1µA, 750MHz - Page 1
Forum section: electronics / general

author: luky315
author_rank: Regular Contributor
timestamp: August 11, 2023, 08:37:40 pm
content: For an experiment (different topic and not really my area of expertise) we need to measure, or more specific, detect the presence of a current signal with a minimum of 1µA (but it can also be much bigger, up to 10mA) which is switched on and of with a bandwidth of approximately 750MHz (can vary a bit).Transimpedance ampifiers are on the limit in this range so the question is: are there oher topologies able to detect such small and fast current signals? A sort of current comparator?

author: Alex Nikitin
author_rank: Super Contributor
timestamp: August 11, 2023, 10:33:45 pm
content: Let's do a simple math first. A 50 Ohm resistor at 25C and for 750MHz bandwidth will have its own RMS noise around 25uV or an equivalent of 0.5uA. That means for 2uA detection limit you probably can get away with just a good low noise amplifier with 50 ohm input. 200 Ohm will give you the current noise around 0.25uA RMS which should be good enough for 1uA detection limit.CheersAlex

author: TimFox
author_rank: Super Contributor
timestamp: August 11, 2023, 11:28:20 pm
content: Do you mean a bandwidth of 750 MHz, or a switching frequency of 750 MHz?If you mean switching frequency, you could improve the SNR by using a tuned amplifier or appropriate band-pass filter to narrow the bandwidth.

author: Marco
author_rank: Super Contributor
timestamp: August 12, 2023, 12:44:34 pm
content: Can't you just use a spectrum analyzer?

author: luky315
author_rank: Regular Contributor
timestamp: August 12, 2023, 02:04:08 pm
content: The pulse can be 0.66ns long with an 0.66ns pause (worst case scenario), but it can also be longer or with a longer off-time, so I don't see how I can use a spectrum analyser to detect the pulse. A bandpass filter is also only of limited use here.I need some sort of current detector with a reaction time <0.66ns.

author: TimFox
author_rank: Super Contributor
timestamp: August 12, 2023, 03:39:19 pm
content: How much longer?The required bandwidth for a valid measurement is a quantitative question.Is it possible for the measurement to synchronize with the pulsing?

author: Marco
author_rank: Super Contributor
timestamp: August 12, 2023, 03:54:56 pm
content: Quote from: luky315 on August 12, 2023, 02:04:08 pmYour first description sounded like a repetetive signal inside a frequency band.What's the capacitance of the current source?PS. transimpedance amplifiers are available for these speeds, but it's all dies. Bit hard to solder.

author: luky315
author_rank: Regular Contributor
timestamp: August 13, 2023, 08:13:57 am
content: We have approximately 2..3pF of parallel capacitance and a very high (GOhm) leakage resistance.The relative high parasitic capacitance i also limiting the use of a descreete TIA based for example on the 8GHz OPA855 or something similar.Unfortunately the pulses are not synchronized. So it is "simply" a question of detecting a current pulse with a minimal high time of 0.66ns. The "low" time can be anything from 0 to a few minutes.I looked for transimpedance amplifiers. There are a few new ones for LIDAR, but they are not fast or not sensitive enough. The old ones for fiber optic LAN (those where the days...) are obsolete or only available as DIE (I don't want that, it makes everything much more complicatet, expensive and I need onla a few working prototypes for the lab...) and even the ones that are available are on the limit here, it looks like I would need something a bit more sensitive then for example the ADN2880 which is specified for input signals > 5µA and is designed for PIN Diodes which have a lower capacitance (usually <1p)

author: dietert1
author_rank: Super Contributor
timestamp: August 13, 2023, 10:12:40 am
content: The maximum amplifier input impedance is 660 psec / 3 pF = 220 Ohm. Times 1 uA gives 220 uV.As Alex Nikitin wrote before, there is a noise limit from the input impedance of the amplifier. For 220 Ohm it should be about 53 uV. As you don't look at RMS noise but you want to detect events, your p2p lower limit is at least 3x more, about 160 uV. Maybe what you want isn't possible, as the amplifier also contributes noise.Probably the lower limit will be at 2 uA or the bandwidth needs to be lower. A TIA - with a short as input impedance - will be the most difficult solution.Regards, Dieter

author: Alex Nikitin
author_rank: Super Contributor
timestamp: August 13, 2023, 11:56:45 am
content: In theory 200 Ohm load +LMH6629 x10 voltage amplifiermight be almost OK. It will saturate at about 1mA and it will be at its limit at detecting 1uA 0.66ns pulse but at least it is easy to try.CheersAlexP.S. - If the input is a vacuum based Faraday plate, than an electron multiplier on the vacuum side might be of some help.

author: Marco
author_rank: Super Contributor
timestamp: August 13, 2023, 09:10:16 pm
content: How about a simple common base transimpedance amplifier with a CEL GaAs FET?

author: luky315
author_rank: Regular Contributor
timestamp: August 14, 2023, 09:01:12 am
content: I have never seen a FET based TIA for high frequencys, only for really high gains in the GOhm range, what would be in practice possible with such a topology?

author: Marco
author_rank: Super Contributor
timestamp: August 15, 2023, 02:59:13 pm
content: In ye olden times before everyone just did hybrid/asic, peoplebuild discrete multistage TIAs for 100s of MHz(this uses a FET for the first stage of a three inverter chain, with BJT for the rest). With the fancy SiGe NPNs available (for) now it's not unthinkable you could stretch that to a GHz..Saw a fast TIA in a solderable package. The Qorvo QPL7434. Meant for differential use, but it should work single ended.Is your current source capacitance parasitic or is it device capacitance? For a device you can gain a bit by bootstrapping the other end to reduce it's apparent capacitance like Phil Hobb's did for his photodiode amplifier.

author: luky315
author_rank: Regular Contributor
timestamp: August 15, 2023, 04:10:55 pm
content: I didn't know that Quorvo is making suitable stuff. Thanks! I will have a look, but the specification is a bit confusing for me, they are talking about dBmV/channel and I haven't found a clear input sensitivity or noise specification.We have a special / experimental APD (avalanche diode) with over 600V of reverse bias, so the usual photodiode circuits are not working here.

author: dietert1
author_rank: Super Contributor
timestamp: August 15, 2023, 05:44:03 pm
content: It eats 1 W! Noise spec is 2.8 pA/sqrt(Hz). This gives 77 nArms into 750 MHz at the input. So the 3 sigma detection threshold can be as low as 0.23 uA. Somehow i can't believe the 6 KOhm transimpedance at 1280 MHz.The application circuit in the datasheet is for RF, yet wideband, so it's certainly worth a try. Mouser offer the evaluation board "Qorvo QPL7434PCK-01".Regards, Dieter

author: Marco
author_rank: Super Contributor
timestamp: August 15, 2023, 06:17:04 pm
content: Quote from: dietert1 on August 15, 2023, 05:44:03 pmDidn't even notice that. Guess you should heatsink and fan it.

author: luky315
author_rank: Regular Contributor
timestamp: August 15, 2023, 07:51:55 pm
content: 1W is not worth a fan, I have an aluminium case anyways and a thermal conductive EMI absorber between PCB and housing.Now I have to understand why they are using inductors in the feedbach and input path. I have never seen that. And I cannot use AC coupling because the input signal is unipolar, this would create problems.Thanks again for pointing me at this device!