OPA818: Differential input voltage limit

Hi Manuel,

Thanks for setting this thread up.

Just to clarify one comment you stated - the expected minimal signal level is ~ 3mV 0-Pk and the max signal level for linear amplification is ~ 350mV 0-Pk, the gain however is not set to 35 but 10 which is set by the gain setting resistors - 301 & 33.3 ohms. The overall gain that appears at the scope input (1M||16p) is only 5 because of the 50 ohm source and termination impedances.

The maximum input is 35V.

I actually think this is an error in the model. I questioned discrepancies between the OPA818 Tina model and datasheet previously and one of the responses stated, "The datasheet currently online is advance information and is subject to change." The response was in relation to OPA8181's Tina model stability compared to that in the datasheet. The Tina model was in the stability case more pessimistic, which was useful to me because I could design the amp to be stable in Tina and it would be better in the real world.

It appears that the same is for this case however the discrepancy is huge and would require more extreme limiting than that which I would prefer to use.

Making a model more pessimistic than the datasheet and hence in the real world is better than making it too optimistic.

However there is a point where the model is so far out that it becomes unrealistic and a real pain to achieve the desired result in the real world.

I would like to invite Sean Cashin to chime in here - we need some answers on this!

Best regards,

Peter

Hi Peter,

maybe my English is not good enough. But what is your actual question?

An input voltage of 35V is way too high for the OPA818 and will barbecue the OPAmp. Please don't violate the absolute maximum ratings, not even briefly! The given maximum currents are only allowed during protection events. You cannot expect standard behaviour when the internal ESD cells are turned-on.

Kai

Hi Hasan,

Thank you so much for your response and temporary fix!

I will go ahead and change the code and test the result.

Looking forward to any more refined adjustments to accurately reflect reality!

It wasn't until I ran the circuit at slower speeds with sinusoidal waveforms that the source of the problem became more evident

I must say it took a bit of detective work - i.e. testing with meters etc, to figure this out because I initially just assumed that the model was correct.

But I guess you can't assume anything. In fact, I do hope I can trust the rest of the model!

Best regards,

Peter

Hi Hasan, Kai,

Thank you for your help on this thread!


Peter,

Please feel free to ask new questions about regarding ISO7310C, OPA818, or other TI devices. We're glad you contacted us!


Respectfully,
Manuel Chavez

Hi Hasan,

I am having great difficulty trying to modify the code in line 268.

For the circuit - 'OPA818-2overload1', which has already been uploaded, I just right click on U1 then click on 'Enter Macro', change code at line 268 then save it and exit.

BUT when I reload the schematic and check the code it hasn't changed!!

What am I doing wrong?

Thanks.

Best regards,

Peter

Hi Peter,

after making the change, click the "Compile" button in the header. Then, click the "Close" button in the header and confirm the change with "Yes" in the popping up box.

Kai

Hi Kai,

Thanks for your help but it doesn't appear to work. I click on "Compile" button under "Analysis" and I get this:-

"Access violation at address 008B936B in module 'TINA.EXE'. Read of address 00004F9"

Hope there is a solution to this?

Best regards,

Peter

Hello Peter,

I haven't encountered this error before, are you able to use the OPA818 model in Kai's schematic?

Hasan Babiker

Hi Kai,

It Worked!!

Thanks so much!

BUT I don't want to have to rely on you every time I need to modify a model - I would like to do it myself.

How did you modify it and why did it not compile successfully?

Hasan indicated that he has never seen the error I encountered before.

Thanks for all your help.

Best regards,

Peter

Hi Hasan,

Yes it did work!!

BUT as I questioned Kai, it would be nice to figure out why TINA would not compile correctly. As indicated, I would like to be able to modify the code myself in future.

Thanks for your help.

Best regards,

Peter

Thanks Hasan! :-)

Hi Peter,

happy to hear that I could help you!

Well, these weird error messages happen from time to time in TINA-TI, unfortunately. Some people reported that a total fresh installation can help sometimes.

I run TINA-TI

Version 9.3.150.4 SF-TI

Kai