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Photodiode TIA - Equivalent Input Noise - OPA847

Albert Nissimoff
Prodigy 20 points
Other Parts Discussed in Thread: OPA847, OPA656, OPA827

Dear Sirs,

I am developing a Transimpedance Amplifier based on the OPA847 for very small currents originating from a photodiode. Hence, I am now trying to fully model the effects of noise on this preamplifier.

According to the datasheet (OPA847), one could use Equation 3 (Page 11) to calculate the equivalent input noise. However, according to my calculations, I am unable to find the given equation exactly.

Could you please tell me:

1) Why is the term (4kT/Rf) squared? (For instance, OPA656's datasheet has this term directly inside the square root, as I think we should have)

2) Why did we divide the last term (En*2*pi*Cd*F) by 3? Could anyone please show me where this comes from?

Thank you in advance,

Albert

  • 0
    TI__Expert 4950 points

    Hello Albert,

    1) You are correct.  The term 4kT/Rf should not be squared.

    2) This term comes from the assumption that the noise of the transimpedance amplifier is bandlimited to a frequency F with F < f-3dB_trabsimpedance.  Under this assumption, the non-inverting input voltage noise term will be constant to a certain frequency then will increase with the gain. [eo = En (1+ sRfCs)].  Finding an average value of the square of this term between 0 and F will give you Eoavg = SQT(En^2 + (En^2 (2.PI*Cs)^2.F^2)/3).

     

  • 0
    Genius 5430 points

    Albert;

    "I am developing a Transimpedance Amplifier based on the OPA847 for very small currents originating from a photodiode...."

    Please look at the OPA847 input bias current spec; it is about 42uA maximum! This is going to make a VERY poor transimpedance amplifier for small photocurrents-- you are probably attracted by the OPA847s very low input voltage noise but what you need in your application is very low input current noise. I'd recommend looking at an OPA827, but I don't know the details of your application.

    Some of our older Technical Seminars covered this quite well but I don't know if those are available on-line. If not, that is a real shame for there is a wealth of information contained in those presentations-- not a sales pitch but real technical tutorials.

    Using TINA software, you can simulate the frequency response of your design and also simulate its noise-- the OPA827 and most "OPAxxx" op amps are well modeled and the noise simulations are very good.

    Regards, Neil P. Albaugh   ex-Burr-Brown

  • 0 in reply to Neil Albaugh
    Prodigy 20 points

    Hi,

    Xavier, thank you very much for your answer. I hope TI corrects the equation on a future revision of the datasheet.

    Neil, you are absolutely right when it comes to ultra high gain and low bandwidth designs. However, my preamplifier has to work at 50 MHz, so the maximum gain I can have with an OPA847 is 20kOhms (~86dB) for Cd~10pF. Also, as I am only interested in the AC component, input bias current is not a problem (42uA * 20 kOhms = 0,84V, far from saturation). 

    Moreover, in high speed designs with OpAmps, voltage noise is usually more important than current noise, as you can see in the examples given on page 11 of OPA847's datasheet.

    Thank you both for your replies,

    Albert

  • 0 in reply to Albert Nissimoff
    Genius 5430 points

    Albert;

    For a sensitive 50MHz transimpedance amplifier with a 10pF photodiode, you will need less than 20k transimpedance. Is there any way of reducing that capacitance? Can you use a lens to make your image size smaller? The noise current of the bipolar amplifier will be the main concern. Try this in TINA.

    If your detector had gain, such as a PMT or APD, then it would be far more sensitive, of course.

    Regards, Neil P. Albaugh   ex-Burr-Brown.

    39MHz TZA.TSC