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LM7721 Transimpedance Configuration

Other Parts Discussed in Thread: LMP7721, TINA-TI

Hello,

     I am constructing a transimpedance amp for a Ion Chamber using the LMP7721 op-amp. I have simulated the design using TINA-TI software with successful outcomes. I am having a few problems building my design and have it functioning currently. The attached .zip TINA file is the design and I also didn't include the guard circuit in my construction.

     The problems I face are the following. There is some negative voltage feedback being applied to the inverting input to the Ion Chamber that I can't explain. No matter what the inputs values are the output will swing to the positive rail even when shorted together to ground.

     Any input on these problems will be greatly appreciated.

Thanks,

Lee

LMP7721-Transimpedance_Amp.TSC

  • Hello Lee,

    Debugging these types of applications is difficult by remote control.

    If you short the "input" (current source end of Rs), then you have created a non-inverting amplifier with a gain of -10Million. 10million x Vos may cause the output to rail. Shorting the input of a trans-impedance amp will cause the output to rail - that is expected.

    You need to break the circuit down to see where the error is occurring.

    How fast does the output swing to the rail? Is it linear or exponential?

    If you disconnect the chamber, does the output still rail? Does the ramp rate change?

    If you remove the feedback capacitor, does it still rail? What type of feedback capacitor are you using?

    What type of feedback resistor are you using? SMT?

    I assume you have thoroughly cleaned and dried the board (as described in the articles and the video). Are you sure you cleaned and flushed underneath the feedback components?

    Do you have a guard/ground surrounding the input stage? Did you "guard' the N/C pins, too?

    Is it possible for you to provide the layout and/or a picture of the circuit? These types of circuits rely more on the physical layout and interaction with the environment then the actual circuit.

    Are you using the 7721 eval board? I have done transimpedance amps with 1G SMT resistors on it and it works fine.

    Regards,

  • Hello Paul,

    Thanks for the response. I won't be able to answer some of your questions till 2 days from now. sorry for the wait. I will answer the ones I can.

    I never thought about measuring the output swing time. When I have measured the output it would swing as soon as I turn on the circuit, but I will take a look at it again to confirm the time.

    When I disconnect the chamber, the output still rails. I will mention if the rate changes in 2 days.

    I haven't tested what would happen if the feedback capacitor was removed. The type of capacitor is 1000pF glass.

    All components except for the Op-amp are through hole. Will this be a problem? I am currently using though hole for the prototype.

    I will take a look at the cleaning articles and video.

    I do not have a guard/ground surrounding the input stage or N/C pins. From the datasheet it sounds like this is key. I will add a guard/ground plane to the input pins. I will post my findings.

    I will provide a picture in 2 days, but it will surely change after I take the picture due to your input on the physical layout being critical.

    I am not using the LMP7721 eval board instead I am using the SMT LMP7721 on a proto-board.

    I will take another look on the layout. It sounds like that is where my problem lies in not having a guard/ground plane.

    Regards,

    Lee