Active MIPI probing (very high speed opamps) advice - Page 1
Forum section: electronics / projects

author: tom66
author_rank: Super Contributor
timestamp: June 05, 2025, 04:55:55 pm
content: We find ourselves needing to probe MIPI signals, 4 lane data plus clock. Not wanting to buy multiple active probes (as wellas design a custom interposer board), I considered the possibility of using a high performance opamp in unity gain configuration to monitor these signals on a custom interposer instead.Our MIPI signals are at 800Mbit/s per lane, so around 400MHz worst case bandwidth. But I want to catch 3rd and ideally 5th harmonic to give good reproduction of the waveform on our 2GHz scope. Typically the high speed signals are around 200mVp-p in a differential configuration, but for added complexity the bus can go into a low power state where signals are around 1.2Vpk (single ended).I ended up selecting the THS4304, which has a 3GHz GBWP and is unity gain stable. I'm running it from a dual supply, because it can't swing all the way to ground.I am wondering if anyone has used such high speed opamps or similar before and if they have any advice or suggestions beyond what's in the TI datasheet, beyond the obvious of don't forget the decoupling caps.

author: Hydron
author_rank: Super Contributor
timestamp: June 06, 2025, 01:48:07 pm
content: This would actually be useful for something I'm working on too (though at more like double your MIPI clock speed, so with added difficulty!)If you're just wanting to look at the data coming out (rather than what the signals look like electrically), then something like a Lattice CrossLink FPGA would let you split the MIPI signal so you can feed one to the video input and one directly to a pair of 50 scope inputs.There's also MIPI decode support (currently only for single lane sadly) for ngscopeclient - I'll be looking at trying to test that too, as a free alternative to a MIPI analyser.

author: nctnico
author_rank: Super Contributor
timestamp: June 06, 2025, 09:50:45 pm
content: I have such probes for sale off-the-shelve.... Another option is to catch just half of the signal through a simple Lo-Z style probe design (470 Ohms in series with a 50 Ohm coax cable) and feed that into an oscilloscope. Recently I measured a 1.25Gbit/s signal with this method so 400MHz is easy.

author: harerod
author_rank: Frequent Contributor
timestamp: June 06, 2025, 11:01:16 pm
content: Quote from: tom66 on June 05, 2025, 04:55:55 pmHow about "don't forget to match your impedances"?Decoupling is important, those opamps will need solid, low impedance supply - at those frequencies.How do you interface your interposer to the scope? I guess you add connectors and directly connect to the scope, keeping everything matched carefully?At any rate, cool project, I'd love to hear, how it worked out for you.

author: tom66
author_rank: Super Contributor
timestamp: June 06, 2025, 11:31:29 pm
content: Plan for connection is 50 ohms to scope via u.FL leads, because I can put 10 x u.FL connectors on the PCB easily (it has to be kind of small to work in the application which I can't really disclose.) So yes op-amp outputs are terminated with 50 ohm series.I've ordered the board now, I'll see how it goes. I will be limited in what I can show off, but I will happily describe trials and tribulations.I did an initial version with Low-Z probing but I found too much rounding of the signal making decoding packets very difficult. The other issue is that MIPI uses active termination, 1.2V idle no termination enabled, then the signals fall in a specific sequence a bit like an I2C start and terminators switch on and all signals are then high speed at ~200mVp-p. The LP drivers have 110 ohms series termination typical, which means if you put 500 ohms to ground, the LP signal will sag to about 950mV. The LP receiver worst case is about 880mV Vih, so it's putting the bus in a precarious position. Might work but getting very close to not working. I could go with 950 ohms series and 50 ohms on the 'scope, but with 1pF scope & cable load (guess, but seems reasonable), I calculate rolloff of ~160MHz. Not good enough really.

author: harerod
author_rank: Frequent Contributor
timestamp: June 07, 2025, 11:53:14 am
content: I may have understood your setup wrong. I imagine the interposer to be a board with MIPI impedance matched connectors on both ends, being spliced into you application. On that board you tap the signal lines into the opamps. Traces being short, no urgent termination issue. Opamp outputs translate to your connectors' and scope's impedance.I wouldn't share a client's ongoing project either. However, if you enjoy the discussion, you could share schematic snippets and maybe partial pictures of the setup. A perfect opportunity to show off.

author: tom66
author_rank: Super Contributor
timestamp: June 07, 2025, 01:07:45 pm
content: Quote from: harerod on June 07, 2025, 11:53:14 amYou got it right. I was responding to nctnico's suggestion of passive probing. I don't think it will work for MIPI. For other interfaces it might work.We have a Siglent SDS6204A for the probing, I'm wondering if anyone knows the 50 ohm input capacitance on that, it's not specified in the manual.

author: tom66
author_rank: Super Contributor
timestamp: June 16, 2025, 04:11:25 pm
content: So, got the board back. Unfortunately, first power up revealed the opamps oscillate their outputs like mad at about 750MHz, which seems to be around their rated GBWP (not sure if this is just a coincidence). However, with a bit of hacking around I found that if I reduced the supply to the opamps and also fit a shorting link directly over the opamp to bypass my 0R feedback resistor on the underside of the board, I am able to get an acceptable trace output. Still a tiny bit of oscillation in the LP phase of the waveform but probably not an issue for decoding it.I guess these opamps are super-duper sensitive to feedback network inductance, even more so than I expected. But it looks like it will work when modded, and it's just a jig/test board so that's fine.I am curious why the supply voltage would a significant impact to stability. I found I got the best stability with only about 2.1V total across the opamp, with the -ve supply at about -0.75V and +ve at 1.35V, negative supply having the most impact on stability. This is actually below the datasheet recommendation (2.7V min)...

author: jbb
author_rank: Super Contributor
timestamp: June 17, 2025, 06:20:10 am
content: If that doesn’t work out, I wonder if a 10:1 divider followed by a BUF802 (from Texas Instruments) would do.

author: tom66
author_rank: Super Contributor
timestamp: June 17, 2025, 09:47:45 am
content: I considered BUF802, but the very high input offset voltage of almost 1V concerned me. It could be nulled out, but might create issues with a MIPI decoder (if we use one) since the signals wouldn't sit at the correct levels for LP and HS-TX.

author: harerod
author_rank: Frequent Contributor
timestamp: June 17, 2025, 12:06:48 pm
content: Quote from: tom66 on June 16, 2025, 04:11:25 pmGreat that you will get some action out of your testboard. I'd love to see your schematics and layout. Looks like you are learning a lot about parasitics. This is certainly a setup where everything enters the equation, so I'll spare you more uneducated feedback.Have you seen Howard Johnson's "Handbook of Black Magic" and its sequel "Advanced Black Magic"?Quote from: tom66 on June 16, 2025, 04:11:25 pmRegarding impact of supply, the following items come to mind first:- headroom increases with supply voltage, resulting in faster rise-time and increased bandwidth. Have a look at the internals of your opamps.- the power supply itself may add a pole to the equation. Most cappas have poor response at 750MHz. I wonder if you could measure the signals at the supply pins, or if you get too much interference. Time to get rid of the crocodile clamps and attach the highspeed GND to the probe. OR solder a homebrew probe into the board (coax cable with 1Meg serial resistor soldered to core and signal, shield soldered to board reference)

author: dietert1
author_rank: Super Contributor
timestamp: June 17, 2025, 03:28:59 pm
content: There is a discussion on GHz probes from 2018. THS4302 and THS4303 were mentioned here:https://www.eevblog.com/forum/testgear/gt-1-ghz-diy-differential-probes/msg1910819/#msg1910819.Regards, Dieter

author: tom66
author_rank: Super Contributor
timestamp: June 17, 2025, 06:33:34 pm
content: Further work: I was noticing that the probe setup was causing too much distortion on the MIPI signals themselves, which was causing my display to reject packets. With a 6GHz active probe, I could see the op-amp output was correct, following the distorted signal on the bus, so decoding the data bus was not possible. I had a 0R placed in series with the non-inverting input of the op-amp; replacing that with 51R (a guess) fixed the distortion. What is odd is that the op-amp is only supposed to contribute about 1.5pF||100kohm, which should not really impact MIPI as it is rated to handle a total of 50pF distributed across the transmission line. So something else is going on here, but I don't know what. Still, it's mostly working now, and tomorrow is another day.

author: tom66
author_rank: Super Contributor
timestamp: June 17, 2025, 06:49:31 pm
content: As for the schematic: it is just a non-inverting amp, but with an 0402 R/C spot placed as a parallel pair for every signal path: non-inverting input, inverting input, and inverting input to ground. Default R is 0R, default C is non-fit. This allows the board to be modded to support various gains as well as add capacitors in the feedback path if needed for stability. But I was hoping to make the default unity gain configuration work as the datasheet specifically outlines that the amplifier is unity gain stable.The current 51R input resistance combined with the 1.5pF input capacitance (of the op-amp) should set the -3dB bandwidth somewhere around 2.1GHz, so it's fine if this is necessary for all channels. Just a little odd.