Slew Rate, Bandwidth, Settling time, rise time for high speed transipmedance amp - Page 1
Forum section: electronics / projects

author: arivalagan13
author_rank: Contributor
timestamp: August 24, 2019, 11:17:26 am
content: Hi allI'm in the process of designing a Transimpedance amplifier for an analytical instrument. The sample waveform (hand-drawn) is attached in the attachments. The requirement is to amplify a pulse kind of signal whose maximum amplitude is around 700 nA and the width of the pulse is around 1 ns. So, I need a high-speed transimpedance amplifier.My question is what are the opamp parameters that decide the speed of the amplifier. I need a concrete relationship between slew rate, settling time, rise time, bandwidth/gain-bandwidth. And particular to the signal which parameter is most important so that I can choose the right opamp for the design.RegardsM Arivalagan

author: Marco
author_rank: Super Contributor
timestamp: August 24, 2019, 01:08:08 pm
content: What's the source capacitance? TI hasa reference design for a 200 MHz TIAwith a link to a reference how to calculate the necessary GBP.Personally though I'd try to dig up some old stock for MAX3266CSA, one of the few GHz range TIAs available in non-die form.

author: arivalagan13
author_rank: Contributor
timestamp: August 24, 2019, 02:21:12 pm
content: The source capacitance is around 20pF

author: Marco
author_rank: Super Contributor
timestamp: August 24, 2019, 04:06:18 pm
content: Really? That's kinda huge for something with a GHz worth of bandwidth and will present an almost insurmountable challenge to any opamp and TIA you can get in a solderable package.

author: awallin
author_rank: Frequent Contributor
timestamp: August 24, 2019, 04:49:57 pm
content: that one seems close(ish) to 1GHz:http://www.ti.com/lit/ug/tidud08/tidud08.pdfit might use this photodiode, specified as 2 pF (with some reverse bias)https://acphotonics.com/downloads/High_Speed_800nm_900nm_PIN_Photodiode.pdf

author: David Hess
author_rank: Super Contributor
timestamp: August 24, 2019, 06:09:11 pm
content: Do an online search for the Burr-Brown application notes which discuss wide bandwidth transimpedance amplifiers.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 24, 2019, 08:05:31 pm
content: Even better question: how large, physically, is that 20pF? If the answer is more than a few cm, end to end, then it'snotgoing to behave as a capacitor, but a transmission line stub of some sort!Note that 1ns * 700nA = 700aC and 700aC / 20pF = 35uV. You're looking to amplify and differentiate a step of this magnitude. Even if you don't need much signal level (low mV?), you're asking for a tremendous amount of gain at high frequency to turn that into a pulse versus a step.There is also a noise associated with capacitance, independent of system resistance, or bandwidth (they cancel out). For 20pF at room temperature, this is 28uV.In general, you are asking for the impossible.If the detector and amplifier are cryogenically cooled, it may still be possible, but you may find it is more productive to fix the detector.Tim

author: Marco
author_rank: Super Contributor
timestamp: August 24, 2019, 08:44:23 pm
content: Quote from: T3sl4co1l on August 24, 2019, 08:05:31 pmIt's not supposed to charge the capacitance, the input resistance of the TIA is supposed to be far lower than the impedance of the capacitance. So in this case a couple ohms.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 24, 2019, 08:55:32 pm
content: Yes, I was implying a conceptual split between the source and the amplifier: the signal and impedance, looking into the source, will be as described. The impedance looking into the amplifier is supposed to be low, and this tells you how low it must be.Which, good luck achieving that at the PCB level -- a few ohms [equivalent inductance] at 1GHz is a trace length of maybe 0.5mm.Also, even on a thin substrate, a trace impedance of 10 ohms is much wider than 0.5mm, so we can't really do anything meaningful at all, at this speed and impedance. Even on HDI where you can stack vias into alternating traces in parallel, you're limited by the amplifier's pins/bond wires.Respectively, 20pF resonates at 1GHz with 1.3nH. That detector better be right on top of the amp!Tim

author: StillTrying
author_rank: Super Contributor
timestamp: August 25, 2019, 01:44:00 pm
content: Don't start any more threads on this subject, 4 is more than enough,vital bits of information on the problem always get left behind.You can always wake-up a previous thread with a new post updating with a bit of new info.https://www.eevblog.com/forum/projects/high-speed-transimpedance-amplifier/msg2489382/#msg2489382

author: Marco
author_rank: Super Contributor
timestamp: August 25, 2019, 05:24:16 pm
content: AFAICS a microchannel plate has 50 Ohm output. You don't need a TIA, you need a RF gain block.If you don't care too much about gain flatness just pop a SPF5189 amplifier in there, you can get modules really cheap. Or for a bit better build qualitythis modulewhich I assume is based on TQP3M9036 doesn't seem a bad deal.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: August 25, 2019, 06:04:41 pm
content: Quote from: T3sl4co1l on August 24, 2019, 08:05:31 pmI always thought pure capacitive or inductive reactances had no noise, though it seems like there should be at least some. How is this calculated?Conrad

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 25, 2019, 10:30:03 pm
content: Quote from: Conrad Hoffman on August 25, 2019, 06:04:41 pmThere must necessarily be a resistance somewhere -- otherwise you can't measure any signal at all (though you might be able to sense it parametrically, with other limitations). That resistance gives the RC time constant and therefore the cutoff frequency, but it's also the noise source and therefore the noise level. Noise over the total bandwidth happens to cancel out, giving the simple result.https://en.wikipedia.org/wiki/Johnson–Nyquist_noise#Thermal_noise_on_capacitorsTim

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 25, 2019, 10:31:52 pm
content: Quote from: Marco on August 25, 2019, 05:24:16 pmAnd for that matter, hopefully the MCP is built in such a way that it has a transmission line structure.For example, it might be a sinuous path over a ground plane, which has the bonus of allowing you to correlate propagation delay to impact location.You're still very limited by noise, and may need to stack another MCP to get higher particle amplification, or something like that.Tim

author: Marco
author_rank: Super Contributor
timestamp: August 26, 2019, 04:17:11 pm
content: In theory you could segment the anode for a MCP in say cm^2 pieces and bring multiple connections outside, that should allow you to use multiple very low impedance TIAs.But given that in the other thread he mentioned connecting it to a Phillips lab amplifier he almost certainly just has coax coming from the device and he needs a RF amplifier. The TQP3M9036 has an almost unbelievably low noise which should bring the noise down by about 2x relative to the Phillips amplifier (for the same bandwidth, but it has far more bandwidth than the Phillips).