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Problems with high speed photodiode amplifier - right amp, right circuit?

Timo Eberle
Prodigy 30 points
Other Parts Discussed in Thread: THS3201, OPA132, OPA140

Hello,

I try to build up a photodiode amplifier with a bandwidth up to 2MHz using a hamamatsu S1226 photodiode. The op amp I chose is a THS3201. The circuit has been designed with TINA (see attached file) and finally build on a simple platine with common parts. The circuit has been adopted from examples with similar tasks which I found in this forum.

However, testing the circuit by the use of ambient light and a laser pointer, the output voltage is not changing. Quite in contrary, the signal is oscillating with about 1MHz between the input voltages of +-5V which brings the supply current above 25mA which is far too much and the amplifier gets hot.

As I am a mechanical engineer, I'm not well versed in opto-electronics and need some help to find a solution. The questions are: Did I use the right amp, and the right circuit? Do I need special resistors and capasitors? The photodiode has to be used for light extinction measurements which means that the light power available will be much less than those of ambient light or a laser pointer. The circuit designed with TINA can handle this, but what's in reality?

I'm looking forward to your suggestions!

Timo

8156.Photo_amp_THS3201.TSC

  • 0
    Guru 21975 points

    Timo,

    The S1226 photodiode comes in several versions with widely differing capacitance. Are you sure you have the correct capacitance value? This is important in optimizing the circuit and avoiding oscillations.

    I recommend that you first determine the light level that you will be measuring. Connect a 50-ohm resistor across the photodiode and measure the voltage in the presence of a constant light level that you want to measure. This will help to determine the feedback resistor value required. This may also affect the best choice of an op amp.

    It's unlikely that the THS3201 is a good choice. The OPA132 or OPA140 may be much better suited to your circuit. The preliminary checks I have recommended will help determine this.

    Regards, Bruce

  • 0 in reply to Bruce Trump
    Prodigy 30 points

    Bruce,

    thank you for your reply. I use the S1226-18BQ with a terminal capacitance of 35pF (Vr = 0V, f = 10Hz). At the moment I can’t determine the light level exactly as I do not know the type of laser and the optical setup yet. But to make a rough estimate: not below 0.1uW. I know, it’s the wrong way to build up the detector before knowing the light level but I have no other choice.

    So do you think replacing the THS3201 with an OPA132 or OPA140 will help? Or do I need a new circuit? Could you give an example?

    And what are the resons for the satisfactory tests with TINA (frequency response, gain, ...) and the failure of the real circuit? Even using the amp without the photodiode (Vin(-) with a resistor to ground) produces these oscillations.

    Thanks!

    Timo

  • 0 in reply to Timo Eberle
    Guru 21975 points

    Timo,

    The THS3201 is a very wide bandwidth current feedback op amp that appears to be unnecessary for this application. These very high speed op amps can be tricky to use and prone to oscillate without very careful layout and circuit connections. Furthermore, this forum does not support this op amp and we are not the best experts on it. If you want to proceed with the THS3201, I will move your questions to the high speed op amp forum.

    It would be wise to use a slower, more docile amplifier unless absolutely necessary. Try the OPA132 with a 30pF feedback capacitor.

    Regards, Bruce.

  • 0
    Prodigy 30 points

    Probably the THS3201 is too fast and the decoupling is not adequate. You should use the same circuit but a different (slower) amplifier with a good decouplig.

    regards

    V. Biliotti

  • 0 in reply to Valdemaro Biliotti
    Prodigy 30 points

    Ok, thanks for your answers. I will try it with the OPA132. As I'm not in love with the THS3201, I will abandon it. I just used it, because it has been recommended for a similar application...

    regards

    Timo