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current drift

GRANGIER jean-marie
Prodigy 30 points
Other Parts Discussed in Thread: OPA4277, TLC274

Hi Every body,

 

I have  a design to measure current about 100uA and to get mean value of it , and then to keep maximum value of mean value. I need to reduce offset , so i'm using OPA4277.

Issue i  have is a quick drift on measured value ( 2s after ), which is not acceptable. Please, see following schematic.  I think problem is coming from capacitor C2 just after Diode D1. Thank's to help me . Jean-Marie

 

  • 0
    Genius 5430 points

    Jean-Marie;

    The schematic is too small to read the text but I suspect that there are two classic sample-hold amplifier problems that may be in play here. First, the input bias current of the op amp follower connected to the hold capacitor causes voltage droop. This is especially severe with small hold capacitors. The second effect that could be a problem is the type of dielectric in the hold capacitor. Dielectric absorption is high in most capacitor types; the best are polystyrene and NPO/COG ceramic. Silver mica is pretty good too, but they are hard to find these days. The same concerns apply to a peak detector.

  • 0 in reply to Neil Albaugh
    Prodigy 30 points

    High Neil ,

    Thank you very much for your answer.  Would you like to continue helping me ?   you can see schematic in a attached file. I hope it will readable for you. I want to describe more my Issue :

    First, i apply  a constant voltage on the input of the board , so ICC through R2//R4 is constant , about 100uA. 

    Second, i Check  voltage on the output of the last Amplifier U1C. Voltage is OK corresponding to the Gain (2000)  of  the system ,and  no offset . But voltage just after 1 second i put voltage on input is going up quicly to the maximum value near  VCC.

    This behaviour is not acceptable right now.

    I think capacitor i'm using should be ok regarding it's dielectric.

    Thank's again for your help.

    jean-Marie

  • 0 in reply to GRANGIER jean-marie
    Genius 5430 points

    Bonjour, Jean-Marie;

    Thank you for the larger schematic .jpg file.

    I think that your problem is your power supply-- the OPA4277 is a very good amplifier but it is not a single-supply op amp. You are supplying it with Vcc to the op amp positive supply pin and ground to the negative supply pin. The data sheet shows the OPA4277 common mode voltage range as +Vcc-2V to -Vcc +2V. You are outside of this range. If you replace the OPA4277 with a good quad single supply op amp or apply a negative power supply of -2V or more to the OPA4277 it should work.

  • 0 in reply to Neil Albaugh
    Prodigy 30 points

    Hi, Neil,

    I'm sorry , schematic is wrong on the following point :  I change Supply of  negative pin to -5V . Positive pin is set to +5V . I use TC7660S to make -5V from +5V. 

    Before i Use OPA4277, i Use TLC274 which is single supply , but Offset was more important , but no drift was observed. What do you think ?

    Is it better to use TLC274 with a device to reduce offset  or continue to use OPA4277 ?

    Thank's again for your help.

    Jean-Marie