A-D conversion of 40 current signals

Hello,

I’m designing a circuit to digitalize 80 analog signal using microcontrollers (in particular, PICs). The analog signal are relative low level current signals, from about 10nA to 10uA. As the dynamic range of the signal is very wide, I was thinking in using a logarithmic amplifier (like the LOG112) and then use an ADC to read the voltage output the amplifier. After that, I can linearize the logarithmic output inside the microcontroller.

In order to reduce cost I’m gonna use only one ADC and 5 analog multiplexers multiplex (like the DG508A) . My initial idea was to use 40 LOG112 and the mux the 40 voltage signal into the ADC, because I have seen some similar circuits in this way (not logarithmic though). However, my second idea is to put the multiplexer before the LOG112, at the input of the current signal. In this way I was only one LOG112, one ADC and I’d mux the 40 current signals into the LOG112.
Does anybody know or think this solution could have some problems or disadvantages respect to use 40 LOG112? Or does anybody know a better solution for this problem?

Thank you very much!

Leakage current and measurement speed according to log amp setup time at low currents may be a problem. Depending on the nature of the current signals, the inactive inputs possibly need to be shorted, so SPDT switches would be needed at the input.

one possible problem i can think of is losses in the multiplexer ..
your signal is very low .. and multiplexer too may induce enough noise into your signal