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AD8220 shift phase

Category: Hardware
Product Number: AD8220

Hello All,

I'm working on RLC meter. I use AD8220 at my project like current sense. I measure voltage drop at the shunt.

I have problem with phase shift in big value of gain from, more than 100.

I would like to measure current with bigger gain due to keep low value of shunt. High value of shunt will move the phase at measured impedance.

I did simulation at LTspice and looks like the same like real measurement.

Could you tell why AD8220 introduce phase shift at the higher value of gain, please?

Thank you in advance.

Best regards,

Mateusz



Simulation added
[edited by: MateuszPL at 3:47 PM (GMT -5) on 1 Feb 2023]
  • Hi Mateusz,

    Good day! Will you be able to share us your schematic to help you on this?

    Best regards,

    Franz

  • Hello Franz,

    thank you for reply.

    Below there is a part of schematic, shunt is selectable:

    Best regards,

    Mateusz

  • Hi Mateusz,

    I am trying to replicate your situation using this schematic that you shared. However, I am unable to replicate the phase shift that you are experiencing. Can you share your LTSpice circuit?

    Best regards,

    Franz

  • Hello Franz,
    below there is a schematic from LTspice:



    Phase shift is between output from AD8220 and V1. I can see that gain has influence on phase shift. I performed a few simulation with different Rg and phase shift follow by that.
    Did you simulate similar circuit? 
    Thank you in advance.

    Best regards,
    Mateusz

  • Hello Franz,
    I will show you this phase shift in detail.

    Schematic:


    Results with Rg=820, Gain=61,24


    Results with Rg=500, Gain=99,8


    Results with Rg=100, Gain=495


    Results with Rg=60, Gain=824,3


    Best regards,
    Mateusz

  • Hi Mateusz,

    I think the phase shift is in the current flowing through R5. Since you measure the voltage drop on R5, this is an output of a simple RC high pass filter formed by R5 and the capacitors connected to it, at least C5 via the small R8 (it is not on your schematic where else the right terminal of R5 is connected, but the MUX also adds its input capacitance). Using your data at gain of 100 it seems that on R5 you have a voltage amplitude of about 160uV. Taking the 400mV input amplitude and the high-pass filter transfer function one gets about 120pF load, so quite close to the value of C5. 

    If you plot V1 and the current of R5, I guess you'll see the phase shift, so I don't think it is due to the AD8220. See this simple simulation that illustrates the above:

    Zoltan

  • Hello Zoltan,

    there is normal than there is phase shift between current and voltage when we have reactance component, but I'm talking about different things. Based on above screenshots from LTspice you can observe that phase shift changed with gain of AD8220.
    Have you got any idea why phase shift with bigger gain of AD8220?
    Thank you in advance.
    Best regards,
    Mateusz

  • Hi Mateusz,

    I recommend you to plot the current of R5 and the output of the inamp simultaneously. Then you'll see more easily how much is the phase shift introduced by the inamp (I do not see much). Use the cursors in LTSpice to measure it accurately and share your results here. So, the timeshift numerical values for the gains you are interested in.

    Zoltan

  • Hello Zoltan,
    Ok, I can see, but come back to my screenshots.
    Could you describe me why there is phase shift change with gain change, please?

    Best regards,
    Mateusz

  • Hi,

    It is too hard to read what is the time difference, because it includes the large shift caused by the high-pass filter and the signals do not have the same DC value. Let's see, how much is the difference, then there is chance to find out the reason. I do not see significant phase shift on the plots other than what is due to the HPF.

    Zoltan