4

I'm relatively new to electronics. I've seen a few circuits online, I just wanted to check that this schematic I drawn would work.

enter image description here

The purpose of the photodiode will hopefully be to read a modulated LED signal. Is the choice of Op-Amp suitable to the job?

JRE
  • 67,678
  • 8
  • 104
  • 179
DaveS
  • 43
  • 4

2 Answers2

6

If this is true: -

DMX is likely 250kbit/s (courtesy of @rdtsc)

Then the op-amp is wholly unsuited for that data rate: -

enter image description here

250 kbits per second requires a bandwidth of 250 kHz in order to get any reasonable clarity for the bit edges and, given that the op-amp proposed has a unity gain bandwidth of 14 kHz, it is highly unlikely to be suitable.

Additionally, if the data rate is 250 kbits per second then the RC feedback network is also a showstopper as it has a 3 dB frequency of 1600 Hz.

However, if it comes to pass that the data rate is below 1.6 kbits per second then both op-amp and feedback network should be OK.

Andy aka
  • 434,556
  • 28
  • 351
  • 777
  • @ Andyaka , Thanks for the information . I've just looked and found that it is 250 kbits. I need to reevaluate the components or would you suggest using the DMX is not possible? – DaveS Apr 08 '20 at 14:45
  • 2
    The BPW34 is well-suited but not the op-amp. You need to be looking for an op-amp with a gain-bandwidth-product of around 10 MHz and, your feedback resistor needs to be significantly lower such as 22 kohm in parallel with 10 to 22 pF. If you must run from 5 volts and 0 volts then look for an op-amp that has rail-to-rail inputs and outputs. – Andy aka Apr 08 '20 at 14:54
  • I've been looking on RS components and found this op-amp MCP6291- https://docs.rs-online.com/972d/0900766b813813ff.pdf. – DaveS Apr 08 '20 at 15:48
  • Yes, that looks more suitable and fig 4.9 shows it being used as a PD amplifier. – Andy aka Apr 08 '20 at 15:53
  • I did notice that the supply voltage 2.4 - 6v ? I couldn't seem to find one with a 0 - 5v , unless I went with a dual supply voltage instead – DaveS Apr 08 '20 at 15:55
  • The supply volts is any voltage between 2.4 volts and 6 volts. – Andy aka Apr 08 '20 at 16:01
  • Thanks , just thought I would make sure with that before I buy the components. Thanks with all the help for this Andy !! – DaveS Apr 08 '20 at 16:09
  • I noticed that the photodiode I have is a BPW21 not 34 . Will the BPW21 still do the job ? – DaveS Apr 08 '20 at 20:17
  • Please link to a data sheet. – Andy aka Apr 08 '20 at 22:22
  • https://www.vishay.com/docs/81519/bpw21r.pdf – DaveS Apr 08 '20 at 22:27
  • Rise time and fall time are both circa 3 micro seconds so that might be a problem. Also, not knowing the main spectral components in the LED make it hard to guess. That's why I asked what the LED was under your question. – Andy aka Apr 09 '20 at 08:20
  • I thought instead of opening a new question about this circuit . I would post it back through here. I recently got the components through and set it all up , however my issue now is the distance that the photodiode can pick up the light has decreased - would it be worth looking into a two stage amplification ? . The light I am using is a DJ DMX light 10W – DaveS Apr 26 '20 at 20:52
  • Start a new question and leave a link. – Andy aka Apr 26 '20 at 23:13
0

The Analog Devices transimpedance amplifier design wizard may be very helpful here:

https://tools.analog.com/en/photodiode/

Note that it has your photodiode built in as one of the examples so it is very simple to try out.

user1850479
  • 14,842
  • 1
  • 21
  • 43