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OPA227 RFI Rectification

Vlad Sergeyev
Prodigy 120 points
Other Parts Discussed in Thread: OPA227, OPA27, OP27, OPA2227, OPA2228, OPA2140, OPA2376, OPA141, OPA376

Hello,

I have a legacy design based on OPA27. The circuitry is prone to RFI rectification problem in high MHz and GHz band. I am planning to replace OPA27 with recommended OPA227. Is it also susceptible to RFI? Is it CMOS or BJT based? Please advise.

Thanks,

Vlad 

  • 0
    TI__Mastermind 44640 points

    Hi Vlad,

    I will get back to you regarding the RFI susecptibility of the OPA227. I can run a conducted RFI test on an OPA227 up to about 6 GHz to see how much rectifcation occurs.

    The OPA227 is a BJT based op-amp.

    Regards,
    Chris 

  • 0 in reply to Christopher Hall
    Prodigy 120 points

    Hi Chris,

    Thank you for the reply. I am looking forward to getting results of the test.

    Could you recommend other compatible opamp based on CMOS or JFET technology?

    Regards,

    Vlad

  • 0 in reply to Vlad Sergeyev
    TI__Mastermind 44640 points

    Vlad,

     

    Below are the graphs showing the results of my conducted RFI test. A 200mVPeak RF sine wave is applied to the OPA227 input while in a buffer configuration. The first graph shows the resulting change in the op-amp DC output voltage due to rectification. The second graph is the EMI rejection ratio (EMIRR), which (like CMRR or PSRR) is the logarithmic ratio of the input amplitude to the output change. Therefore, for an EMIRR of 80 dB, the op-amp DC rectification is 1/10,000th of the peak RFI signal amplitude.

     

    The RFI signal was only applied to the non-inverting input terminal for this test. The op-amp could rectify signals received by other terminals; however, you can expect the amount of rectification due to RF applied to other terminals to be less than or equal to that of the non-inverting input. The op-amp inputs are known to be the most sensitive to RFI rectification and both inputs should be the same since their layouts are typically symmetrical.

    When it comes to recommending a replacement op-amp, what are the critical parameters or operational needs for your circuit design?

    Best regards,

    Chris

  • 0 in reply to Christopher Hall
    Prodigy 120 points

    Hi Chris,

    Thank you for the valuable data.

    The critical parameters for  the replacement are:

    • Low noise, compatible with OP27
    • Low offset, compatible with OP27
    • Low susceptibility to RFI rectification
    • Availability of 2 op-amps in one pack is preferable

    Bandwidth is irrelevant, since the input signal of interest <7kHz

    Regards,

    Vlad

  • 0 in reply to Vlad Sergeyev
    TI__Mastermind 44640 points

    Vlad,

    Thanks for that information! In addition to the OPA2227 (the dual of the OPA227), I would look at:

    • OPA2228 (Bipolar)
    • OPA2140 (JFET)
    • OPA2376 (CMOS)

    Regarding EMI rectification, I have found JFET and CMOS devices to perform better in general. I have done the same measurements for the OPA376 and OPA141 (which is similar to the 140) and the EMIRR ranges from around 40 dB in the 10's of MHz frequency up to 100 dB in the GHz frequency range for both devices.

    I hope that information is helpful. Let me know if I can assist you in any other way,
    Chris

  • 0 in reply to Christopher Hall
    Prodigy 120 points

    Chris,

    Thanks for recommended replacements. I'll try JFET.

    Regards,

    Vlad