multiplex multiple photodiodes to one transimpedance amp

Hello

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 affect 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 ?

The 60 Ohms isn't a resistor, it is the effective resistance of the switch in the 'on' state. The resistance itself isn't an issue but in such a sensitive part of the circuit you really can't afford noise pickup. The best solution would be to put an amplifier at each photodiode then multiplex the amplifier outputs. Doing that ensures the signal levels are high enough at the mux that wiring and switching noise is small compared to the signal you want.

Brian.

Hi
thanks for reply

if I use a TIA for each photodiode then my project cost will be very expensive ...

I found one receiver board of IR frame touch panel , and circuit designer use just one TIA and seven analog multiplexers for 42 photodiodes !!

please help :-|

Hi,

you say up to 40 photodiodes.

I recommend to use one TIA for every 8 or 16 photodiodes.
There are cheap 8 channel muxes. And there are some 16 channel muxes.
I don´t think there are 40 channel muxes.

***
So what about 5 TIAs with an 8 channel MUX each.

***
The photodiode acts like a capacitor, so avoid that they charge when not selected ... or let the TIA settle for some time after you changed the channel.

Klaus

Hi
thanks for reply

your opinion is very good but my final project price is very important for me...


please say me your thought about this issue

I checked the pcb of receiver board of IR frame with multimeter in buzzer mode .... It has 42 photodiodes and 7 analog mux(8 channel) and one TIA .... if we assume designer select all photodiodes one by one , and then pass selected photodiode output to TIA input , designer need 6 analog mux (6*8 = 48 > 42) ....... and no 7 analog mux (one is extra)

I examined the traces , designer connect each output of 6 analog mux to each input of the last analog mux , and the last analog mux is very close to TIA


can you say me some suggestion about this ?

Hi,

you need very careful PCB design to get useful results.
All wiring from the photodiodes to the TIA is critical. Even the common connection of all photodiodes.
I assume you need good guarding and shieldin on the PCB and you need to have a shielding case for the overall circut.
Else it will catch more noise than real signal.

Klaus

Hi ,

yes ... I think I should design 4 layer pcb for guarding and shielding

thank you very much