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OPA548: OPA548

DRUILHE yvan
Intellectual 560 points

Part Number: OPA548
Other Parts Discussed in Thread: OPA187, , OPA827

Tool/software:

Hello,

Please find attached a diagram using a Howland current pump.
Input voltage +/-10V
Output current +/-12mA
Resistive and inductive load connected between Iout and Iin varying from 10R<R <970R and L=12mH r=130R
Current feedback via R580 and OPA187IDR
Output tolerance over the whole range +/-160uA
I don't have perfect linearity between 940R and 950R for -10V input the current tends to increase from -12.2mA to -12.3mA.
I can't explain this problem.

Is it a problem with the measuring amplifier OPA187?


Thank you for your help.

Howland_Pump.docx

  • 0
    TI__Genius 16146 points

    Hi Druilhe,

    Do you see this problem at any other resistances? Does this appear when trying positive current at the same 940R?

    Do you have a graph for error vs RL?

    When you say negative current in your design, we are talking about current flowing into the IOUT pin, correct?

    It might be helpful to monitor pin 6 of the OPA548 while doing the resistance test. Can you probe pin 6 of the OPA548 to measure DC value with a linear resistance and with R = 940R? This will help me compare output error with good value and with non working value. 

    Thanks,

    Jacob

  • 0 in reply to Jacob Nogaj
    Intellectual 560 points

    Hi,

    Do you see this problem at any other resistances? No

    Does this appear when trying positive current at the same 940R? No only in negative

    Do you have a graph for error vs RL? No

    When you say negative current in your design, we are talking about current flowing into the IOUT pin, correct? yes and return pin IIN

    At the moment I don't have the equipment to monitor pin 6 of the OPA548, so as soon as possible I'll do it.

    But basically the voltage should oscillate between 13V068 and 13V284.

    (130R+950)*12.1mA=13V068

    (130R+950)*12.3mA= 13V284

    Why do we have this instability of the assembly for this resistance value of 950R +/-10R ?

    Thanks

  • 0 in reply to DRUILHE yvan
    TI__Genius 16146 points

    Hi,

    When you are able to check pin 6 of OPA548 with a DMM/scope, I recommend also checking pin 6 and pin 3 of OPA827.

    We are currently violating the VCM spec when we are running at higher resistance values:

    OPA827 VCM spec:

    Violating this spec will likely cause the output of the feedback amplifier to go non linear, causing a change in the voltage setting of the OPA548 amp. This feedback delay may cause the OPA548 to have an apparent oscillation due to the time constant formed between OPA827 going non-linear to linear. 

    It may be necessary to select a different amplifier for this function, or ensure we are not violating the common mode input specification for this device. 

    Verifying with measured data may help us better understand where this problem is coming from. 

    An easy way to check may be to increase the supply rail of OPA827 slightly just to see if the problem is eliminated. 

    Thanks,

    Jacob