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INA290: After filtering the output of the op amp, the sampling value is different from that before filtering

user5988908
Prodigy 40 points
Part Number: INA290

Hello,

    I have a question about INA290A4Q(200V/V).As shown in the figure,I add a RC filter(R=100ohm,C=1uF) for the output,and then to the DSP(280049).But the sample voltage between iL_sam and iL_samf is different.iL_sam is about 200mV,but iL_samf is about 500mV.The detected currrent is about 1A ,and the shunt resistor is 1mΩ .

Best Regards.

  • 0
    TI__Mastermind 28620 points

    Hello user,

    Can you increase the Rf and drop the Cf?  This can help with the output impedance driving the 1µF capacitor.  I would suggest Rf = 1k and Cf = 0.1uF.  This will improve the output load seen by the INA290.  

    Also what is you sample rate and internal components of you ADC?  Are you comparing the voltage measurement vs ADC output or what is measured at the IL_samf node?

    From your numbers is seems that IL_sam node is correct. So I would assume it is the total load (RC and ADC) or sampling rate of the ADC that is the issue.

  • 0 in reply to Javier Contreras4
    Prodigy 40 points

    okay,I will try Rf = 1k and Cf = 0.1uF. My sample rate is 200kHz,and the ADC output is equal to the IL_samf node.

  • 0 in reply to user5988908
    Guru 140200 points

    Hi,

    what charge kickback filter is the datasheet of ADC recommending?

    Kai

  • 0 in reply to kai klaas69
    Prodigy 40 points

    Hello,Kai,

           In 280049 Technical Reference Manual, there is an example that choosing  Rf = 180Ω and Cf = 150pF for the RC filter.Actually,I think the RC parameters are changeable which depends on your need,and then you should set the different acquisition window.

  • 0 in reply to Javier Contreras4
    Prodigy 40 points

    Hello Javier，

        Thanks for your answer.I have tried Rf = 1k and Cf = 0.1uF for the filter,but  IL_samf is more than 500mV.It seems to not work. Then,in the INA290 datasheet,the parameter called Maximum capacitive load is 1nF.Does it mean  the capacitor applied to the filter is too large?And this paramater's test conditions is no sustained oscillations and no isolation resistor.What's mean of the isolation resistor?Is the isolation resistor equal to the Rf?

        Looking forward to your reply,

        Leaso.

  • 0 in reply to user5988908
    TI__Mastermind 28620 points

    Leaso,

    The max capacitive load would be without a Rf(isolation) resistor.  You can increase the capacitance with increasing the Rf but this will also depend on the load you see.  The additional load seen on your diagram is the ADC.  I gave a recommendation that should work for the INA290 and not casue an oscillations at the INA290 based on load. 

    With no oscillations you could be loading it with either the ADC or something else. If you disconnect the ADC and measure IL_samf then you can for sure know the issue is driving the ADC.  You can also do this by making sure the ADC does not convert for this channel and measure the voltages.

    Did you verify the two nodes for oscillations with a scope?  Did you verify both nodes with a meter as well?  For the voltage to be that different there must be current flowing.  This could be into the ADC.

    Also your filter is 1.3kHz and your sampling is 200kHz.  You may need to change the capacitor based on the ADC requirement and switching.  We have some training on driving a SAR ADC but here there are links.  From this link you will find other driving ADC training

    Link to calculator in presentation you can use to drive an ADC

    Calculator

  • 0 in reply to user5988908
    Guru 140200 points

    Hi,

    do you need such a high sampling rate?

    Kai