Analog amplifiers for pulsed LIDAR

[mr_byte31]

-- My first post

Hi 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 stage
1- 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 PCB

I appreciate if any expert here give some advices. I am also open to any other solutions/ICs.

[MarkT]

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?

[jwet]

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.

[mr_byte31]

Many thanks for the Book and the advice. Highly appreciated.
I will definitely read the book and try to build its circuits.

[mr_byte31]

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 ?

[tggzzz]

I am SW engineer with limited experience in analog/RF circuits.

I am designing a laser radar.

That's a worrying starting point

How exactly are you going to ensure this doesn't blind someone? That answer must include definition of failure modes and why those cannot cause dangerous radiation.

Fundamentally, I really don'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.

[mr_byte31]

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.

[tggzzz]

it is just postgraduate research project for "Terrestrial Laser Range Finding Systems". It is not planned to be used against humans or animals.

Nobody cares about the planned use.

What matters is the actual use.

It doesn't matter how many other people 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 from any surface that happened to be around.

[mr_byte31]

Thx for the advice. I will keep that in mind.

[Revky27]

it is just postgraduate research project for "Terrestrial Laser Range Finding Systems". It is not planned to be used against humans or animals.

Nobody cares about the planned use.

What matters is the actual use.

It doesn't matter how many other people 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 from any surface that happened to be around.

Damn 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.

[Revky27]

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 ?

Hi,

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-01187
3. 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.

[tggzzz]

it is just postgraduate research project for "Terrestrial Laser Range Finding Systems". It is not planned to be used against humans or animals.

Nobody cares about the planned use.

What matters is the actual use.

It doesn't matter how many other people 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 from any surface that happened to be around.

Damn man you need to take a chill pill. Second time I see you going off on your safety craze.

No, 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.

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.

Strawman 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 safety
• the OP almost certainly won't realise all the relevant facets that compromise safety - and therefore won't tell us
• it is the OP's responsibility to ensure safety, not mine

Remember: beginners think about how things work, but professional also think about how things fail

[Revky27]

it is just postgraduate research project for "Terrestrial Laser Range Finding Systems". It is not planned to be used against humans or animals.

Nobody cares about the planned use.

What matters is the actual use.

It doesn't matter how many other people 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 from any surface that happened to be around.

Damn man you need to take a chill pill. Second time I see you going off on your safety craze.

No, 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.

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.

Strawman 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 safety
• the OP almost certainly won't realise all the relevant facets that compromise safety - and therefore won't tell us
• it is the OP's responsibility to ensure safety, not mine

Remember: beginners think about how things work, but professional also think about how things fail

Again, 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. soldering
2. drilling/cutting
3. Lifting heavy electronics equipment
4. Aggressive and angry forum posters

So 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

[Marco]

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.

[tggzzz]

it is just postgraduate research project for "Terrestrial Laser Range Finding Systems". It is not planned to be used against humans or animals.

Nobody cares about the planned use.

What matters is the actual use.

It doesn't matter how many other people 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 from any surface that happened to be around.

Damn man you need to take a chill pill. Second time I see you going off on your safety craze.

No, 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.

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.

Strawman 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 safety
• the OP almost certainly won't realise all the relevant facets that compromise safety - and therefore won't tell us
• it is the OP's responsibility to ensure safety, not mine

Remember: beginners think about how things work, but professional also think about how things fail

Again, 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.

More strawman arguments, similarly unimpressive.

Again, as you said its op-s responsibility not yours so why are you here doing this warning?

Obvious, to anybody that thinks.

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.

If so, how come you don't warn for the following things which can cause serious damage to you or others around you:
1. soldering
2. drilling/cutting
3. Lifting heavy electronics equipment
4. Aggressive and angry forum posters

So 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

Now 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?

[Revky27]

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.

[tggzzz]

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.

It is assertive rather than aggressive. Assertion is necessary since too many people want to actively ignore risks.

Yes, I can do something, and I should do something: alert the OP to the risks.

Your position is that if you can't do everything then 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!

[Revky27]

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!

[tggzzz]

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!

Based on your points we can conclude that if you saw a child about to run into the road without looking, you wouldn't warn them unless you 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.

[TimFox]

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.

[dmendesf]

Now we have a new way to start every post about lasers: "tggzzz, no need to comment. Please go away."

[tggzzz]

Now we have a new way to start every post about lasers: "tggzzz, no need to comment. Please go away."

As long as you make the point I would have made, that's perfectly sane

Basically every time I see a child about to run into the road without looking, I will warn them. What would other people do? What should other people do?

[Revky27]

Now we have a new way to start every post about lasers: "tggzzz, no need to comment. Please go away."

As long as you make the point I would have made, that's perfectly sane

Basically every time I see a child about to run into the road without looking, I will warn them. What would other people do? What should other people do?

Perfect 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?

[tggzzz]

Now we have a new way to start every post about lasers: "tggzzz, no need to comment. Please go away."

As long as you make the point I would have made, that's perfectly sane

Basically every time I see a child about to run into the road without looking, I will warn them. What would other people do? What should other people do?

Perfect 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?

I'm not feeding this particular troll anymore.

[mr_byte31]

Many thanks all for the hints.

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.

can 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.