Visjnoe wrote on Jul 6th, 2010, 11:33pm:Dear Rob,
Thanks for your suggestions.
In your experience, are those effects typically well modeled, meaning
you can observe them in simulation?
Regards
Tz got it right imo... hard to see in simulation unless you go looking for them, but there are things you can do to test some things. Noise gets coupled in through parasitic caps, and the caps on the bipolar should be modeled. Run a
transient simulation with noise of varying frequencies on the supplies and substrate and see how much it takes to shift the voltage. I guess you could also change the clock frequency and see if it affects the measured results...
The base resistance
should be modeled, but bipolars aren't easy to model. Base resistance is also dependent on layout so if you didn't use exactly the same layout it can be different. You can get a feel for how close you are by increasing the current and seeing if it affects the curve. generally it is a problem if your bipolars are biased too high and will be worse at cold. You can also have problems if they are biased too low, but the problems kick in a higher temps and shouldn't cause differences at room temperature.
I forgot to mention the startup circuit if you use one. It might not be turning off.
There are so many things that can go wrong with PTAT circuits it is hard to guess without seeing the results and the circuit. They are circuits that teach you about what can go wrong. Are the results random around zero? Probably mismatch. Is opamp offset important? Opamp offset drift might be as much as 10uV/C, but it should have a mean of zero. Offset from the bipolars should be very small, but I don't know how big your PTAT voltage is to begin with. Any current density ratio less than 8 is an automatic do-over because the signal is very small
![Wink Wink](https://designers-guide.org/forum/Templates/Forum/default/wink.gif)
.
Package stress is a problem with plastic, but not ceramic. Suspect package stress if it is different each time you cycle the temperature.
Are there circuits on-board that change the on-board temp? Especially if they create a temperature gradient.
Is the nonlinearity worse at hot or cold? What do you mean by a "linear error trend?"
So many things... Those are just the obvious ones. If you put up your circuit we can see if there is an obvious gotcha.