OPA847: PCB design questions my transimpedance amplifier - Page 1
Forum section: electronics / projects

author: JoeGt
author_rank: Newbie
timestamp: October 03, 2025, 05:35:27 pm
content: Hello I'm new here and haven't found an amplifier yet so if I am not at the right place don't hesitate,I’m currently working on a 4-layer PCB for a transimpedance amplifier design, and I’m looking for resources and feedback on best practices.I’ve already reviewed some documentation on the topic, but I still have a few open questions:Regarding the copper-free areas around the amplifier:- What is the recommended clearance distance?- Should the ±5 V decoupling capacitors be placed inside or outside this area? (Inside keeps them close to the amplifier, but outside respects the guard zone while increasing the distance from the inputs).For the feedback components (resistors and capacitors):- Is it better to keep them on the same layer as the amplifier, even if that means placing them slightly farther away, or to use via-in-pad (In–, Vout) and mount them directly on the opposite side?Regarding via stitching:- At what stage should it be added to the layout?- What are the recommended rules for placing vias around the amplifier and sensitive areas?My design also includes a 400 V high-voltage supply for the APD. What would be the best practices to properly isolate this HV section from the TIA, in order to minimize coupling while ensuring both safety and analog performance?Finally, I’d greatly appreciate a general review of my PCB design, highlighting any critical points or potential mistakes I should be aware of.Thanks !Edit :Add a schematic and some figures:Amp used = OPA847APD used = Hamamatsu S16453-30kOpamp Gain Bandwidth Product (GBP) = 3900.00 MHzFeedback Resistance (RF) =2.00 kOhmFeedback Capacitance (CF)= 2 pFInput Capacitance (CIN) = 27.00 pFClosed-loop TIA Bandwidth (f-3dB) = 50 MHzPhotocurrent: 1 – 10 µA (peak)Optical event rate: 1 – 10 MHz (so detect pulses up to ~10 MHz)Supply rails: ±5 V for TIAAPD bias: 400 V (handled separately)Noise :- Excess noise factor F=2.94 i used k=0.02 to calculate it but I have no idea where to find it ... This is the ref of the APDS16453-30k(https://www.hamamatsu.com/content/dam/hamamatsu-photonics/sites/documents/99_SALES_LIBRARY/ssd/s16453_series_kapd1086e.pdf)- Shot noise : ishot=1.09e-10 A/Hz ( ishot_rms_MHz=420nA)- idark_shot=3.3nA- Johnson noise Irms=11.2nA- Amp current noise = 9.68 nA-> i_tot_rms=420.6nA-> Vrms =0.841mV, Vdc= 0.5V-> SNR = 55.5dB

author: jwet
author_rank: Frequent Contributor
timestamp: October 03, 2025, 06:06:57 pm
content: A schematic would be a help. Also the overall transfer function, ie transimpedance and speed and perhaps noise tolerance.The very best book on Photodiode Amps is by Graeme. Post a schematic and I'll take a look.

author: JoeGt
author_rank: Newbie
timestamp: October 03, 2025, 08:52:43 pm
content: Hello jwet and thanks for your answer !I updated my post to add a schematic, some figures and noise i compute with my basics knowledge.Ok thanks for your book suggestion i will see if i can find it.Joe

author: jwet
author_rank: Frequent Contributor
timestamp: October 04, 2025, 06:12:16 am
content: Excellent details- I have no excuses now. The book by Jerry Graeme is the only book I've seen that covers the topic end to end. If you're doing these kinds of circuits, it should be required reading.The left side of that 1 Meg R goes to 400V at anode of your detector?How did you settle on the 847, its kind of low voltage noise grunty ADC driver amp. The old standard for this application was the OPA656- Fet input Wideband amp. The bias current on the 847 is pretty high.I'll study some more but I'm not sure if I'm following your parts selection.

author: Terry Bites
author_rank: Super Contributor
timestamp: October 04, 2025, 02:06:13 pm
content: Why 4 layers?See dis app note from TI.Use thishttps://tools.analog.com/en/photodiode/And this. Do your maths!Factor in trace or cable capacitance from the PD to the TIA.

author: JoeGt
author_rank: Newbie
timestamp: October 06, 2025, 08:07:40 am
content: Hello and thanks !My thoughts when I went to 4 layers were :Continuous ground plane (inner layer): clean return path, reducing current loops and ground noise around the TIA.Power separation: a dedicated VCC VEE layer to reduce distribution impedances and facilitate decoupling.Easy routing to keep sensitive tracks short.Shielding and isolation: internal ground plane protects the TIA from interference from the HV converter (switching).I am, of course, open to comments if you have any points to raise.Thanks for the document and the tool (very useful !), I have already use the second one to design my TIA. What do you mean by do you math ? What have I missed ?Thanks for the document on clearance and creepage

author: JoeGt
author_rank: Newbie
timestamp: October 06, 2025, 08:40:43 am
content: Hello,The left side of that 1 Meg R goes to 400V at anode of your detector?-> Yes (and the circuit to simulate an APD with a function generator).How did you settle on the 847, its kind of low voltage noise grunty ADC driver amp. The old standard for this application was the OPA656- Fet input Wideband amp. The bias current on the 847 is pretty high.-> Good point, I’ve ordered an OPA847 and an OPA657 for my tests (similar to the 656).What I’m about to say might not be 100% accurate, so please feel free to correct me.To keep it simple, based on my previous calculations, with an OPA847 the dominant noise is from the APD, not the op-amp, so it’s sufficient for my case. If I were limited by current noise (e.g., an APD with very low photocurrent or a TIA with very high impedance), I would indeed have chosen a FET input op-amp like the 656.To go into a bit more detail:My APD provides a current on the order of a few µA. The OPA847 is sufficient here (current noise around ~2.5 pA/√Hz). The 656 or 657 would be relevant for measuring very low currents, on the order of pA (current noise ~2 fA/√Hz).Of course, there is a non-negligible bias current that creates an offset voltage across Rf, but this offset is not problematic (the offset will be at most 10 µA × 2 kΩ = 20 mV — small and easy to correct). My current noise would convert to only a few tens of µV RMS at the output (for 40 MHz).Additionally, I’m aiming for a wide bandwidth. The OPA657, with its lower GBP, makes it difficult to reach the required 40 MHz. At low Rf values (a few kΩ), the higher voltage noise of the OPA657 becomes limiting, and from experience, it would oscillate at low Rf for reasons I’m not entirely sure of.Yes, no problem, if you have something better, I'm interested!I just want to understand.

author: jwet
author_rank: Frequent Contributor
timestamp: October 06, 2025, 04:01:26 pm
content: You might have a point in the APD case- I'm thinking of straight PD's.I think the bias current becomes a shot noise source like PD leakage. In=sqrt(2*q*I). I'll have to look at Graeme again.The 656 is obsolete I guess but there are replacements. (the 657 is just a decomp'd version). I won't dwell here since I have to study more- could be a non problem.

author: Someone
author_rank: Super Contributor
timestamp: October 07, 2025, 12:38:13 am
content: Quote from: JoeGt on October 06, 2025, 08:40:43 amYour 1M input resistance is going to add more challenges for seeking bandwidth than the amplifier choices!Series resistance added to a current source (which a photodiode usually is) is generally avoided.

author: Andree Henkel
author_rank: Regular Contributor
timestamp: October 09, 2025, 03:45:17 pm
content: Quote from: Someone on October 07, 2025, 12:38:13 amI second thisI also doubt use of OPA847I would advice use of JFET input type with specially optimized footprint for TIA applicationfor +/-5V there are Analog Devices ADA4817-1ACPZ and Texas Instruments OPA818IDR they have same SON 3x3mm package, special footprintwith additional feedback pin (internally connected to out) right besides the minus input, sharing same pinoutThe lower Bandwidth of the two JFET will give you theoretically lower TIA Bandwidth as OPA847, but I doubt it later will give workable stable (well behaved frequency responce) design. In case JFET solution misses some Bandwidth reduce TIA resistance and add second gain stage.your Input capacitance is also pretty large, being problematic for TIA stability, on other hand your feedback resistor is quite low so feedback cap is still at workable size without needing to go into Tee feedback network

author: Andree Henkel
author_rank: Regular Contributor
timestamp: October 09, 2025, 04:27:39 pm
content: Assuming the 1Mohm resistor has a 0.5pF parasitic parallel capacitance, you would get a corner frequency of 318kHz with high pass caracteristics, below that you get theoretically signal swing of 420mV accross this resistor, 500 times more than at your TIA output, this basically will not work. you would not need a TIA, if it where that easy to get a voltage from photocurrent combined with high bandwidth.Also I doubt why you AC Couple your input current. Why?Ok, you probably mean parasitic detector capacitiy.connect APD Bias voltage ground without galvanic isolation with ground of your OPAMP bipolar supply,such the point that is at your OPA plus input (via 2k resistor).Seems you want a positive signal at TIA output, so you need a negative APD Bias Voltage at anode and connect cathode to TIA.I designed predecessor of Thorlabs APD130C versions with and without temperature compensation, as fas as I remember (it was more than 15 years ago) yes OPA847 was used, but no 1M, no ac coupling at input.

author: Andree Henkel
author_rank: Regular Contributor
timestamp: October 10, 2025, 09:02:42 am
content: you probably have the 1M resistor there for quenching / current limiting purposeconnect a like 1n (50 times your parasitic photodiode capacitance) capacitor after it (closer to TIA) to ground,so at AC small signal model you now have the APD capacity directly to ground, no longer in series to 1M resistor

author: JoeGt
author_rank: Newbie
timestamp: October 10, 2025, 02:55:20 pm
content: Hello @Someone,I am truly sorry for the late reply. I hope you will see it.I put a 1MR resistor here because I wanted to simulate my photodiode with a function gene. This allowed me to simulate a small, low current. I am an idiot; I forgot to remove it. I updated my first message.Like all the values in red, this is only when I was using the function block to simulate my APD or test values.

author: JoeGt
author_rank: Newbie
timestamp: October 10, 2025, 03:12:10 pm
content: Hello Andree Henkel, and sorry for the late reply — I had notifications enabled, but I only received the first ones and not the newer ones, I’m not sure why...As I said to @Someone : I put a 1MR resistor here because I wanted to simulate my photodiode with a function gene. This allowed me to simulate a small, low current. I am an idiot; I forgot to remove it. I updated my first messageI don’t know if you saw my reply to jwet about the choice of the OPA847.That said, you’re right — using an OPA818, which has an FB pin, would definitely simplify my layout. I should consider that; I’ll order some to try.I managed to get something stable with the 847 on a prototype board. When I tested with the 657, though, it went haywire. But I’ll give it another shot then.Yes, I still want to try to avoid using a Tee feedback network.

author: JoeGt
author_rank: Newbie
timestamp: October 10, 2025, 03:47:48 pm
content: I'm definitely really clumsy. I should have paid more attention to that 1MR resistor ^^.For the AC coupling, it was also a test residue that I was advised to use to simulate my PD, but it doesn't seem very wise, even for a test.In my updated diagram, I also added my HV output. Do you think that's okay?Regarding adding a 1nF capacitor between the IN- terminal of my TIA and the GND. I've seen that before, you're right. Why take 50 times the value of the PD capacitor? Is it experience that makes you say that? I also added that to my diagram in the original post.

author: StillTrying
author_rank: Super Contributor
timestamp: October 10, 2025, 04:00:32 pm
content: I guessed the 1M was some test.Here's what a LT sim and some guesswork gives.It detects 10uA 10ns APD pulses but only 18 mV on the output.

author: JoeGt
author_rank: Newbie
timestamp: October 10, 2025, 04:26:37 pm
content: Thanks @StillTrying !Yes I have done some simulations also and I get 20mV output with a SNR of min =15dB to max= 35dB.I thought that's enough because I then boost my signal to 500mV with an amplification stage. What do you think?Your LTSpice looks great. Is it possible to share it? I've always used TI tools, but I think I need to move on, and this would be a good foundation for me. Thank you.If you don't mind, I'll manage with your image

author: StillTrying
author_rank: Super Contributor
timestamp: October 10, 2025, 04:54:34 pm
content: I've got OPA847.lib just in my schems folder.The pin 'Dis' has to be edited/removed from it to use it with the opamp2 symbol..SUBCKT OPA847_Model + - V+ V- OutDis

author: Andree Henkel
author_rank: Regular Contributor
timestamp: October 13, 2025, 12:48:01 pm
content: the explanation regarding the 1M and AC coupling being for test with function generator enlighten that.No you shouldn´t put 1n Cap directly at minus input of OP, but it may be good value for C_filter1

author: StillTrying
author_rank: Super Contributor
timestamp: October 13, 2025, 01:17:54 pm
content: Quote from: JoeGt on October 10, 2025, 04:26:37 pmYou don't say what you want from the detected pulses, such as keeping some of their amplitude or width information, or just a count of the pulses above the noise.Boosting 10ns pulses from 18mV to 500mV wouldn't be easy.The LT TIA sim is very stable. compaired with a much faster TIA one I'm doing, so you could peak up the 10ns pulses to around 80mV for easier detection by increasing the feedback resistor and and reducing the cap to around 0.8pF.Did anyone get my LT version to work.

author: JoeGt
author_rank: Newbie
timestamp: October 14, 2025, 11:12:17 am
content: Thanks@StillTryingfor the simulation! It's very nice, it works perfectly.It's true that I didn't specify that. Yes, ideally I want to keep the amplitude and width.So I'm playing around with your simulation and adjusting the parameters as I did before.I wonder if it's possible to simulate the noise and stability of my setup with LTSpice?I've seen that with the simulation, you can actually increase the gain and maintain good bandwidth. I'll try all that.