Objective :

Converting laser pulses from photo diode ranging from 10nA - 100mA to digital format, for measuring pulse width exactly.

Pulse width 10ns-150ns and repeating at a rate of >20us up to 1second.

Observations :

I have seen pretty encouraging application notes like this for a wide dynamic range TIA requirements, but my doubt is how fast they can be? The normal diodes can't be very fast as 2ns (considering my photo diode rise time,) how about a GHz BJT as feedback element or a schottky diode in feedback? ( I did not find people trying such.) I have found a few integrated designs like LOG112 but I could not find high frequency log amplifier modules.

More than bandwidth, it is the pulse response of such log amplifiers, for example MAX4206 matches all requirements but the response time is very poor, my pulse being 10ns the rise time of photo diode is 2ns.


As suggested I have also seen very high bandwidth log amps like SDLVA which have ns response time but oriented for different applications and are very much advanced than my requirements and too costly, surprisingly achieving only 5ns response time!

Problem Statement :

Is it possible to sense ns pulses using log amplifiers? If so how? I want to construct from off the shelf components only.


Here I have one flexibility. I have two channels - that is two diodes, so I can break this 10nA-100mA range in to two, one being 70nA-70uA from one channel and 70uA-70mA in another channel, so even if the log amp covers this range it would be satisfactory, I understand how difficult its to sense 10nA even if 100nA is sensed that is a good job.

  • \$\begingroup\$ Yes, but could you please separate your observations and parameters from actual questions. You have them jumbled into a wall of text that is difficult to understand. \$\endgroup\$
    – user105652
    May 16, 2016 at 3:35
  • \$\begingroup\$ The LOG112 is only good to 10KHZ even if it has 5 decades of dynamic range. If you find such an IC it is likely to be expensive. You could try GHZ op-amps wired as a log amplifier. 10nA and 10nS are pushing the limits of off-the-shelf products. \$\endgroup\$
    – user105652
    May 16, 2016 at 3:45
  • 3
    \$\begingroup\$ If you want real blazing logging performance check out SDLVAs as the first result from a Famous Search Engine states "Analog Devices successive detection log video amplifiers (SDLVAs) include an operating range of 0.1 GHz to 20 GHz", log video amplifiers give you the amplitude of a signal (irrespective of frequency - well, to some degree anyway) and can have output rise times of 5nS or less, ranges of 50+ dB is pretty common too. \$\endgroup\$
    – Sam
    May 16, 2016 at 11:03
  • \$\begingroup\$ @Tom i have already seen them, i dont require that high end one, my requirement is 5-100Mhz, even if go, i would be wasting somany bucks, i require 19 such :p, each is > 1000$, i will look if any low range are there, thanks \$\endgroup\$
    – kakeh
    May 16, 2016 at 13:06
  • 1
    \$\begingroup\$ Ok, so you don't have just a comparator after the amp, you also have some other device for the power measurement part (ADC, or whatever) \$\endgroup\$
    – dim
    Jun 7, 2016 at 7:46

1 Answer 1


Another way to look at it is if you can get the high speed and low sensitivity and IF the device doesn't saturate or slow down if you load it heavily then you need not have to have the log aspect of it.

Look at the Analog devices ADN2880 which is a SiGe based 2.5 GHz Integrated TIA.

And the HMC6590 (which might be too costly for you) also is blazing fast (faster in fact) and can saturate.

  • \$\begingroup\$ yeah i have also thought of this kind of solution, ADN2880 kind of solution as stage one and , i want the log amplifier to be followed by a TIA which will be very useful to cover the dynamic range if i fix my limits to 1uA-4mA lets say(as ADN says 300nA is noise itself ), i may not digitalize properly the lower currents, if suppose the TIA like ADN2880 is followed by a log amp that will even stop the second stage amp to go in to saturation even, when i try to amplify 4.4mV(4400V/A is ADN) by 10V/V and results 40mV which helps me to put a ref of 25mV atleast, 4.4V should not override the stage2 \$\endgroup\$
    – kakeh
    Jun 11, 2016 at 3:47
  • \$\begingroup\$ @kakeh You can't follow the log amp with a TIA, you are getting the speed out of the photodiode by the fact that you are operating it in that fashion. \$\endgroup\$ Jun 11, 2016 at 3:52
  • \$\begingroup\$ i did not understand, do you mean a slow log amp will under perform and distort if its followed by TIA ?? if that is so, its true and i want the log amp to be fast enough to be followed by a TIA, correct me if my perception is wrong \$\endgroup\$
    – kakeh
    Jun 11, 2016 at 3:59
  • \$\begingroup\$ i also did not get why the filter is used un-necessarily, it seems like a LPF, but i need a HPF of 4MHz, as mine being a pulse waveform the filter would bring distortion i feel \$\endgroup\$
    – kakeh
    Jun 14, 2016 at 11:26
  • \$\begingroup\$ @kakeh Your Photodiode will be biased at a constant voltage and will generate current. If you put the log amplifier first then it should be a current mode amplifier. The TIA converts from current to Voltage through the capacitor in the feedback loop. It is a tricky circuit to get to run fast. \$\endgroup\$ Jun 14, 2016 at 15:49

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