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OPA2111kp_replacement

Mehdi Noormohammadi
Prodigy 70 points
Other Parts Discussed in Thread: OPA211, LMP7721

Hi,

I am using an OPA2111KP as a transimpedance amplifier (TIA) with a very large resistor (1 giga-ohm) in  feedback for fluorescence signal detection applications.  I have attached the TIA schematic's picture. I am using a phototransistor as a detector, thus its voltage need to be set to 0.8V as shown in the schematic. I set the terminal three to 0.8V. Furthermore, I can not use double supply voltage (-/+V), only the positive supply voltage is allowed in the system. 

The circuit does not work, its output remains at 0V because feedback does not operate to regulate the terminal input voltage, and the terminal voltages (terminal 2 and 3) are in different voltage.

I am looking for a small footprint opamp to work with a single supply voltage in a TIA configuration, with a large resistor (about 1 giga-ohm) at a bandwidth of 5 KHz or more, at a mid-supply range output common mode voltage, input common mode voltage at around 0.8V, and low-noise high-precision. I would apprecaite it if you guide me how I can select such an opamp for the special application. 

Regards, 

Mehdi

  • 0
    TI__Guru 63135 points

    Hi Medhi,

    A 1GOHM feedback resistor is an extremely large value to use in practical op amp circuits.  The input bias current through the feedback resistor will create a large DC offset that will saturate the output stage.  In the case of the OPA211 the 60nA input bias current would create a +/-60V signal through a 1GOhm feedback resistor. 

    I suggest you limit the TIA gain to less than ~10MOhms and then use a post amplification stage to increase the signal amplitude further. 

  • 0 in reply to Collin Wells
    TI__Guru 63135 points
    I didn't mention it in the previous post but keep in mind that the OPA211 is not a rail-to-rail input device and the 0.8V common-mode voltage is too low in your circuit drawing. The minimum common-mode voltage would be 2V in your circuit (V- + 2V).
  • 0 in reply to Collin Wells
    Prodigy 70 points

    Thanks Collin,

    It was very helpful and I think I did not carefully make an attention into the device. However, if you look at its datasheet, there is a schematic which shows a 1Gohm resistor utility. 

    My input signal is a very weak (less than one pA) and need a very low input noise input TIA. To the best of my knowledge, the input referred current noise is inversely proportional to the feedback resistor value. Thus, I chose the large value resistor to achieve the target.  

    Could you please introduce me an opamp to work with input common mode  of 0.8V or less,  the large resistor with a bandwidth of about 2KHz or more, and only need a single supply voltage (+V)?

    Regards, Mehdi

  • 0 in reply to Mehdi Noormohammadi
    TI__Guru 63135 points

    Hi Medhi,

    I scanned the OPA211 datasheet but can't find the schematic showing the 1GOhm resistor.  Could you point it out to us?   Also, I think you're a little backwards as the input referred current noise is directly proportional to the feedback resistance (V = I*R).  These subjects are covered in detail in the TI Precision Labs videos on op amp noise.

    Consider using the LMP7721.  It's one of the lowest input bias current amplifiers available and my work in your application.  

  • 0 in reply to Collin Wells
    Prodigy 70 points

    You can find in page 10 of the datasheet in the following link:

    www.ti.com/.../opa2111.pdf

    Meanwhile, its schematic is attached.

    Thanks 

    Mehdi

  • 0 in reply to Collin Wells
    Prodigy 70 points
    I forgot to ask you if LMP7721 works with low input common mode voltage?
    Thanks Mehdi
  • 0 in reply to Mehdi Noormohammadi
    TI__Guru 63135 points
    Sorry, I misread your initial post and was looking at the OPA211 core, while you were asking about the OPA111 core (dual is OPA2111). Many of the same comments apply although the bias current of the OPA111 is only 2pA which would function in this circuit. However the OPA111 requires a minimum supply voltage of +/-5V and does not feature a rail-to-rail input stage.

    The LMP7721 has an input common-mode range from (V- - 0.3V) to 1.5V, so 0.8V is acceptable.
  • 0 in reply to Mehdi Noormohammadi
    Prodigy 70 points
    Thank Collin for you help,
    They were helpful and I will use LMP7721 and I think it would work for my case.
    Regards,
    Mehdi
  • 0 in reply to Mehdi Noormohammadi
    TI__Guru 54606 points
    Hello Mehdi,

    Also note that the minimum supply voltage for the OPA2111 is 10V (±5V). At +5V, you are running it at half it's MINIMUM supply voltage!

    The LMP7721 is a 1.8 to 5V device , and has a common mode range down to to V-, so it will happily function with a +5V supply and 0.8V common mode.

    Most CMOS devices have their lowest bias current around mid-supply. One of the LMP7721 features is that the bias current stays relatively flat down to V- due to the internal bootstraped ESD diodes, so the bias current will be lower than most devices at 0.8V CMV.

    Do note that the LMP7721 has a special pinout (for low leakage), so it will not simply drop in place of a standard single.

    Regards,
  • 0 in reply to Mehdi Noormohammadi
    Prodigy 70 points
    Hi Collin,

    The input bias current of OPA2111KP is 3pA and input common mode range is -/+11V. Hence, it seems that the OPAMP must not have a problem with 1Gohm resistor. I do not know why the OPAMP get saturated and does not work and I do not know if I develop a new board with LMP7721, it will work.

    For single supply voltage, as you said for an input common mode voltage of 0.8V, OPA2111KP would not work. I imagine that -/+ 11V mean that it works with 0.8V.

    Please give me an answer for those.
    Thanks Mehdi
  • 0 in reply to Mehdi Noormohammadi
    TI__Guru 63135 points

    Yes, we mentioned both of those points a few posts back:

    Many of the same comments apply although the bias current of the OPA111 is only 2pA which would function in this circuit. However the OPA111 requires a minimum supply voltage of +/-5V and does not feature a rail-to-rail input stage. 

    The +/-11V input common-mode range is only valid for a +/-15V supply voltage.  The minimum supply voltage for the OPA111 is +/-5V so with a 0-5V supply your system is trying to operate the OPA111 with less than 1/2 the required supply voltage as Paul pointed out.