transimpedance amplifier

Hi,

I need help in selecting an opamp for transimpedance amplifier. I need a very low current noise density in the order of fA/sqrtHz.Single supply is prefered with a bandwidth greater than 12KHz and gain of 10^6.I have already seen AD8627.

Thanks in advance

Nisha Joshy

  • Nisha,

      Take a look at the newer ADA4610-2.

    Harry

  • Harry,

    Thanks for the reply but there is nothing in the data sheet which mentions about the current noise density and I am more concerned about that than the voltage noise density.Could you please help me with  any other suggestion.

    Regards,

    Nisha

  • Nisha,

    If you have a look at the parametric opamp chooser on my website at http://www.sevensoftltd.com/OpAmp.aspx you can filter on current noise density in the "noise I" column. Choosing <= 2fA leaves 483 devices, so you've got plenty of scope for more filtering. The TLC2652 comes to the top of the pile.

    If you enter <= 0 in the "Vin CM-" column you will filter to leave devices that work with the inputs at the negative supply, which is probably what you need for single supply operation.

    I don't know why you are bothered by gain as it doesn't make much difference in a transimpedance configuration (and I don't list it). A GBW of 12kHz minimum seems terribly low, and you may have stability problems caused by input capacitance. What value feedback resistor do you intend to use?

    PS I disagree with Harry, I don't think the ADA4610-2 is a good choice.

  • Nisha,

      Current noise is extremely difficult to measure.  The JFETs are usually 1-2 fA/rt-Hz or less.

    Harry

  • Hi, Nisha.

    Good day! I just want to follow-up on your query. Did you already find the amplifier that you need? There are other op amps available in ADI with fA/rtHz current noise density and has a bandwidth of greater than 12KHz that you can use as transimpedance amplifier. You can visit this link for the Op amp selection table http://www.analog.com/ps/psthandler.aspx?pstid=10347&la=en. In this link, you just have to add the current noise density parameter column under 'add/remove parameter' for you to see the current noise density values of each part.

    ADI also has Transimpedance amplifier products for fiber optic and photo detector applications but these parts have pA/rtHz range only of current noise density. You can take a look at this link http://www.analog.com/ps/psthandler.aspx?pstid=10348&la=en.

    As additional information on making a transimpedance amplifier, you have to consider the input capacitance of the amplifier and the value of the feedback capacitor (as compensation capacitor) in order to make your output more stable. As for the gain (when an op amp is used), the feedback resistor determines it that's why it's called transimpedance wherein the unit of the gain for this case is ohms (output = voltage; input = current). 

    Hope this helps.

    Regards,

    Anna