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EMIRR of OPA378 vs LMV831

John Tillotson
Prodigy 50 points
Other Parts Discussed in Thread: OPA378, OPA347, OPA369, OPA334, OPA313, OPA376, OPA330, OPA333, OPA188, OPA180, OPA170, OPA171

Narional Semiconductor developed a method of measuring the EMI rejection ratio (EMIRR) of OpAmps in AN-1698. (This application note was authored by Gerrit de Wagt and Arie van Staveren in 2007.)

It would be useful for engineers to know what the EMIRR is of all opamps. 

The OPA378 has internal 25 mhz EMI filters. Has the OPA378 been tested to the methods in AN-1698? How can I find this data?

  • 0
    TI__Genius 12325 points

    Hi John,

    TI duplicated National's efforts in EMI-RR characterization a few years ago. Our characterization method is described in this application note: http://www.ti.com/lit/an/sboa128/sboa128.pdf. We're currently working to characterize all of our amplifier catalog and we post this supplemental information on our website in reports on the product pages if it does not already appear in the datasheet:

    Looking at the OPA378 product page, I don't believe a report has been generated yet, however I know that the part has been tested. I pulled the data from our internal server and plotted it below:

    Hopefully this helps!

  • 0 in reply to John Caldwell
    Prodigy 50 points

    Thanks John.

    For the EMIRR curve you supplied on the OPA378, I need to know what the input level was. (ie -10 or -20db)

    Additionally, if you can find an EMIRR curve for the OPA347, that is another device I am considering.

    Appreciate the help.

  • 0 in reply to John Tillotson
    TI__Genius 12325 points

    John,

    That curve was taken at -10dBm, I should have noted that in my original response. The OPA347 has not been tested yet but I will try to test it this week and update this thread when I have results.

  • 0 in reply to John Tillotson
    TI__Genius 12325 points

    John,

    I was able to measure the EMI-RR of the OPA347 this afternoon:

    I should note that the OPA347 is an unprotected device (no input EMI filter) and so its EMI rejection will be much lower than the OPA378. 

  • 0 in reply to John Caldwell
    Prodigy 50 points

    Thankyou very much for the super fast EMIRR test on the OPA347.   Are you aware of a selection guide for EMI protected opamps?

  • 0 in reply to John Tillotson
    TI__Genius 12325 points

    We don't have a selection guide for op amps with input EMI filters yet, however I can ask that a column be added to the op amp selection guide. I don't have a complete list of the SVA products (formerly National Semiconductor) which include EMI filters, but here is a list of the TI Precision Analog parts that include them:

    Low Voltage Op Amps:

    OPA313, OPA330, OPA333, OPA334/335, OPA369, OPA376/377, OPA378

    36V Op Amps:

    OPA170, OPA171, OPA180/2180/4180, OPA188/2188/4188

    I should point out that not all input EMI filters are created equal. Because this filter is actually inside the feedback loop of the op amp, the filter corner frequency is dependent upon the bandwidth of the amplifier. The corner frequency of the input filter must be well above the unity gain bandwidth of the amplifier in order to avoid stability issues. That means, that a 20MHz GBW amplifier with an input EMI filter will most likely exhibit worse EMI-RR that a 1MHz GBW amplifier. The 20MHz amplifier may have a input EMI filter with a corner frequency of above 60MHz, while the 1 MHz amplifier can have an input EMI filter near 3 MHz. There are also other limitations on the corner frequency such as input noise and die area. Finally, different amplifier types are more susceptible to EMI. Bipolar amplifiers tend to be the most sensitive and chopper amplifiers the least.