Electronic – How to construct high speed logarithmic amplifiers

high speedoperational-amplifier

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.

EDIT:

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.

EDIT:

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.

Best Answer

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.