EEVblog Electronics Community Forum
Electronics => Repair => Topic started by: Darkover on November 15, 2025, 01:11:15 pm
-
In this thread I like to show you a huge problem with the R&S RTB2004 and probably some other scopes and how to repair it.
But history first!
Some years ago I bought a new and shiny RTB2004. After two years I noticed that I had a huge offset drift in one or two channel. I don't
understand the reason, but it was just in warranty, so I send it back and R&S repaired it. Repairing means, they exchange
the mainboard and calibrate it. So you got a more or less new scope.
Another two year later I found the offsetproblem again. But this time I understand the reason. I used a selfmade current probe to
measure current of my DUT. In most cases I use this probes on channel 3 or 4, and more seldom on CH1.
So my current measurement destroyed the offset. But why?
My probe has a OPV at the output (Vcc=3.3V) and delivers 0-2.5V for any current range. I am measuring the current consumption
of some microcontroller board. So I put the GND line at the bottom of the screen and the 2.5V at the top.
Most of the time the scope is now measuring 2V on the top of the screen in DC mode.
The internal PGA inside the scope a LMH6518 has to withstand the offset for some time, but it can't.
TI find it is to much for the PGA. They changed the datasheet later. They did not alow a huge DC offset
anymore. Unfortunatly the developer at R&S did not know it.
I had a diskussion with TI about it:
https://e2e.ti.com/support/amplifiers-group/amplifiers/f/amplifiers-forum/1569255/lmh6518-offset-problem-with-lmh6518
However there are some more discussion about this problem, because I am not the first victim.
This time R&S did not repair it in warranty because they only give 1 year of warranty for there repairs.
So I decide it to repair it by myself! That means replacing the LMH6418
But before, have a look of some picture I did BEFORE repairing. It is important to memoryze it to understand what happens at the end.
The picture show you the huge offset drift before repairing and the that the internal offset compensation stops with an error
after 25%. That is important!
Olaf
-
Now, we know the problem, we know what we have to do. We are removing the LMH6518!
Unfortunatly removing is not as easy as you might think, because inside the board there is much copper and
the LMH draws more than 1W of power so it need a perfect connection to the copper.
At first I put the board on a big hotplate at 200°C. Unfortunatly that did not work, because the parts of
the bottom made a higher distance of the PCB it becomes not hot enought and I did not like to increase
the hot plate at more than 200°C.
Than I used a heat gun and heated the bottom side and a 858D (at 370°C, Fan at 5) to heat the chip.
That did not work either, but it destroyed the chip. Now the PCB looks like after an impact of an asteroid. >:D
The next step was using the heat gun at the bottom and a Weller Pyropen at the top. This is a gas heat gun
that is MUCH more powerful than 858D. It can easily toast a PCB to black cole!
But it worked. It was possible to remove the LMH6518 and after cleaning the PCB looks good.
Olaf
-
Okay the board is now fine, we have to solder.
Unfortunatly we can not use the big gun this time, because the LMH should survive!
So I used two different type of solder. A special low mealting solder with wismut inside on the exposed pad,
and normal lead-solder on the outer pads. I put also some lead-solder to some GND point. So I notice,
when the lead-solder mealted at 183°C the wismut solder below the chips is melted, too.
It is preheated with the big heat gun and solder with the 858D (370°C/Fan5) from the top.
That works surprisingly well! Soldering was MUCH and I mean MUCH! easier than desolder the old part.
Unfortunatly I lost one 0402 capacitor. But I measured it from an other channel. It is 250pF and I solde a bigger 0603 for testing purpose, because I hace to buy the 250...
Olaf
-
Hmm, poor show by TI for not fixing the LMH6518 with new updated replacement part.
Does using ac coupling reduce risk of damage to LMH6518?
-
Okay, everything is in place, but it is working? :-/O
After I switched on the scope I made the same offset control screen than I made before I replaced the chip.
It looks the same! CH1 has a smaller offset, CH3 and CH4 hat a huge one. CH2 looks okay.
I started the offset calibration in the setup menue. This time it tooks much longer time!
But it stoped at 67% and told me CH2 bad! Because I expect that every Channel needs 25%
of the work load and 67% is calibrating CH3. I am guessing that this is a firmware bug (V3.0),
perhaps a copy&paste error. ;D
So I expect now that CH1 and CH2 are okay and CH3 and CH4 have still the offsetproblem,
because I only replaced the LMH from CH1 until now!
But at least at my testscreen CH3 and 4 looking good now. :-DD
I will use the scope now for some time and write again if something happens. 8)
Olaf
-
Does using ac coupling reduce risk of damage to LMH6518?
Yes, AC is fine. Only DC is the problem. I guess they can not fix the chip because the power
consumption is to high. I think it is only possible so solved by R&S with a different firmware
that used lower voltage level in DC mode, but we have to pay for higher noise in this case.
But I think R&S did not like to talk about this, because it can mean that many user will knock
on there door with a broken scope.
Olaf
-
Try FW version 2.0 if you need it to get through the entire calibration process. Then go and look through the (huge) log to see how much offset was needed for each channel.
As for fixing this properly, TI only recently actually fessed up as to how the problem actually occurs. That said now that it's obvious where the fault is, R&S really should be pulling this from the market (assuming it's the same design with the "new" RTB2x scopes) given it's now a known fatal flaw.
-
Looks like an open heart surgery on an expensive patient :-)
I still wonder how many other scopes use the same frontend IC and if some consider it somehow in terms of frontend settings and saturation checks.
This IC is quite tempting to use compared to making something discrete.
-
Looks like an open heart surgery on an expensive patient :-)
I still wonder how many other scopes use the same frontend IC and if some consider it somehow in terms of frontend settings and saturation checks.
This IC is quite tempting to use compared to making something discrete.
I believe at least some Siglent scopes use it too, and I'm sure there'll be others (it's basically what the IC is designed for). Question is how high each design let the output signal get - datasheet didn't give a limit until many years after the IC was released, so this will probably depend on the input range of the ADC used rather than the PGA damage limit.
-
https://www.eevblog.com/forum/testgear/rigol-ds2xx2a-and-ds2xx2a-s/ (https://www.eevblog.com/forum/testgear/rigol-ds2xx2a-and-ds2xx2a-s/)
Some Rigol have them, too.
Olaf
-
LMH6518 also in the R&S HMO1000 series
https://www.eevblog.com/forum/blog/eevblog-879-rs-1202-scope-bandwidth-hack-investigation/ (https://www.eevblog.com/forum/blog/eevblog-879-rs-1202-scope-bandwidth-hack-investigation/)
..and in GWInstek GDS2000A
https://www.eevblog.com/forum/blog/eevblog-475-gw-instek-gds-2000a-oscilloscope-teardown/ (https://www.eevblog.com/forum/blog/eevblog-475-gw-instek-gds-2000a-oscilloscope-teardown/)
-
I think my hmo2024 has it too from what I remember having seen.
What about Batronix Magnova? Just bought one... spec wise it could also use that IC, though it is of course not clear if they operate it also to such amplitudes in their y/div vs. Gain settings. Don't wanna open that one and could not find teardowns :-)
-
Btw: Reducing the front gain scaling by balancing analog vs. Digital gain will not result in 0 risk. Scope inputs are typically spec'd with an abs. Max. input voltage independent of vertical setting.
I guess most of us have seldomly also already driven a frontend willingly to saturation to view small gnd level artifacts on larger scale signals. OK, not often and for long times and of course signal distortions can happen then.
But getting to "a german accuracy level" of spec., a 10V DC input signal with y/div set to 1mV and 0V Y offset is safe in the way typical scopes inputs are spec'd and shall not result in something that stresses abs max ratings of the internal signal chain. Practically speaking this case is not so relevant, but spec wise it is OK and will result in some unwanted permanent drift.
A protection could though be done in software by detecting a long term saturating DC, throwing a warning and then setting the frontend gain to a very low one.
-
You are never ready, until you are ready! |O
Today I switched on the scope after calibration yesterday and I found it forgot the result of the calibration!
I think the reason is the error at the end of the calibration. I guess it did not store the calibration value
permanently until it will run without an error.
Okay, I expected that I had to replace two more LMHs, but now it is clear!
Olaf
-
Just in case someone likes to repeat my steps, here are the stl files
for a Steinel to ArcaSwiss Adapter to connect a Steinel heatgun to a tripod.
Olaf
-
I have another important notice for the brave hearted who decide to open the scope. :-BROKE
I am just in the process to remove the mainboard a second time to exchange the LMH for channel 3 and 4.
I found that two screw that hold the mainboard in the area of the channel connector are seized. When I install
the mainboard last time I did NOT use cordless screwdriver! All was done by hand with lower force! And I am not Hulks bigger brother.
But R&S, a company that always had to take care for there money, had used self taping steel screw in aluminum
that is prone for seizing. So it is probably a good idea to use anti seize paste or even better normal screws in
case we have to open the scope more often.
BTW: I am still thinking if it is possible to install a 50R switch a least in one channel......
Oh..and another hint. There is an expensive high quality and high density connector to connect the lcd to
the mainboard. It has a locking metal bracket. Turn it bei 180degree and than use it to pull the connector, so it
is very easily removable.
Olaf
-
Okay, I removed the parts from CH3 and CH4.
This time it was easier, because I had some experience and the parts are closer to the border. So the board sucks less heat.
Even the removed LMH6518 looks good now. Perhaps I can sell them at ebay as untestet. :-DD
I think soldering the new PGAs will happen next week. I orderd some Yorkshire tea from Internet for my mental health, I guess that works better than German beer!
Olaf
-
Ooh happy to see that CH4 is probably the easiest, still have to find time to fix mine (again), thankfully that's the channel that went this time.
Fingers crossed yours is all fixed once you put the new ones on, I pinged the R&S guy about the issue too btw.
-
Keep CH1,2,3 as AC coupling only, because CH4 easy to replace ;D
-
Unfortunatly we can not use the big gun this time, because the LMH should survive!
So I used two different type of solder. A special low mealting solder with wismut inside on the exposed pad,
and normal lead-solder on the outer pads. I put also some lead-solder to some GND point. So I notice,
when the lead-solder mealted at 183°C the wismut solder below the chips is melted, too.
Oh, no…
(FYI, it’s “bismuth” in English.)
You must never mix bismuth and lead solders! Even small amounts of lead combine with bismuth and tin to make an alloy with an extremely low melting point (worst case, 95°C!!!). Additionally, the bismuth-lead-tin combination has terrible mechanical properties, making the solder much more brittle.
Overall, your technique is not very good. The reason you’re resorting to low-temp solder is because you’re not preheating the board properly. Rather than a single point of hot air on the underside (which means one spot getting very hot, but the rest of the board acting as a big heatsink), you want weaker heat all over. 1 minute of preheating is not nearly enough IMHO. When I work on heavy multilayer boards, I like to preheat the board from the edge, moving around the edge of the board around and around, so that the edges can’t act as huge heatsinks. The center gets heated as it steals heat from the edge. This can take a solid 3-4 minutes to do, but avoids overheating one spot. Alternatively, an oven is a great way to preheat the board uniformly. Then, after you secure the board to work on it, keep gentle heat on the bottom.
Also, your copper foil should be stuck down, so that hot air doesn’t sneak underneath and blow small components away; gaps can actually end up focusing heat exactly on the components you’re trying to protect!
-
Keep CH1,2,3 as AC coupling only, because CH4 easy to replace ;D
Aehem yes, this comes to my mind, too. 8)
Olaf
-
Even small amounts of lead combine with bismuth and tin to make an alloy with an extremely low melting point (worst case, 95°C!!!).
I know that, but I did not care so much about it. And I did not mix it with lead. There is pure bismut/tin in the center. lead is only outside.
Additionally, the bismuth-lead-tin combination has terrible mechanical properties, making the solder much more brittle.
Of course, but this is also true for bismut/tin. This is the reason why I only used it on the EP.
Alternatively, an oven is a great way to preheat the board uniformly. Then, after you secure the board to work on it, keep gentle heat on the bottom.
There are always better ways, but I am not in my company. It has to work on my private desktop!
Also, your copper foil should be stuck down, so that hot air doesn’t sneak underneath and blow small components away; gaps can actually end up focusing heat exactly on the components you’re trying to protect!
This did not happen, because the copper tape is glued on the PCB. All part stay where they are. Oh and the Weller Pyropen did not blow very much. It has no fan like a normal heatgun. I think it is more radiant heat.
Olaf
-
You're missing a BGA soldering machine. Replacing it with an LMH6518 would be a piece of cake, and the soldering result would be perfect.
You can use 559 soldering fluid, which can lower the temperature and make chip removal easier.
-
You're missing a BGA soldering machine. Replacing it with an LMH6518 would be a piece of cake, and the soldering result would be perfect.
It is not my idea to make a living from it. ;)
You can use 559 soldering fluid, which can lower the temperature and make chip removal easier.
Nobody, at least not me, know what a 559 is. We only know a 555. ;D
But removal can not be easier because you has to deal with the solder used by R&S when they build
the board. For the next time it will be easier, because of the solder I had lying around in my fridge.
Olaf
-
To be frank, your operation is very dangerous and can easily cause PCB bulges, that is, insulation layer bulges
Perhaps each region has a different name, so I can only send a picture. I hope you understand what I'm saying
-
To be frank, your operation is very dangerous and can easily cause PCB bulges, that is, insulation layer bulges
I don't think so, because I checked the temperature carefully and I do things like this often.
Perhaps each region has a different name, so I can only send a picture. I hope you understand what I'm saying
Of course, but your solder is strange. I did not find any datasheet for it. And 559 is, from my understanding only the flux and not the solder.
I used solder with 150°C-160°C for EP and 183°C for the outer pads. What is youre melting point?
Olaf
-
I'm referring to flux.Applying a little of this will significantly lower the temperature, improve flexibility, and enhance the fluidity of the solder.
The original manufacturer uses lead-free solder, so a little flux must be added to aid soldering; otherwise, it will be difficult for the solder to melt. Increasing the temperature is not the optimal solution.
I usually use S-SN60PbAA solder.
Although it's not environmentally friendly, I still think it's excellent.They are now very hard to buy.
-
It is working! This time I soldered CH3 and CH4. So now all in all CH1, 3 and 4 are changed, CH2 is not changed.
As you can see in the picture, the offset of CH3 and CH4 has moved very much compared to the value before calibration. From -2V to +0.5V
This time the calibration runs to 100% without an error. And than all channel had 0V offset. The calibration tooks around 10min with many funny picture and movement on the screen. You feel like in a scary movie! :popcorn:
Perhaps you should do that calibration with youre device now to enjoy this movie, too! :-DD
I also think it is now clear that the CH2 error that was chown by the last calibration is a firmware bug. It said CH2 when it mean CH3. Probably someone from R&S can write this in there bug tracking system!
It also wrote the new calibration value to internal memory, so after switch off/on everything looks good again.
Now I think it is repaired!
Olaf
-
Party time :-) Maybe next time you repair it there're pin and function compatible AFEs available without drift issue :-)
-
When there is a faulty channel the calibration will cause offset on ALL channels - this happened to my unit too. I assume it gives up and leave all the channels with offset not corrected.
I have another scope on hand now to use while the RTB is in pieces so will try and fix mine this weekend.
-
I have another scope on hand now to use while the RTB is in pieces so will try and fix mine this weekend.
[X] send pics!
Olaf
-
Just ran self-alignment - mine also said the wrong channel (ch3 when the issue is really on ch4).
Although it errored out the scope seems to actually have had it's offsets corrected - I think the issue I saw last time (where all channels got big offsets) might have been to do with trying to adjust the "zero offset" after the self alignment. That option is now definitely broken - all channels show 0V as the min/max offset, whereas they used to allow adjustment. Will still fix the hardware issue but nice to know that (at least sometimes) the scope can calibrate out the offset, even if it doesn't finish it's procedure.
-
Will still fix the hardware issue but nice to know that (at least sometimes) the scope can calibrate out the offset, even if it doesn't finish it's procedure.
I found that, too. when it stoped with the CH2-Error it fixed the huge offset for CH3/4, wich is strange by themself, but it only store the result save for a powercycle until it arrived at 100%
Hm..I think we need the sourcecode of the firmware to do some improvement. ;D
-
Yes you are right, offset comes straight back on reboot.
Just swapped the IC on my channel 4, had to put a reasonable amount of heat into it but with a hotplate underneath there were no dramas at all. New chip went on super easy with a thin layer of 63/37 tin/lead solder paste underneath - given how easy it soldered down with leaded solder I don't think mucking around with bismuth alloys is necessary for future rework.
Honestly the biggest issue is that the glue on the copper tape I used for shielding has made a huge gunky mess where it got a bit cooked - should have used kapton instead. Will need quite a bit of cleaning, I'll get that done and test the scope, will post results and add some pics once that's done (and I've eaten dinner).
-
I found CH4, and also CH3 was much easier compared to CH1. And you are right, for CH3/4 there is probably no special solder necessary, but I have 1kg of it in my fridge, sigh.
I had no problem with the glue of my copper tape. I used the cheapest tape I could buy at Aliexpress. It is also a very thin copper. In my company we have more expensive copper tape that is much thicker, but I think for this job the thin one is the best.
For the future I worry a little bit for the electrolytic capacitor on the bottom side, because they are cooked for some time above 100°C. Probably not the best to expand there livetime. :D
-
Well fuck, I put the PGA on in the wrong orientation |O :palm: :-BROKE. Put the national semiconductor symbol in the pin 1 position instead of the dot.
Gonna need to go buy a new one, and fingers crossed I didn't do any damage. Will report back in a week or so.
Guess I'm gonna see how easy it is to desolder with leaded solder quicker than I planned :-BROKE
-
My deepest condolences for your chip. :'(
I think I checked this at least three times by comparing it with a brother channel.
-
So applying a continuous 2 volt DC voltage to a Rohde & Schwarz RTB2004 digital storage oscilloscope damages it because of the LMH6518? That sure seems like a problem Rohde & Schwarz should be handling.
-
So applying a continuous 2 volt DC voltage to a Rohde & Schwarz RTB2004 digital storage oscilloscope damages it because of the LMH6518? That sure seems like a problem Rohde & Schwarz should be handling.
It is not that easy! It only happens if you move the ground to bottom and use an amplification that the 2V are on the top of the screen. So the LMH6518 has a huge DC value to withstand at the output. And probably you have to measure this for some time, so something in the chip is overheating. This is of course an allowed measuring, but probably not many people do it.
That sure seems like a problem Rohde & Schwarz should be handling.
I can not disagree, but R&S is astonishing quiet isn't? I guess they believe they can sit it out like a politician ass.
-
So applying a continuous 2 volt DC voltage to a Rohde & Schwarz RTB2004 digital storage oscilloscope damages it because of the LMH6518? That sure seems like a problem Rohde & Schwarz should be handling.
It is not that easy! It only happens if you move the ground to bottom and use an amplification that the 2V are on the top of the screen. So the LMH6518 has a huge DC value to withstand at the output. And probably you have to measure this for some time, so something in the chip is overheating. This is of course an allowed measuring, but probably not many people do it.
Seems pretty easy to me given that what the screen is showing is exactly the digitizer input, which is exactly the LMH6518 output. So configuring the display where the ground at the bottom, and the DC coupled trace is at the top, should always result in damaging the LMH6518.
How is this mode of operation exceeding any possible oscilloscope specification? I do it all of the time, but this will break a Rohde & Schwarz RTB2000 series DSO?
The RTB2000 series is discontinued, but do any of their other oscilloscopes use the LMH6518 in the same way? What about other manufacturers?
Texas Instruments has a history of mistakes like this, which they then ignored or covered up. I remember they had a new ADC which was *advertised* to specifically resolve the +/- zero issue, which was a big deal back then. It did not and it still returned +/- zero, and they *never* updated the datasheet and continued to advertise that it resolved +/- zero properly. They also have a history of datasheets with bogus specifications and characteristic curves.
That sure seems like a problem Rohde & Schwarz should be handling.
I can not disagree, but R&S is astonishing quiet isn't? I guess they believe they can sit it out like a politician ass.
I have now removed Rohde & Schwarz from my consideration to purchase list. No company should be rewarded for this kind of behavior, and their products cannot be trusted.
-
The RTB2000 series is discontinued, but do any of their other oscilloscopes use the LMH6518 in the same way? What about other manufacturers?
The RTB2000 is still in production, although they changed the name to RTB2:
https://www.rohde-schwarz.com/us/products/test-and-measurement/oscilloscopes/rs-rtb-2-oscilloscope_334282.html (https://www.rohde-schwarz.com/us/products/test-and-measurement/oscilloscopes/rs-rtb-2-oscilloscope_334282.html)
Yes, the LMH6518 is used in other scopes, e.g. GW Instek GDS-2000A/GDS-2000E series.
-
The RTB2000 series is discontinued, but do any of their other oscilloscopes use the LMH6518 in the same way? What about other manufacturers?
The RTB2000 is still in production, although they changed the name to RTB2:
https://www.rohde-schwarz.com/us/products/test-and-measurement/oscilloscopes/rs-rtb-2-oscilloscope_334282.html (https://www.rohde-schwarz.com/us/products/test-and-measurement/oscilloscopes/rs-rtb-2-oscilloscope_334282.html)
I wonder if they resolved this problem.
Yes, the LMH6518 is used in other scopes, e.g. GW Instek GDS-2000A/GDS-2000E series.
Probably a lot of DSOs use the LMH6518, but if their ADC input range was restricted, then they might not have a problem.
-
I wonder if they resolved this problem.
This is the open question isn't? >:D
I guess they did not change the PGA, because in this case they would not made the RTB2 firmware compatible with the RTB2000.
However, and this is only a guess, it may be possible to solve this problem in firmware only if they use different/smaller amplifikation values.
If this is possible they may have done this in V3.0 or will do in V3.x. But we did not know.
I had an exchange in warranty for the mainboard two years ago and I believe there are a few more customer and if someone at
R&S tracked it down why they had so many problem, they should know about it a few years earlier then us.
On the other hand, what is the reason for changing RTB2000 to RTB2? Only marketing bullshit?
However, there is now many guessing and in this big companys people on different floors made there own wars and hugs about topics that are not understandable by normal people.
And perhaps, now they need all there engineers to make Germans army great again. :-DD
-
A spec comparison of rtb2 vs. Rtb2004 should ideally show deltas if the frontend design was changed, but too lazy as I already bought the Magnova now and promissed myself not to look into other scopes for at least 2 years :-)
-
A spec comparison of rtb2 vs. Rtb2004 should ideally show deltas if the frontend design was changed, but too lazy as I already bought the Magnova now and promissed myself not to look into other scopes for at least 2 years :-)
I had short look at the RTB2 specs yesterday and I did not found something interesting. Oh..and the Magnova hast 5 year of warranty, so you should close your eyes a little bit longer. ;)
-
I went through the LMH6518 datasheet and did not find any way to adjust the clamp levels. The only way to avoid damaging overload is to restrict the output range by clamping the input and adjusting the attenuator and gain settings, or to decrease the output loading.
-
I made an interesting investigation!
Look what Dave show us at 3:18
https://www.youtube.com/watch?v=3THvWEKGOwE (https://www.youtube.com/watch?v=3THvWEKGOwE)
There is an LMH6518 in the HMO1202.
I did not look inside, but I have an HMO2024 at company and a HMO2022 at home, to. I guess the LMH is used there, too.
Of course I did not wrote it down, but I know that I did the same measurement that kills my RTB with both of the devices,
and they survived. So the reason is probably a different attenuation level. Perhaps they had to change something because
of the 10bit of the RTB. It should be interesting to do a comparision. From my memory the layout I see at 3:18 looks
very close to the RTB.
And perhaps it is interesting to check what parts they saved when they removed the 50R input....
-
Ups, my guess was wrong. I opend my old HMO2022 without R&S branding. The inputamplifier looks much different from the HMO1202. There is no LMH6518 in this scope.
But there is some dust. :o
Perhaps they changed the hardware when they bought the Hameg company.
-
Of course I did not wrote it down, but I know that I did the same measurement that kills my RTB with both of the devices,
and they survived. So the reason is probably a different attenuation level. Perhaps they had to change something because
of the 10bit of the RTB. It should be interesting to do a comparision. From my memory the layout I see at 3:18 looks
very close to the RTB.
A lot depends on the ADC input range and impedance on the output side of the LMH6518 . With proper control of the LMH6518 , a smaller ADC input range will avoid damaging the LMH6518 by limiting power dissipation.
-
My HMO is branded R&S and Hameg :-)
No LMH in there too.
-
OK so I found a half hour to solder in the new LMH6518 and we are back in business - wasn't very hard to resolder (prep and cleaning took a lot more time and effort than the actual soldering); CH4 is the best case but I suspect leaded solder will be fine for them all given good pre-heating, without a need to go with 138C bismuth stuff.
I also did a quick check looking at input vs ADC voltage (only at a couple of input levels, 1.25 and 2.5V) - it appears that ~330mV is full scale high/low on the ADC input, so you need to overdrive it by about 5x to get into the danger zone. Problem is that I don't think there's a way for the scope to tell how much it's clipping by, so probably no way to do a firmware based safety limit (if there was a way to tell how heavily it was clipping there could be a failsafe to change gain and avoid sustained heavy clipping).
I will admit that I expected the ADC input to be a bit closer to the LMH6518 limit - you wouldn't normally leave it clipping by 5x for long, but maybe the damage occurs pretty quickly (fuck TI for not documenting this properly!). I also wonder if there is another factor at play - there is a clipping circuit (some RF schottkys and a DAC/opamp, the autocal routine has a step to calibrate it) that clearly kicks in at some point, maybe it starts sinking more current than the LMH6518 likes as it starts limiting the signal? I'm hesitant to push it further testing this, don't want to have to change yet another PGA, at least while I don't have stock of them or time to do the work!
Anyway I think the name of the game is to try and avoid sustained heavy clipping, which unfortunately means needing to be much more careful than should be strictly necessary going by the datasheet input limits :-BROKE
Edit: abs max is 1700mV, so danger zone is 5x full scale, not 3x I stated originally.
-
suggest you communicate in English。(建议你最好使用英语进行沟通交流)
-
Anyway I think the name of the game is to try and avoid sustained heavy clipping, which unfortunately means needing to be much more careful than should be strictly necessary going by the datasheet input limits :-BROKE
I think so, too. I faced with this problem when I use the fullscreen for my measurement, it was my idea to stay away one div away from top and bottom when I do it the next time.
Olaf