Analog amplifiers for pulsed LIDAR - Page 1
Forum section: electronics / rf-microwave

author: mr_byte31
author_rank: Regular Contributor
timestamp: February 09, 2024, 02:47:42 pm
content: -- My first postHi All,I am SW engineer with limited experience in analog/RF circuits.I am designing a laser radar. The Tx will send a very short electrical pulse (pulse width =5ns, current = 35A ,wavelength = 905 um) to laser diode (MPPC). The RX shall consist of photo detector diode , amplification stage and range circuit (TDC-GP21).This is the recommended Rx configuration from the datasheets :I read many articles on the internet about how much noise exists in such Rx circuits and I would like to avoid such things.I have two options for the amplification stage1- two cascaded OPA855 each one has gain of 20x . First stage is transimpedance configuration.2- Use one IC which make all amplification without any external components such as ADA8282 (LNA+PGA), it has 4 channels and I was planning to use two of them in series.The Rx will be implemented on 2-layer PCBI appreciate if any expert here give some advices. I am also open to any other solutions/ICs.

author: MarkT
author_rank: Frequent Contributor
timestamp: February 09, 2024, 09:23:38 pm
content: That's a confusing circuit - normally the first stage is explicitly differential (an opamp), and the diode bias is negative so that the first stage output is positive. And I don't get the capacitor on the input of the amp. Which datasheet?

author: jwet
author_rank: Frequent Contributor
timestamp: February 11, 2024, 04:45:38 am
content: There is a great book by Jerald Graeme called "PhotoDiode Amplifiers"- it is the Bible for designing this stuff. A good EE library would have it.Another good resource is the data sheet for the OPA656, a wideband amp used in these apps. The OPA858 is another stellar part to look at. The configuration you show in your block is quite approximate but "generally" ok. The cap to ground is a parasitic C- its there in reality but is a nuisance, there certainly shouldn't be a physcial cap installed there! These are circuits of real finesse, I respect your trying but its a tough design. Read Graeme's book cover to cover and you've got a chance.You may want to look at purpose built LIDAR receivers from Maxim (now ADI) and others- the MAX40658-60. These are what are used in industry.

author: mr_byte31
author_rank: Regular Contributor
timestamp: February 15, 2024, 02:36:56 pm
content: Many thanks for the Book and the advice. Highly appreciated.I will definitely read the book and try to build its circuits.

author: mr_byte31
author_rank: Regular Contributor
timestamp: March 04, 2024, 01:57:20 pm
content: I went through the book. I learned something new. I am missing one thing! --> All circuits has fixed gain.I was thinking to add a variable feedback Resistor which I could control via I2C to achieve variable gain receiver.is there a better approach for doing so ?

author: tggzzz
author_rank: Super Contributor
timestamp: March 04, 2024, 03:11:33 pm
content: Quote from: mr_byte31 on February 09, 2024, 02:47:42 pmThat's a worrying starting pointHowexactlyare you going to ensure this doesn't blind someone?That answermustincludedefinition of failure modesand why thosecannotcause dangerous radiation.Fundamentally, Ireallydon't care if it blinds you, but I do care about random people passing by.I am rather sensitive to that, since I am very deaf. If I become blind I will have to find a way to commit suicide. Seriously.

author: mr_byte31
author_rank: Regular Contributor
timestamp: March 04, 2024, 03:24:02 pm
content: it is just postgraduate research project for "Terrestrial Laser Range Finding Systems". It is not planned to be used against humans or animals.just open IEEE and you will find tons of research paper about high power Lidars. Unfortunately those papers don't add much details about minor implementation details so I came here for help.

author: tggzzz
author_rank: Super Contributor
timestamp: March 04, 2024, 05:12:48 pm
content: Quote from: mr_byte31 on March 04, 2024, 03:24:02 pmNobody cares about theplanneduse.What matters is theactualuse.It doesn't matter how manyotherpeople do something.What does matter is whether someone is blinded. "Oops, I didn't mean to blind you" is not a sufficient response.To aide recall of safety briefings, in the 80s BT Research Labs used to have posters up "Do not look at laser with remaining eye". Their safety briefings made special note of the consequences of specular reflections fromany surfacethat happened to be around.

author: mr_byte31
author_rank: Regular Contributor
timestamp: March 05, 2024, 12:51:29 am
content: Thx for the advice. I will keep that in mind.

author: Revky27
author_rank: Contributor
timestamp: March 27, 2024, 06:41:26 am
content: Quote from: tggzzz on March 04, 2024, 05:12:48 pmDamn man you need to take a chill pill. Second time I see you going off on your safety craze.You don't know power or where he is going to use the laser. Why do you immediately assume he will do it in public? Do you do all your electronics in a room full or random people?Rather than be so agressive ask some basic questions to see if the laser he building is is indeed harmful to anyone.

author: Revky27
author_rank: Contributor
timestamp: March 27, 2024, 06:46:45 am
content: Quote from: mr_byte31 on March 04, 2024, 01:57:20 pmHi,The book that was recommended while interesting it actually deals with relatively low frrequencies.For lidar you need some faster amplifiers and here are some things you can look into:1. You bias your diode correctly and you can use MMIC amplifier (distributed or other) This has very wideband response and since you're doing lidar your DC component is not that critical.2. You get useful application notes from components manufacturers. I recommend you google what you want i.e. 'LIDAR Receiver' with 'reference circuit' or 'application notes'. Then you check what they do and their target and this gives you idea of either circuit topologies to use or parts that are at your speed grade. Here is one example that i found:https://www.ti.com/tool/TIDA-011873. For gain you usually want to switch resistor in the feedback path of the amplifier. you have to check stability of your circuit for all gains and feedback configuration. You could also implement a voltage controlled resistor with a fet but that is significantly more tricky in the feedback of a fast amp.

author: tggzzz
author_rank: Super Contributor
timestamp: March 27, 2024, 08:10:31 am
content: Quote from: Revky27 on March 27, 2024, 06:41:26 amNo, I will not.I don't care if he blinds himself. I do care if he blinds someone else.Why? I am very deaf, and if I am blinded I will have to find a way to commit suicide. Seriously.QuoteStrawman arguments are unimpressive. Why do you assume the OP will do his electronics in complete isolation?Why not ask questions? Easy:I am not qualified to make as statement about safetythe OP almost certainly won't realise all the relevant facets that compromise safety - and therefore won't tell usit is the OP's responsibility to ensure safety, not mineRemember: beginners think about how things work, but professional also think about how things fail

author: Revky27
author_rank: Contributor
timestamp: March 27, 2024, 08:33:18 am
content: Quote from: tggzzz on March 27, 2024, 08:10:31 amAgain, your personal circumstances are yours no one on this forum should care and the fact that you're so aggressive and confrontational its completely un-helpful. I don't care and neither does anyone else. You don't see me crying over my personal circumstances because they are just that: personal.Again, as you said its op-s responsibility not yours so why are you here doing this warning?If so, how come you don't warn for the following things which can cause serious damage to you or others around you:1. soldering2. drilling/cutting3. Lifting heavy electronics equipment4. Aggressive and angry forum postersSo then if you have a need to point to people out these sorts of things do the full job not a half job make sure you cover all aspects. As you imply in your post 'professionals think how things fail' well actually professionals make a risk assessment where you consider both severity and likelihood so really you should be doing a proper professional job

author: Marco
author_rank: Super Contributor
timestamp: March 27, 2024, 08:45:59 am
content: Assuming the current from the photodiode is sufficient with a 50 Ohm load you can use a RF amplifier IC as the first stage, easier than transimpedance. Not sure if PGA makes much sense, to me it seems easier to make signal detection mostly gain independent. Say a differentiator + ZCD gated by a threshold detector.

author: tggzzz
author_rank: Super Contributor
timestamp: March 27, 2024, 08:52:08 am
content: Quote from: Revky27 on March 27, 2024, 08:33:18 amMore strawman arguments, similarly unimpressive.QuoteObvious, to anybody thatthinks.People new to a field frequently don't realise the ways things can be dangerous. That is particularly true for things that are outside the realm of "everyday life" - examples include lasers, high voltages, RF, etc, etc.QuoteNow you are being a twit.All aspects such a lifting heavy equipment? Are you really expecting me to point out the dangers involved in working at heights, with biohazards, etc, etc?Are you really expecting me to travel to the OP (since risk assessments cannot be done remotely), and get indemnity insurance?Or are you just trolling?

author: Revky27
author_rank: Contributor
timestamp: March 27, 2024, 12:32:07 pm
content: Exactly - then why do you cry about the lasers in such an aggressive way without knowing the circumstances?you can't do anything, and you don't warn about all dangers, so I don't understand what your goal is apart from having a chip on your shoulder.I wasn't going to say anything but it's not the first post I saw your comment. As I said if you are genuinely worried about risks you should point all risks otherwise if it's an outlet for your frustration (which it sounds like). This is an electronics forum not a discipling and condescending posts forum.

author: tggzzz
author_rank: Super Contributor
timestamp: March 27, 2024, 01:04:43 pm
content: Quote from: Revky27 on March 27, 2024, 12:32:07 pmIt is assertive rather than aggressive. Assertion isnecessarysince too many people want toactively ignorerisks.Yes, Icando something, and Ishoulddo something: alert the OP to the risks.Your position is that if you can't doeverythingthen you shouldn't do anything. That's defeatest nonsense, of course.As for the character of this forum, you have 19 posts, I have 19000 posts. Contribute more information before whining. Hint: nobody else has whined about this!

author: Revky27
author_rank: Contributor
timestamp: March 27, 2024, 02:11:49 pm
content: Oh, so you decide what people do even though you don't know them. Assert at being a nicer person not some frustrated old guy angry at life.I understand to ward when they say they are about to do something, but you come out of nowhere and as I said you can do something about back pain from lifting electronics equipment, but you don't which actually affect way more people that eye laser damage. You come here with your hero complex then do a good job not a half assed job. You can alert op for more risks. Why don't you?Maybe it's the attitude of people like you which is stopping many from interacting like me.And it's not the number is quality of the posts. I saw you make this post on 3 separate threads which brings exactly zero value. But anyways I tried you clearly think you are doing good so keep it up!

author: tggzzz
author_rank: Super Contributor
timestamp: March 27, 2024, 04:50:48 pm
content: Quote from: Revky27 on March 27, 2024, 02:11:49 pmBased on your points we can conclude that if you saw a child about to run into the road without looking, you wouldn't warn themunlessyou could also warn them about flowing water, heights, snowstorms, etc. If you saw a different child about to do the same 10 minutes later, you wouldn't warn them because you had already warned a different child. None of that makes sense.OK, so you are the kind of person that misses the key point, whines, asks unimportant and unperceptive questions, and ignores the answers.Can I suggest you might investigate stackexchange and edaboard. They may be more to your taste; ISTR they allow downvoting.Plonk.

author: TimFox
author_rank: Super Contributor
timestamp: March 27, 2024, 08:31:12 pm
content: It is very important to understand the difference between obvious risks, including those which result in immediate painful feedback such as soldering irons, heavy machinery, and cutting tools (on the one hand) and non-obvious risks such as ionizing radiation (e.g., x rays), non-odorous poison gases (e.g., CO), and invisible blinding high-power light (e.g., lasers).You cannot sense the damage you are receiving from the latter hazards.

author: dmendesf
author_rank: Frequent Contributor
timestamp: March 27, 2024, 09:44:12 pm
content: Now we have a new way to start every post about lasers: "tggzzz, no need to comment. Please go away."

author: tggzzz
author_rank: Super Contributor
timestamp: March 27, 2024, 10:58:24 pm
content: Quote from: dmendesf on March 27, 2024, 09:44:12 pmAs long as you make the point I would have made, that's perfectly saneBasicallyeverytime I see a child about to run into the road without looking, Iwillwarn them. What would other people do? Whatshouldother people do?

author: Revky27
author_rank: Contributor
timestamp: March 28, 2024, 07:08:07 am
content: Quote from: tggzzz on March 27, 2024, 10:58:24 pmPerfect let's analyze again what you said in the light of your recent analogy. It really shows the difference between your post and what you think you are doing.in your original post you quote" Fundamentally, I really don't care if it blinds you, but I do care about random people passing by. "So effectively in the light of your analogy what you are actually doing is warning the kid running into the road because you care about the cars and the people driving, not the child itself. Or is it not the person working with the laser at highest risk of being damaged by set laser?Or are these not your comments?

author: tggzzz
author_rank: Super Contributor
timestamp: March 28, 2024, 08:50:27 am
content: Quote from: Revky27 on March 28, 2024, 07:08:07 amI'm not feeding this particular troll anymore.

author: mr_byte31
author_rank: Regular Contributor
timestamp: April 12, 2024, 02:53:56 pm
content: Many thanks all for the hints.Quote from: Marco on March 27, 2024, 08:45:59 amcan you please recommend RF amplifier IC ? I see a lot of expensive ICs from Analog and I am not aware about popular and cheap options.

author: Marco
author_rank: Super Contributor
timestamp: April 12, 2024, 03:06:36 pm
content: https://eu.mouser.com/c/semiconductors/wireless-rf-integrated-circuits/rf-amplifier/?sort=pricingI didn't mean RF frequency opamp, but actual RF amplifiers. AC coupled, 50 Ohm matched gain blocks with pretty terrible transfer functions if you're used to opamp based circuits, but good enough and generally low noise.