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THS3491: data on the GD2/HD3 validation

Part Number: THS3491
Other Parts Discussed in Thread: OPA2677

Hi Team,

Please advise on the following question.

I consider to use THS3491 in a system  which requires stringent  HD2 and HD3 performance.

The THS3491 datasheet insists HD2 and HD3: Less Than –75 dBc (50 MHz, VO =10 VPP, 100-Ω Load)

It is also stated HD2 -78dBc (typ)@f20MHz in the Electrical Characteristics table.


I want/need to know how much the HD2, HD3 validated depending on the lot validation.

Q1. Don’t you have any data on the HD2/HD3 validation?

Q2. If so, can you share the distribution profile (es: Gaussian or other curve) and its standard deviation?

Best Regards,


  • Hello Nishie-san,

      1)  We do not physically test for lot variation for distortion performance (HD2 and HD3). The variation we do test come from our DC specifications via ATE prior to unit distribution. AC specifications are difficult to physically test lot wise unless specified in the datasheet usually with open-loop gain or bandwidth. 

      2) I can look into if we have data on a sample size of THS3491 for distortion. However, this is unlikely, and in the original e2e thread the distribution profile (Gaussian) is based on design simulation and statistics rather than physical data. Therefore, you can expect around 99% of the device to be within +/-5 standard deviation from the listed typical in the datasheet. 

    This e2e thread has really good information on gaussian distribution of our specifications. The same explanation would follow our HD2/HD3 specification. 

    Thank you,



  • Sima, I do not believe a large sample size was used to get a range on HD for the THS3491. I have done that work in the past (Including screening drivers for distortion for HP AWGs)  and the HD is more like a Poisson distribution being a cancellation effect. Using Guassian on a HD spec predicts a much too conservative min level. It is also requisite that you do your statistics using uV/V kind of data instead of dB data. We did put some min AC specs into some of the Burr Brown high speed parts, see for instance the OPA2677 spec tables (This device was for xDSL line drivers where HD is also critical)  - this approach was abandoned with the TI buyout.

  • Hello Michael,

       It looks like that yep, we do not have a large sample size on HD.

       Thanks for the information! That is really good to know that there was data backing a specific distribution. For Burr Brown, how did they determine the min AC spec? Was this via generalized Poisson distribution based on your findings or was this via design simulation? 

    Thank you,

  • Well Simi, in development the PDK's (Process Design Kits) always have process and temp variation in the models. The OPA2677 (and others at the time) were simulated by design over process and temp in CBC10 to build that spec table. The same could be done for more recent products but it is very time consuming (expensive). 

    Working from uV/V type HD data, the HD2 shows a Poisson distribution - it is a cancellation effect up and down through the balanced op amp structure - sometimes you get perfect cancellation showing a very long tail going low in uV/V. The HD3 is really more Gaussian being curvature through zero crossing. 

    I have seen really tragic cases of test groups doing this in dB terms are grossly underselling their performance. 

  • Hi Sima-san, Michael-san,

    Thank you for your support.

    Q3.  I understand that the HD2 distribution follows Poisson distribution  and HD3 profile is Gaussian.

              Can you advise me how much performance degradation do you count on when less than 100 ppm failure rate is expected?

    Best Regards,


  • Hello Michael,

      Sounds like that would time consuming especially since it is over temperature. Thank you for the additional information! Since the spec range is not a guarantee, it does tend to undersell device performance through worst-case estimations. But, using dB terms seems to greatly affect this range. 

    Hello Nishie-san,

      For performance degradation, is this referring to end of life or operating close to the abs max rating of the device? If so, this video and blog on estimated changes in amplifier specification in the long-term (typical life test). The device's MTBF/FIT estimates, Qualification summary, and Ongoing reliability monitoring can be found under the ordering & quality tab of the device product folder. i.e. THS3491IRGTR. These cover overall device performance for various conditions. 

      Feel free to let us know if understanding above is correct and if clarification is needed from above links!

    Thank you,

  • Hi Sima-san,

    I am sorry that my explanation is not sufficient.

    My intended question is how the HD2 and HD3 characteristics vary from device to device.
    I thought that if we knew the distribution of the variation, we would be able to know the probability of including samples that did not meet the minimum HD2 and HD3 characteristics required by the customer.

    Best Regards,


  • Hi Team,

    Please allow me to supplement Nishie-san’s question.


    Each THS3491 shows different HD2/HD3 performance.

    Some THS3491 shows better than –75 dBc of HD2 performance and some shows poor performance than it.


    In the meantime, my system requires better than, says -70 dBc  of HD2 characteristics to achieve expected system goal.


    The performance validation may follow statistical theory.

    When you purchase million pieces of THS3491s, some of them do not meet this criterion.

    From costumer’s stand point of view, they are “failure”. (The poor performance devices may be through away)

    I want to know how much the HD2 and HD3 performance degradation you need to count on, to maintain less than

    100 ppm failure rate.


    I also want to know how much the HD2 and HD3 performance degradation depending on the supply voltage change (+/- 5%)

    and temperature change (-40 to 85 degrees).


  • Hi Mita,

    I'm sorry but your questions are way too far-reaching. The manufacturer cannot give absolute maximum and minimum specifications for every single parameter including all process variations, temperature ranges, supply voltages, loads, gains, a.s.o. This is just impossible because the number of specificaions would rise exponentially. The manufacturer has to restsrict himself to a set of most important parameters. Everything what is going beyond what the manufacturer is specifiyng in the datasheet has to be checked by the customer himself. This is part of the developping phase of any circuit.

    So, if you find that the OPAmp shows too high fluctuations in HD2 from chip to chip then you have to find a better OPAmp providing a much higher headroom for HD2.

    And be on your guard against statistics. That a parameter fluctuation is following a Gaussian or Poisson distribution is always a first order approximation, if at all. But real life is much more complex and you must always watch out for outliers. A statistical distribution allows any outlier at any rate of occurence. The only remedy against outliers is either to test every single board you sell or to choose an OPAmp offering as much as possible headroom for the parameter you are interested in.


  • All correct Kai, 

    Also, the actual nominal HD is very dependent on application configuration. The datasheet plots on HD are more indicative than definitive. This device has about the best HD for high output power and speeds of anything out there - I had worked on this as the next gen AWG output stage, so it has to be better than what was already being used. However, the plots are 100ohm load plots - if for instance the intended load were 500ohm, the HD will drop lower by quite a lot giving more margin. Somewhere I have a really good tech days slide deck covering all these issues. haven't found it yet, however, in the meantime, if the customer could provide their intended application schematic, some comments might be pertinent from that. 

    If in fact they were going to buy million pieces, it is always possible to implement a outgoing screen - quite possible, just how much can you afford. (one of my favorite design manager quotes was to never say no, but instead how much?)

    Over the decades of shipping out high speed amplifiers I have seen many AC tests implemented where the customer was willing to pay for it. I once set up an automated HD screen for the CLC142 going into HP AWG output stages, but that was a decades ago and they were willing to pay for it. 

  • It's soooooooo good to have you here, dear Michael Relaxed

    I enjoy every post you write and learn, learn and learn...


  • Thanks Kai, 

  • And, I did find that presentation from a 2004 dynamic range - 

    here you go, the inner workings of distortion, 


  • Kai-san, Michael-san,


    Thank you for your answer.

    I understand that it is not feasible to estimate the relationship between HD2/3 minimum performance and failure rate.

    I will try to find high speed AMP which should have adequate HD2/3 performance to meet system requests.



  • Hi Mita,

    an OPAmp with a much higher supply current is a good candidate for lower distortion. A higher supply current allows the class AB output stage (and other internal stages) to be furtherly driven into the class A region.

    But keep in mind that an OPAmp with a much higher supply current will hardly allow supply voltages up to 32V, because of the enormous heat dissipation. +/-15V times the 17mA of THS3491 is 510mW !

    To be honest, the THS3491 appears to be a very good compromise between maximum supply voltage, load current, bandwidth, distortion and supply current. It will be hard to find a better OPAmp.


  • And, it will be even harder to find someone to provide this kind of requested data - easy to ask for, quite difficult to extract.