Well, from what I understand about log amps, it is a device which calculates the log of an input signal's envelope. (Refer "here("")":http://www.analog.com/en/analog-dialogue/articles/logarithmic-amplifiers-explained.html for more info on log amps) With regards to the topology, a general low-pass filter is used in filtering this log amps. There are two topologies discussed on TI's pages. Kindly refer to them: https://focus.ti.com/lit/ml/sloa088/sloa088.pdf
If you are using it as a photodetector, then adding anything other than a photodiode to the input is not likely to be a good idea(consider leakage current, for example), filtering should probably be applied on the output. If you are wanting to use it with something other than a photodiode, then further explanation would be helpful.Cheers, richard
