LVDS to LVCMOS clock signal - Page 1
Forum section: electronics / projects

author: stefan_o
author_rank: Regular Contributor
timestamp: June 08, 2025, 07:02:08 pm
content: Hello,I'm working on a project with multiple (different) ADCs connected to an FPGA. I would like to use a CDCM6208 as a low jitter clock generator. It has 8 outputs, but 4 of them only differential. As there is up to 15cm distance between the clock and an ADC I assume differential is better anyways, but I need to convert the LVDS to LVCMOS for the ADCs. Of course there are LVDS->LVCMOS chips, but how much jitter would they introduce?Is there a recommended way to convert a LVDS to a LVCMOS clock signal?Best regardsStefan

author: jbb
author_rank: Super Contributor
timestamp: June 08, 2025, 08:22:34 pm
content: Some of the LVDS - single ended chips have jitter specifications.For best jitter, maybe a high speed comparator would be best?

author: stefan_o
author_rank: Regular Contributor
timestamp: June 08, 2025, 08:52:54 pm
content: Quote from: jbb on June 08, 2025, 08:22:34 pmCan you point me to one of them? I found the ADN4662 and numerous TI parts, non of them mentions jitter. There is a value for "Differential Pulse Skew" (around 100ps), not sure if that is related.The CDCM6208 software tells me with my PLL settings I can expect a jitter of 1.5ps. I would like a part that is at least in the same order.Quote from: jbb on June 08, 2025, 08:22:34 pmIs there an example that measures or at least calculates the jitter somewhere?

author: mtwieg
author_rank: Frequent Contributor
timestamp: June 08, 2025, 10:42:33 pm
content: Lowest jitter option would be to use a balun circuit. I believe minicircuits, pulse, and macom make transformers suitable for this.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: June 09, 2025, 01:26:32 am
content: Quote from: stefan_o on June 08, 2025, 08:52:54 pmSee OnSemi NB3N4666C 3.3 V Quad LVCMOS Differential Line Receiver Translator - specs 161fs jitter and calculations.

author: stefan_o
author_rank: Regular Contributor
timestamp: June 09, 2025, 11:08:32 pm
content: Quote from: mtwieg on June 08, 2025, 10:42:33 pmI had look, I couldn't find anything. LVDS impedance is 100 Ohm, couldn't find anything for that.Quote from: PCB.Wiz on June 09, 2025, 01:26:32 am161fs jitter would be perfectly acceptable. Unfortunately this is a 4 channel receiver, I don't want to place a 4 channel receiver next to every ADC using only channel (it's a 9€ part, really expensive compared to other LVDS receivers). I had a look for a single channel from onsemi, and that mentions 3.5ps jitter without any explanation on how it was measured. TI and Analog either don't mention it or it's also like 4ps. How did you find the NB3N4666C? It advertises it's low jitter nowhere, I wonder if other parts would perform similar if measured the same way.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: June 10, 2025, 01:14:01 am
content: Quote from: stefan_o on June 09, 2025, 11:08:32 pmI just used google.What is your actual clock speed ?There are also parts like TI's TLV3601DCKR, or ADI's similar LTC6752, which are fast comparator that specs ~4ps jitter with a low 100mV sine signal.The LTC6752 includes a graph of jitter vs sine amplitude.Addit : Maybe the PI6C49CB01 from Diodes ? Specs 90fs typ and looks to be stocked ~ 56c

author: ch_scr
author_rank: Frequent Contributor
timestamp: June 10, 2025, 09:08:23 am
content: Quote from: stefan_o on June 09, 2025, 11:08:32 pmWell this just means they measured the datasheet figures in a 50 Ohm system on the driving side ("1" side in a 1:4 xformer).IMHO 100 Ohm isn't unreasonably far away from that, as to suspect a totally different behavior.Also, with a balun driven from LVDS feeding a CMOS input, one might want to think about what actually happens there anyway - won't the drive be bipolar then? And would you need termination?And what will the effective impedance of the capacitive input be at the clock frequency (even if not additionally resistively terminated). And will the voltage swing then be high enough?It's a bit moot as even the cheaper ones are almost 3€/piece anyway, so the Diodes part PCB.Wiz found looks really attractive.

author: mtwieg
author_rank: Frequent Contributor
timestamp: June 10, 2025, 10:51:01 am
content: Yeah the passive option with a balun would look something like this. Need a bias T on the LVCMOS side, and some series termination will likely be helpful:Most of the distance would be covered by the LVDS pair. The rest of the circuitry would be compact, right next to the LVCMOS input.Here I used a 1:4 impedance transformer to get a ~2.5V output. For other amplitudes you could choose a different impedance ratio in the transformer, or adjust the differential trace Z0 and termination resistance. For the transformer I would look at parts like MABAES0061 or TC4-1T+.Downside is the parts count and the need for some fine tuning of component values. But jitter performance and power consumption is basically ideal.

author: stefan_o
author_rank: Regular Contributor
timestamp: June 10, 2025, 03:51:35 pm
content: Quote from: PCB.Wiz on June 10, 2025, 01:14:01 amThe clock speeds are up to 40 MHz.Maybe I searched with the wrong terms, I always looked for something like LVDS to LVCMOS low jitter clock buffer. The PI6C49CB01 seems to be exactly the part I'm looking for, but it's expensive at JLCPCB/LCSC (almost 5€), much cheaper at Mouser/Digikey.Quote from: ch_scr on June 10, 2025, 09:08:23 amI see, well the Diodes part is actually more expensive, but this solution seems to be a lot more complex.Quote from: mtwieg on June 10, 2025, 10:51:01 amThanks for your solution, but the tuning part is what really scares me.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: June 10, 2025, 11:42:54 pm
content: Quote from: stefan_o on June 10, 2025, 03:51:35 pmIf you want to use lcsc, then maybe do a dual footprint layout, and see if you can see any difference in operation?You could place both the PI6C49CB01, which does give jitter numbers and the 'very-similar other specs' parts like FIN1002M5X and DS90LV012AFrom lcsc another candidate could be the MX3501T23 comparator, (~10c) specs show 50MHz and 100MHz drive waveforms.