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I want to drive a VCSEL with the MAX3795 driver, but this ship has a very complicated circuit on it pins.

Is there is another available driver with a simpler circuit that i can use ?

optical
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1 Answers1

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Driving a VCSEL (or other laser diode) can be done with no IC at all. The circuit can be as simple as a bias tee:

schematic

simulate this circuit – Schematic created using CircuitLab

What the Maxim chip does is reduce cost by eliminating the inductor from the circuit, and gives a way to adjust the bias and modulation currents programmatically (with external digitally adjustable resistors). It also provides feedback features that enable building a standard fiber optic transceiver module (such as an SFP module), and a shutdown pin that would be helpful for designing a product with eye safety requirements.

The Photon
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  • thank you, but i want to send a PRBS with the VCSEL through a tissue, receive it on the other side with a PIN photodiode and calculate the BER. So it wont be possible with this circuit i think. – optical Jul 05 '13 at 20:13
  • Replace V1 with a PRBS sequence generator and it should work fine. You may need to work out a better value for L1 (or just buy an off-the-shelf bias tee from a place like MiniCircuits). – The Photon Jul 05 '13 at 21:07
  • You can calculate BER with pseudo or a fixed waveform. If the signal to noise ratio is low it doesn't matter what you put through regards data. – Andy aka Jul 05 '13 at 21:09
  • Do you really need L1 if I1 is a half decent constant current generator? After all it's just about modulating the current through the diode (laser or VCSEL)? – Andy aka Jul 05 '13 at 21:12
  • @Andyaka, From the specs of the Maxim chip OP linked, I am assuming he is working in the 1 - 4 Gb/s range. you can get the current source within 1.5 cm of the laser, you could probably get away with no inductor. If you're building a PCB, that's doable, but if you're using a benchtop supply for the current source, probably not. – The Photon Jul 05 '13 at 21:20
  • @ThePhoton OK undertood... and, he doesn't need the chip if the VCSEL is lowish power and even if it's not he can use the inbuilt photodiode to control laser output safely if it's a test bench thing. – Andy aka Jul 05 '13 at 21:41
  • why the MAX 3795 has a limitation between 1Gbps and 4.25Gbps ? can we use it for lower bit rate ? – optical Jul 22 '13 at 17:10
  • @optical, at very low speeds, the modulation could start to interact with the power control circuit (if you use automatic power control), leading to unpredictable behavior. Other than that I don't see a physical reason for a lower limit in what Maxim published --- but Maxim won't have tested the performance at lower speeds, and isn't promising anything about its suitability for use at lower speeds. – The Photon Jul 22 '13 at 17:25
  • @ThePhoton ok i see, so it would be better not to use this driver if i want to send a much lower bitrate, like 100 Mbps ? – optical Jul 22 '13 at 19:13
  • It's probably not the best solution. But depending on what you're trying to do it might be a "good enough" solution. – The Photon Jul 22 '13 at 20:28