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OPA830: Differential Input voltage absolute limit

Part Number: OPA830
Other Parts Discussed in Thread: OPA837, LMH6609, OPA838

I am in a situation where I need to replace an obsolete op amp with a new op amp. I intended to use the OPA830 as this looks to be the best op amp as a replacement however during board test this op amp may see a differential voltage of +5V which is greater than the maximum stated in the datasheet. If the current is limited at the input will the op amp be protected from damage? If so is <30mA protection enough?

  • Hello Tim,

    Normally these parts have back to back diodes across their inputs to avoid Vebo breakdowns in the input stage transistors. Not clearly shown in this case but that is the question you are asking. The 30mA number comes from the ESD steering diodes on each pin related to inputs going beyond the supplies - not the same thing it would seem that you are asking.

    The TINA model might show clamping to input diff voltage overdrive? That would be a good clue to if they are there, but then the max current is still a question.
  • Hello Tim, I did run a model test for input clamping - not in the model, but that is not conclusive.

    Just run transient here and the 100Hz +/-2V input to the open loop model does not clamp lower as input clamping diodes would show.

    OPA830 input stage diode test.TSC

  • Thanks for the prompt reply Michael. I have run some tests in the lab and the voltage is not clamped therefore I am leaning towards the conclusion that the voltage limits are due to low internal breakdown voltages and cannot be safely exceeded whether the current is limited or not.
  • That seems like a design miss then if the inputs are not protected - I know more recent parts like the OPA837 definitely have protection. Yes, you definitely do not want breakdown the input transistors - that normally causes irreversible increase in low frequency noise (higher 1/f corners).

    While I worked on the OPA830 release and it is a great part, maybe there is another with protection - what obsolete device are you trying to drop into (I assume drop in).
  • It is a replacement for the EL5131 which went obsolete some time ago but we had a large last time buy. You may remember I spoke to you about this particular op amp back it 2011 and 2013 when you were at Intersil, small world. The position is not critical in any way and I have identified some more alternatives eg LMH6657MF/NOPB which I think will be suitable. I will test this op amp.
  • Sorry I meant the LMH6609
  • Yes Tim, that EL5131 was a really nice part - tried hard at the time to keep it alive but the management at the time was not that concerned about continuity of supply for their legacy products. This was essentially a supply chain issue where the cost of migrating fabs did not cover the forecasted revenue. Did a lot of work to remodel it and put it into the online tools I was building at the time. Recognizing the ephemeral nature of the portfolio, I had set the part selection up with a simple flag to check if the part was still available (or not) - switching that off removed it from the tools as time went on - I have noticed more parts dropping out of the original tables on those tools.

    Let me build some parametric comparisons and make sure which parameters are must have and which are flexible.
    I did find a datasheet, the EL5131 is a 5pin SOT23 decompensated device offering 1.8nV input noise with up to 13V total supplies.

    Before I start sorting, what supplies are you using and is that low noise and high output drive critical - if you had a schematic that would help a lot. (higher gain in the app., or transimpedance perhaps).
  • Still waiting for more details Tim, but if you application can handle a 5V solution, the latest and greatest in low power, low noise decomp is this OPA838 we brought out a couple of years ago.
  • Hey Tim,

    Was the LMH6609 sufficient for you or are you still looking for a replacement part?

    Thanks!
    -Karan
  • Incidentally, the LMH6609 is what we call a high transconductance input stage to get unity gain VFA with CFA type slew rate. That does not look like a good cross to the EL5131 noise wise. I tried to describe the different ways we push slew rate up in VFA - and decomp is one of them, in this article.

    www.planetanalog.com/author.asp
  • Hi Michael, for the location in the circuit where I intend to replace the EL5131 with the LMH6609 the noise performance is not critical as the signal has been sufficiently amplified. I have tested the LMH6609 in circuit and it does not impact the performance therefore we will go with this op amp. Thanks for your help.