04.080 My name is Avital. 00:04.200 --> 00:09.960 In the previous video we talked about differential signals its advantage over single ended signals with 00:10.000 --> 00:12.520 two factors that affects differential impedance. 00:13.360 --> 00:19.040 Now in this video I am going to discuss very popular differential signaling scheme which is LVDS, also 00:19.040 --> 00:21.120 known as low voltage differential signaling. 00:21.560 --> 00:29.400 I will talk about LVDS differential voltage, frequency data rate, etc. and how it is better than differential 00:29.400 --> 00:34.240 signals, along with couple of simulations on Sigrity topology explorer. 00:34.560 --> 00:39.760 So in short, I am going to talk about everything a hardware designer should know about LVDS. 00:40.280 --> 00:41.480 So let's get started. 00:44.280 --> 00:49.880 Let's start with very first question what is LVDS or low voltage differential signaling? 00:50.560 --> 00:56.760 So LVDS is high speed long distance digital interface for serial communication over two wires. 00:57.920 --> 01:01.460 That means we send or receive data one bit at a time. 01:02.700 --> 01:08.940 Now, to understand LVDS signals electrically, we need to recall the working of differential signals. 01:09.060 --> 01:10.340 From our last video. 01:10.660 --> 01:16.860 So differential signals contrast to traditional single ended signals in two transmission lines which 01:16.860 --> 01:19.460 are complementary to each other anyways. 01:19.500 --> 01:26.620 So we send serial bits on these transmission lines and at the RX side comparator will generate the output 01:26.620 --> 01:27.140 voltage. 01:27.900 --> 01:33.740 So this is the general differential signaling scheme which provide noise immunity and common mode rejection. 01:34.380 --> 01:39.260 And we have already seen this using SPI simulations on last video. 01:40.060 --> 01:46.980 Now in case of LVDS transmitter, it consists of a current mode driver which provides a certain amount 01:46.980 --> 01:47.740 of current. 01:47.940 --> 01:54.780 So as per Ansi standards, it is 3.5 milliamps through the transmission lines of differential pairs. 01:55.900 --> 02:01.640 Now at receiver, there is a 100 ohm termination resistor which is used to used to match the differential 02:01.640 --> 02:05.480 impedance of transmission line which connects driver and receiver. 02:05.880 --> 02:11.560 Now, as you can see, this termination resistor also provides a path between complementary signals 02:12.320 --> 02:14.720 and high input impedance of receiver. 02:14.840 --> 02:23.120 Cause the 3.5mA current coming from driver to flow through this 100 ohm termination resistor, and resulting 02:23.120 --> 02:27.760 in 350 millivolt voltage difference between input terminal of receivers. 02:30.240 --> 02:34.840 Let's see these three cases to understand the working of LVDS model. 02:35.440 --> 02:42.280 So case one is, which is a default case when we are not sending any bit on differential pair signals. 02:42.440 --> 02:50.200 So by default we'll have 1.2V common voltage or common mode voltage at the differential line. 02:50.320 --> 02:56.720 As you can see in the waveform, let's see the second case when positive MOSFETs are closed. 02:57.120 --> 03:03.820 So as soon as positive MOSFETs are closed Old 3.5 milliamps of current will start flowing on differential 03:03.820 --> 03:13.300 pair and create 350 millivolt voltage difference between terminals of R x and this sort of waveform 03:13.300 --> 03:14.100 will be formed. 03:14.460 --> 03:21.820 So where you will see a clearly rise on non-inverting terminal, and it will rise up to 350 divided 03:21.820 --> 03:22.780 by two volt. 03:23.460 --> 03:26.780 And the inverting terminal will go low with the same voltage. 03:27.580 --> 03:27.980 Let's see. 03:28.020 --> 03:32.340 Case three when all the negative MOSFETs are closed and positive MOSFETs are open. 03:32.700 --> 03:39.500 Current 3.5 milliamps will flow in opposite direction, and the waveform on differential pair signals 03:39.500 --> 03:40.460 will be opposite. 03:40.740 --> 03:47.660 So as you have seen on case two, inverting or complementary signal was going low and non-inverting 03:47.660 --> 03:49.940 or true signal was going high. 03:49.940 --> 03:53.260 In case of case three the exact opposite will happen. 03:53.260 --> 04:00.880 The true signal will go low to 350 divided by two millivolt and the complementary signal will go high 04:01.120 --> 04:02.080 for same voltage. 04:03.120 --> 04:11.320 Now in this, if we say true signal is V1 and complimentary signal is V2, then v one equal to v plus 04:11.320 --> 04:17.680 one by two of v differential and v two equal to v common minus one by two of v differential. 04:18.360 --> 04:27.120 So in this case v common is 1.2V, which is the average of this waveform and v differential is 3.5 millivolt. 04:28.400 --> 04:29.000 All right. 04:29.200 --> 04:30.280 So two more things. 04:30.400 --> 04:32.240 Then we will go for the simulation part. 04:32.640 --> 04:34.840 Now what will be the output of receiver. 04:35.800 --> 04:40.760 To derive that we need to separate differential and common part from the signal. 04:41.120 --> 04:45.640 So as you can see the common voltage will be at 1.2V. 04:45.640 --> 04:52.680 And differential signal will swing between 0.17 five volt to -0.175V. 04:52.720 --> 04:55.520 That means the total swing will be 3.5 millivolt. 04:57.040 --> 04:59.520 Now here what receiver will see? 04:59.660 --> 05:00.140 We'll see what? 05:00.140 --> 05:00.540 We'll see. 05:00.540 --> 05:03.580 The difference of incoming differential waveforms. 05:04.340 --> 05:09.540 That means the output voltage will be equal to V1 differential minus v2 differential. 05:10.140 --> 05:17.340 So in first half cycle it will see 0.175 minus -0.175. 05:18.740 --> 05:21.820 Now it will add this it will be 0.35V. 05:22.660 --> 05:24.820 Right now let's see the other half cycle. 05:25.020 --> 05:28.100 So on other half cycle we'll see minus. 05:28.220 --> 05:31.380 So v1 differential will be -0.175. 05:31.660 --> 05:34.180 And V2 differential will be 0.175. 05:34.340 --> 05:38.420 And if we subtract that we'll get -0.35V. 05:39.100 --> 05:39.580 All right. 05:39.580 --> 05:42.060 So this will be the output of receiver side. 05:42.260 --> 05:45.780 And we'll see total 700 millivolts of swing. 05:49.540 --> 05:50.060 All right. 05:50.180 --> 05:52.860 So now let's see the quick demonstration of LVDS. 05:52.980 --> 05:59.340 For that I am using Sigrity Topology Explorer and Ibis models of LVDS drivers and receiver. 06:00.800 --> 06:06.960 So I'm using two chips from TI, ds LVDS 1047 and 1048. 06:07.960 --> 06:08.240 All right. 06:08.280 --> 06:10.520 For that open topology explorer first. 06:11.920 --> 06:14.080 And this is the topology. 06:14.520 --> 06:16.880 And as you can see we have a transmitter. 06:17.480 --> 06:22.160 This is differential signal of 100.9Ω differential impedance. 06:23.200 --> 06:25.800 We'll also have 100 ohm termination resistors. 06:25.800 --> 06:27.760 And we have discussed on whiteboard. 06:28.720 --> 06:30.840 And then at last we have receiver. 06:32.080 --> 06:38.600 Now where we need to enter all the information for these driver and receiver and trace properties. 06:39.680 --> 06:42.840 So you can enter all this information in this property section. 06:43.080 --> 06:45.240 As you can see here I have assigned the Ibis file. 06:45.720 --> 06:50.840 If you click over this Ibis file you can just add all the Pin assignment, all the V differential voltage 06:50.880 --> 06:51.920 time delay, etc.. 06:52.400 --> 06:55.560 All right I'll talk about this in very detail in next video. 06:56.880 --> 07:01.340 And similarly for trace impedance you can add your stack of information here. 07:02.100 --> 07:08.660 And as you can see as per this stack up I'm getting differential impedance of 100.90. 07:08.700 --> 07:09.020 All right. 07:09.020 --> 07:18.420 And where we enter all the data rate and bit pattern for that you have to click over Set analysis option. 07:20.340 --> 07:22.780 Here you will find this option of adding data rate. 07:23.540 --> 07:25.580 So as per data sheet of driver. 07:26.140 --> 07:33.620 This particular driver which is DS LVDS 1047 supports up to 0.4GB. 07:34.220 --> 07:34.780 All right. 07:34.980 --> 07:40.940 And here I am sending 32 random bit patterns to get the simulation results. 07:41.340 --> 07:43.020 Here you can set the corner frequency. 07:43.180 --> 07:48.540 So either you can go for fast mode slow or fast slow slow fast I'm going for typical as of now. 07:49.900 --> 07:53.900 And another thing which is important to check connectivity. 07:54.100 --> 08:00.160 So before running the simulation you have to check the connectivity between driver and receiver, right? 08:00.200 --> 08:01.720 It is properly connected or not. 08:01.760 --> 08:05.200 To do that, you have to just run this tool and click over check button. 08:06.280 --> 08:07.720 So from here it will verify. 08:07.760 --> 08:08.560 Okay now it is ready. 08:08.560 --> 08:10.240 Otherwise you will get a cross sign here. 08:10.240 --> 08:18.640 That driver or receiver's pin two comma one is not connected to transmitter ten nine right. 08:18.680 --> 08:21.840 But in our case it is connected. 08:21.920 --> 08:23.360 Now let's run the simulation. 08:23.760 --> 08:25.160 So first let's close this. 08:26.200 --> 08:30.280 To run the transient analysis we have to just click over this start transient analysis. 08:32.800 --> 08:39.080 So as you can see this is the waveform of R in one plus minus r in one minus okay. 08:39.120 --> 08:45.680 So this is the same waveform that we have discussed for output of receiver. 08:45.720 --> 08:46.360 All right. 08:46.680 --> 08:51.080 And if you wanted to do the measurement you have just right click and set up some marker. 08:52.440 --> 08:54.880 I just want to measure it vertically. 08:55.760 --> 08:58.280 And just set this marker at the edge of this one. 08:58.280 --> 09:02.060 If you want you can you can just zoom in and set those markers precisely. 09:02.060 --> 09:08.020 But as you can see, the difference is very much close to 0.67V. 09:08.580 --> 09:09.260 All right. 09:09.300 --> 09:17.180 Similarly, if you want the these are the differential waveform which is coming at the input of receiver. 09:17.940 --> 09:18.500 All right. 09:18.660 --> 09:20.620 So similarly you can measure these as well. 09:20.940 --> 09:24.580 And now if I wanted to compare. 09:24.780 --> 09:27.580 So we have a transmission line between driver and receiver. 09:27.740 --> 09:32.100 I wanted to compare what I am sending and what I am receiving at the receiver side. 09:32.140 --> 09:32.820 All right. 09:32.860 --> 09:39.860 To do that, firstly I'm just going to disable these and to zoom in just click over here and drag it 09:39.860 --> 09:40.540 like this. 09:41.740 --> 09:42.100 Okay. 09:43.300 --> 09:47.820 Now I'm just going to compare R in one positive with. 09:49.940 --> 09:53.700 Driver positive which is D out for positive. 09:53.740 --> 09:54.420 All right. 09:54.900 --> 10:02.720 And as you can see we have a very clear Time delay because of lengthy transmission line. 10:03.280 --> 10:03.600 All right. 10:03.640 --> 10:05.520 And if you want, you can just measure it. 10:05.720 --> 10:08.960 To do that you have just again go to the measure difference. 10:09.360 --> 10:13.760 And this time disable the vertical okay. 10:14.960 --> 10:19.400 And you can just drag it here I'm just setting it at peak here. 10:21.560 --> 10:23.040 And we can see the difference. 10:24.440 --> 10:35.840 So it is around 780 1.4 picosecond time delay between driver signal and receiver signal. 10:36.840 --> 10:37.480 All right. 10:37.880 --> 10:44.400 And similarly if you want you can analyze the AI waveform for R in one plus and R in one minus. 10:44.800 --> 10:50.440 To do that you have to just select the waveform and just uncheck all other waveforms and click over 10:50.440 --> 10:51.680 this I button. 10:53.000 --> 10:58.040 Once the AI waveform is there you have to add the mask. 10:58.200 --> 11:03.060 So this mask information you will get from either from the standard or sometimes it is given on the 11:03.060 --> 11:03.740 data sheet. 11:03.900 --> 11:06.620 You have to fill all these details okay. 11:06.820 --> 11:12.860 And you will simply apply the mask and see your AI waveform is within the limit or it is exceeding the 11:12.860 --> 11:13.460 limit. 11:13.660 --> 11:19.180 So then you can do other changes on your design and you know, make it working in your case. 11:19.380 --> 11:20.020 All right. 11:20.060 --> 11:21.500 So I'll explain that as well. 11:21.500 --> 11:27.260 How to apply the mask and how to correct the waveform if you are getting some some kind of over limits. 11:27.580 --> 11:30.980 So that's it for this, uh, simulation part. 11:31.020 --> 11:37.020 Now in the next step, I'm just going to talk about some standards for LVDS, which is very important 11:37.020 --> 11:40.900 to understand if we need to apply those for different applications. 11:42.020 --> 11:45.980 So this is LVDS owners manual I'll attach this document in the description. 11:46.300 --> 11:53.460 And here you'll find all the Ansi standards and the minimum and maximum limits for voltage frequency 11:53.540 --> 11:55.380 and time delay etc.. 11:55.940 --> 11:56.420 All right. 11:56.540 --> 11:58.020 So you can go through with this document. 11:58.060 --> 12:03.200 It's like 5 or 6 page and it will give you enough brief about this standard. 12:04.080 --> 12:06.080 Another one is IEEE standard. 12:06.280 --> 12:15.120 And on page number 12 and page number 13 you will get a detailed table of voltage, current and frequency 12:15.120 --> 12:16.880 and time delay specifications. 12:17.120 --> 12:19.480 So you can go through with this document as well. 12:19.840 --> 12:25.560 And based on these two documents we basically decide which chip we need to use for our application. 12:25.560 --> 12:28.840 Like which standard is perfect fit for our application. 12:28.840 --> 12:32.080 Either we should go for Ansi or IEEE. 12:32.600 --> 12:34.480 I'll attach the project document as well. 12:34.680 --> 12:42.000 So let me know in the comment section if you want me to discuss how I have created this topology and 12:42.000 --> 12:45.160 how I have assigned the Ibis model, or edited the Ibis models. 12:46.120 --> 12:47.480 So that's it for this video. 12:47.760 --> 12:52.880 I hope you got the overview of LVDS, how to do different measurements and what are different terminologies 12:52.880 --> 12:54.400 we use for LVDS, right? 12:54.920 --> 12:58.920 Let me know in the comment section in case of any doubt and see you in the next video.