High Speed Transimpedance Amplifier - Page 1
Forum section: electronics / projects

author: arivalagan13
author_rank: Contributor
timestamp: June 10, 2019, 03:55:57 pm
content: Hello all,I'm in the process of designing a transimpedance amplifier for an analytical instrument. The requirement is such that the process generates a very low current which is multiplied by microchannel plate. Even after this, the current is between 1 to 5 pA and the bandwidth requirement is around 300 MHz. The transient response requirement is also very high.I've designed a transimpedance amplifier with OPA656 which is a 500MHz unity gain-bandwidth product opamp. I've installed a 1Gohm feedback resistor..I'm about to sweep this resistor value in order to find the correct resistor value to faithfully amplify the current signal.For testing purposes, I use an old photodiode..whose specifications I don't know clearly.My questions are:what are the systematic steps I should follow to design the amplifier?2. I don't have ready-made access to sophisticated measuring instruments like keysight/Tektronics DSO and the like.Having said that suppose I want to characterize the frequency response and transient response of the amplifier.But, function generators can only generate voltage sources...or how do I generate current sources in order to measure the frequency response of the TIA.....or whether the facility of generating current sources is available with the function generator?3.I've the same question for measuring the transient response of the TIAto put in other words: How do I measure the frequency response of the transimpedance amplifier? and How do I generate such a small current in order to measure the frequency response and the transient response..Kindly provide necessary support in this regard.Regards,M ArivalaganResearch ScholarAnna UniversityChennaiTamilnaduPh.:+91 75023 98999email: arivalagan13@gmail.com

author: StillTrying
author_rank: Super Contributor
timestamp: June 10, 2019, 04:29:52 pm
content: Quote from: arivalagan13 on June 10, 2019, 03:55:57 pmI think you're beyond what can be done by a factor of about 10 million, hopefully I'm wrong though.

author: awallin
author_rank: Frequent Contributor
timestamp: June 10, 2019, 05:18:11 pm
content: gotta learn to walk before you run... try this for a start:https://electrooptical.net/static/oldsite/www/frontends/frontends.pdf

author: DaJMasta
author_rank: Super Contributor
timestamp: June 10, 2019, 05:33:27 pm
content: Hamamatsu also has some designs for high speed photodiode circuits available as app notes with some of the important considerations mentioned.A lot has to do with parasitic capacitance of the system, the photodiode itself is a significant component so using any old photodiode may not give you the performance of the rest of the circuit. Of course, the capacitance of the amp matters, but I believe the Hamamatsu app notes also mention that breaking the feedback resistor into a series string of feedback resistors also helps reduce the capacitance on the path.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 10, 2019, 06:49:49 pm
content: For a fast system the FB resistor should only be as large as needed to get the noise low enough.I would start with increasing the gain of the micro-channel plate to get more signal to start with.I would suspect that one may need to go discrete fets and possibly not silicon based any more. If small and thus low capacitance, one may not need a TIA anymore.

author: D Straney
author_rank: Frequent Contributor
timestamp: June 10, 2019, 07:26:12 pm
content: One thing which is really useful for this type of project is the ability to do very quick ballpark calculations to see if something is (a) easy with no special effort, (b) possible but difficult with off-the-shelf parts, or (c) completely impossible without government-lab levels of funding.In this case, with a transimpedance amplifier, one thing besides the amplifier that limits the bandwidth is going to be the capacitance in parallel with the feedback resistor. Think of an inverting amplifier with a capacitor in parallel with the feedback resistor, as a low-pass filter (seethisat the "Frequency Response Curve" section, or work through the math from an ideal op-amp), then replace the input voltage across input resistor (which acts as a current source) with a pure current source, and you have a transimpedance amplifier. You'll find that the feedback resistor and its parallel capacitance adds a pole to the frequency response at 1/(2*pi*Rfbk*C).Anyways, that was just showing you where my math is coming from. Let's assume the op-amp's bandwidth isn't even a problem, and look for what capacitance is allowed in parallel with the 1Gohm resistor to give you a pole at 300 Mhz or higher. Knowing that 300 Mhz=1/(2*pi*1Gohm*C), the parallel capacitance has to be < 5.3E-19 F, or 0.00000053 pF! Considering that capacitances like 0.01pF - 0.1pF are seen just by traces on the same board, expecting to achieve 0.00000053 pF of stray capacitance across your feedback resistor is wildly unrealistic. Even if you made the resistor a mile long for almost zero stray capacitance, there's way more capacitance than that just between the op-amp's pins.So maybe it would be more realistic to have multiple cascaded stages, with lower gain in each one? In that case, each one would need > 300 Mhz bandwidth, but just as an easy example let's suppose we can survive with 300 Mhz bandwidth in two separate stages. Let's try and split up the gain between the stages, with the first stage having a gain of 1E4 and the second stage having a gain of 1E5:2-stage exampleNow with a 10K feedback resistor in the first stage (a transimpedance amplifier) you'll need "only" < 0.05 pF across that resistor. This is still difficult, and will require some special feedback-resistor-strings like DaJMasta mentioned, but at least it isn't wildly unrealistic.The second stage with a gain of 1E5 will be a voltage amplifier (because your transimpedance amplifier is now producing a voltage output), so let's suppose that an inverting amplifier: if you use an input resistor of 1 ohm (you can do that here because the input voltage will be so tiny that the current through 1 ohm will still be very small), your feedback resistor will be 100K. To give the second stage a bandwidth of 300 Mhz, it can't have more than 0.005 pF (5 fF) across its feedback resistor...uh oh, this is starting to look unrealistic again, especially because to get 300 Mhz overall bandwidth, you'll need > 300 Mhz bandwidth in each stage.3-stage exampleMaybe splitting it into 3 separate stages will help, each with a gain of 1E3. I did a quick LTSpice simulation, and it turns out that 3 stages of 600 Mhz bandwidth each gives you a total bandwidth of 300 Mhz (see attached image).So the first stage will have a 1K feedback resistor, and its stray capacitance will have to be 1/(2*pi*600E6*1E3) = 0.27 pF or less. The second and third stages can then be voltage amplifiers with input resistors of 1 ohm, and feedback resistors of 1K, with the same stray capacitance requirement on each one (< 0.27 pF). This is sounding fairly realistic.Even though using a 3-stage solution this way solves the stray-feedback-capacitance problem, it adds a noise issue though. A 1-5 pA input will give you 1-5 nV at the input to your second stage. This is an absolutely tiny signal, and even the lowest-noise op-amps have input-referred noise levels of about 1 nV/sqrt(Hz). I don't want to overwhelm you here by going into a full noise analysis tutorial, but basically if you solve your stray-capacitance problem, your signal is going to be completely swamped in noise (even with 2 stages probably). There are measurement instruments that can measure nV-level signals, but that is usually done with methods thatdon'tallow high frequency (chopper amplifiers, heavy averaging, etc.).Anyways, I hope that wasn't too much informationJust wanted to:1. Show how someone else would think about a problem like this, and2. Demonstrate how useful some quick feasibility calculations are - it's much easier to spend an extra day doing some easy math, than to order parts and build things immediately, before finding out that what you're trying is impossible by a factor of a million!My best advice overall is that you won't be able to have both low current (single pA) and high bandwidth at the same time. See if there's a different way to make the application with only one of those things.

author: Marco
author_rank: Super Contributor
timestamp: June 10, 2019, 11:46:08 pm
content: Phil Hobbs has an interesting factoid inhis allusions to his pHEMT current amplifier.QuoteOr in other words, at 1 to 5 pA with a bandwidth requirement of around 300 MHz you're not really measuring a signal ... you're electron counting. Also he considers nA with that bandwidth state of the art. I imagine you're going to be looking at something more exotic and liquid helium cooled.

author: David Hess
author_rank: Super Contributor
timestamp: June 11, 2019, 03:25:02 pm
content: Burr-Brown (now TI) published detailed application guides on high speed transimpedance amplifier design. Search for them online. I think you are going to need a composite design rather than just a fast operational amplifier.The large value feedback resistor will need to be replaced with a t-network.

author: AndyC_772
author_rank: Super Contributor
timestamp: June 11, 2019, 05:18:24 pm
content: Even if you could build such an amplifier, I'm not sure how you'd ever use it without the output coupling back into the input to such an extent that it would go unstable.

author: duak
author_rank: Super Contributor
timestamp: June 11, 2019, 06:23:05 pm
content: Jerald Graeme's book Photodiode Amplifiers is worth reading. It is from 1995 and may not be up to date on device developments, but the concepts are useful. Much of the information is also on the Burr-Brown/Texas Instruments, Analog Devices (and other) websites.Just to understand the problem better, are you expecting a DC to 300 MHz bandwidth? ie., do you need DC or is there a lower frequency limit? Is the input single shot or repetitive? Is there a high level signal synchronous to any particular event or feature in the signal to allow for sampling?

author: Marco
author_rank: Super Contributor
timestamp: June 11, 2019, 06:26:35 pm
content: If those are his real specs he's electron counting rather than measuring what can be treated as a continuous signal. At pA 100 MHz range you just get the occasional electron, most of the time there's nothing ... I'm pretty sure this isn't going to work at room temperature, unless he has a repetitive signal he can rescue from noise double digit dB stronger than the signal.

author: David Hess
author_rank: Super Contributor
timestamp: June 11, 2019, 06:38:25 pm
content: Quote from: AndyC_772 on June 11, 2019, 05:18:24 pmTransimpedance amplifiers with this kind of performance exist in various forms. But they are extensions of the commonly studied operational amplifier based transimpedance amplifier.

author: AndyC_772
author_rank: Super Contributor
timestamp: June 11, 2019, 07:13:46 pm
content: Any chance of a link to some more information on them please?

author: cur8xgo
author_rank: Regular Contributor
timestamp: June 11, 2019, 07:31:27 pm
content: Not sure if this was posted already:"High-Speed, Linear Transimpedance Amplifier Reference Design"Photo diode included!http://www.ti.com/lit/ug/tidud08/tidud08.pdf> 500 MhzCurrent to voltage gain 500 to 10000 ohmMaybe this could be the front end?

author: Marco
author_rank: Super Contributor
timestamp: June 11, 2019, 08:08:20 pm
content: Quote from: cur8xgo on June 11, 2019, 07:31:27 pmCouple pA/rtHz noise current, signal is only 80 or so dB below the noise

author: StillTrying
author_rank: Super Contributor
timestamp: June 11, 2019, 08:08:46 pm
content: http://www.ti.com/lit/ug/tidud08/tidud08.pdfThey've got about 900uA in the fast PD, which is about 200 million times higher than the OP's 5pA.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: June 12, 2019, 01:14:42 pm
content: OK, a bit of a reality check.An MCP can have (conservatively) a gain of ~1000. For each electron entering it will produce ~1000 electrons output . That is the minimum charge you need to detect. If the output pulse width is ~2ns it creates a current of~ 30uA for that duration, detectable even on a good fast scope with a 50 Ohm input as a ~1.5mV pulse.~80nA, 1pA to 5pA output current will be averaged from these fast current pulses.100 pulses / second will create an average current of ~6pA. The only way to measure this current reliably is to either integrate it over a long time (i.e. seconds) or count the pulses. What the TS needs is not the current measurement as such, I suspect, but the timing of pulses. That is not a difficult task and you only need a low gain transimpedance amp (or even just a resistor load and a fast voltage amplifier), so the bandwidth is not a problem either.CheersAlexP.S. - corrected the calculations, had a senior moment apparently

author: Marco
author_rank: Super Contributor
timestamp: June 12, 2019, 03:08:06 pm
content: That's fine, but that's not his spec. It does make sense though, why use an electron multiplier and then spec an amplifier for single electrons within your bandwidth.

author: arivalagan13
author_rank: Contributor
timestamp: June 16, 2019, 03:15:04 pm
content: Thank you all for the response...I would like to add a few more information so that I can get more information from you.I've installed 1Gohm resistor...but while testing with the instrument in the lab I will sweep the resistors (desolder and solder) with values ranging from 10ohm, 1kilo ohm, 2.5kilo ohm and many more...I'll do this exercise to understand the system requirements more precisely...it is one of the govt. labs which use the analytical instrument(time-of-flight mass spectrometer)... I am attaching the graph for your reference...Also, the lab at present uses the amplifier 6950ds(attached) from Phillips Scientific. I need a better performance than this...I'll go iteratively...One of my questions is like suppose I want to test the amplifier in the lab with a function generator (which generates a voltage signal) how do I generate a current signal for testing purposes..I never used or have access to advanced measuring instruments and so I am asking about itKindly give me further suggestions and input..also, suggest some information regarding noise and input capacitance of the opamp

author: arivalagan13
author_rank: Contributor
timestamp: June 16, 2019, 03:18:30 pm
content: Adding one more graph for your reference

author: StillTrying
author_rank: Super Contributor
timestamp: June 16, 2019, 03:40:04 pm
content: "Also, the lab at present uses the amplifier 6950ds(attached) from Phillips Scientific. I need a better performance than this...""Input : One; 50 ohms""Wideband Noise : Less than 25 μVolts RMS; Referred to the input. (1.5nV/ÖHz.)""Even after this, the current is between 1 to 5 pA"I can't see how your 1 to 5pA could be correct as input to that amp.

author: arivalagan13
author_rank: Contributor
timestamp: June 16, 2019, 04:22:35 pm
content: The stage previous to the amplifier is the microchannel plate which generates a current..as shown in the graph...and I need a TIA...The microchannel plate generates current in the range of 1 to 5pA....Did I answered your question or I'm mistaken?

author: Marco
author_rank: Super Contributor
timestamp: June 16, 2019, 06:00:08 pm
content: Quote from: arivalagan13 on June 16, 2019, 04:22:35 pmYou didn't address Alex Nikitin's post ... you will never see 5 pA unless you average over some huge period like a second. With a high bandwidth amplifier you will always see massively higher currents because the electrons arrive in bunches, so worrying about noise is silly when working at high bandwidth. You just need enough SNR to distinguish one event from multiple near simultaneous events.

author: StillTrying
author_rank: Super Contributor
timestamp: June 16, 2019, 06:07:52 pm
content: Quote from: arivalagan13 on June 16, 2019, 04:22:35 pmYes, you confirmed only 1-5pA.It looks to me that software records the time that the a 1-5pA pulses arrive, and their amplitude is not that important, which woud help a little bit but not much.But, I don't much about currents that low, or how these types of things work, or how the signal is extracted from the noise.https://en.wikipedia.org/wiki/Time-of-flight_mass_spectrometryI can't see how the OPA656's 1*500MHz could ever improve on the Phillips's 10*300MHz.But hopefully someone will have a much better idea than me.

author: Marco
author_rank: Super Contributor
timestamp: June 16, 2019, 06:27:35 pm
content: Quote from: arivalagan13 on June 16, 2019, 03:15:04 pmThis isn't a transimpedance amplifier by the way and I don't think you will be able to use one at the moment (ie. with the coax connector). Where you do you even measure 5pA? You're never going to be dealing with currents at the end of a piece of coax at these frequencies ... somewhere deep inside the measurement device there's current, then it goes into coax and from that point on all you have to worry about is the voltage on the 50 Ohm termination.Lets say you get 10000 electrons in a single event from a single stage MCP, lets say the time domain response of your digitizer is 5 ns, so the average current is 10000*1.6e-19/5e-9 =~ 0.3 uA. Times 50 Ohm is 15 uV. The Johnson noise for 50 Ohm over 100 MHz is ~ 6 uV ... so this is kinda pushing it. The Philips amplifier is indeed not good enough to detect a single event within these specifications. A TIA would help, but you can't connect that through coax in this case, because the necessary bandwidth is way too high for a piece of improperly terminated coax. A TIA needs to be at mm distance to the MCP.How fast is your digitizer BTW? It's rather relevant ...PS. when I say TIA I actually mean 10k resistor and voltage buffer, I doubt the MCP cares much about the load resistance.PPS. oh that's not right, I underestimated the capacitance of the MCP ... guess you'll need a proper TIA.

author: arivalagan13
author_rank: Contributor
timestamp: September 17, 2019, 03:43:24 pm
content: Dear all,I'm reraising the thread for the design of high-speed transimpedance amplifier (TIA) with some updated information.My requirement is to design a TIA for ToF-MS. I said the instrument uses MCP (Microchannel Plate) and I was not sure about the information about the current and quoted it to be in 1 - 5 pA range. But, I have some updates towards this as mentioned below.Updates:The ToF-MS does not use MCP. Instead, it uses ET - Secondary Electron Multiplier (SEM) model number AF880(part number 14880).I'm attaching the datasheet of this part number with this as an attachment. Now that I've got the datasheet I wan to design TIA for this device.My questions are:1. How do I model this SEM electrically, particularly how do I find (or extract) the capacitance information so that I can calculate the capacitor values and design appropriate capacitor and resistor values?2. How do I go about choosing the important parameters of the target opamp to be used in the design of TIA (I believe closed-loop bandwidth, stability, noise and slew rate parameters are important). How do I go and relate the SEM parameters and the opamp parameters?

author: Marco
author_rank: Super Contributor
timestamp: September 17, 2019, 06:22:07 pm
content: With that sensor you can't really use a transimpedance amplifier, because it's 50 Ohm matched. You don't really need one either, because of the 10^7 or more gain the pulses will be easily above the noise floor with a good RF amplifier. I mentioneda modulebased on TQP3M9036 in the other thread.TIAs at high frequencies are for things like photodiodes, tiny devices right next to the TIA. Often even wirebonded.