TIA = transimpedence

I have lots of photodiodes for barrier detection, I tested one photodiode with one TIA op amp, and it was OK

1) Now I want to multiplex multiple photodiodes for my project, and I use 74HC4051 IC for that, I read in datasheet that between each of inputs to output 60 Ohm resistor, what's the effect of this resistor ?

2) I read this application note for TIA pcb design, and in this article author implied that the photodiode should be as close as possible to TIA because of the very low current of photodiode and TIA input bias current, It's very good for one photodiode, But when I use multiple photodiodes, I'm forced to place some photodiodes far away from TIA (40 photodiodes), what should I do for this in circuit design ?

  • \$\begingroup\$ I’d be concerned about the bias currents from each pin of the 4051. Have you considered their effect. \$\endgroup\$ – Andy aka Jul 28 '18 at 13:41
  • \$\begingroup\$ no , what should I do for 4051 bias current ? \$\endgroup\$ – mohammadmehdi golmeshki Jul 30 '18 at 8:42
  • \$\begingroup\$ Bias currents create an error signal that will vary with local temperature and might even produce a non-linear effect with your signal. If important, then consider a better device than the 4051. I'm sure there are better devices BTW. \$\endgroup\$ – Andy aka Jul 30 '18 at 8:57

1) In the case of a photodiode with very low current output, the effect of the 60ohms series resistance of the HC4051 multiplexer will be negligible.

2) A long distance between the photodiode and amplifier increases the capacitance of that node. That may make your amplifier unstable if it has no additional capacitance across its feedback resistor, add a few pFs here to make sure it's stable.

It also gives greater opportunity for pickup of stray signals on that very sensitive node, which you can avoid with shielded cable.


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