I have been testing an LM35 sensor and a common npn transistor.
The difference in normal conditions is that the pnp transistor shows approximately ~3-4oC degrees more than LM35.
When I heat them with a thermal source, I see that the common transistor shows a larger increase in temperature than the LM35. This difference can go about ~10oC.
So, while 3oC degrees difference is not that much, the question is how the common transistor will behave, when it is close to a thermal source (for example an active unit, generating heat when operating).
I guess this needs to be tested in actual conditions, to see the actual behavior.
Hi,
I feel you have seen with the above experimentation pretty much what is to be expected. On-chip temperature sensors are - from what I've read - still not a perfect method as even single packages have hot-spots, cooler parts, etc, and a pcb will be more noticable in this respect.
The LM35 is very accurate, however, there is a section in the datasheet offering advice on layout, as it also senses temp. through pins, and if heat-shrinked, you can get it to go up quite a few degrees by holding it by the pins, not just the plastic IC housing.
Related, somewhat, playing around with discrete bandgaps some time ago, either BJT or Brokaw op amp one - not worth the time for a serious application: one BJT had suffciently different temp. to one half a centimeter away - with no heat creating devices nearby... and blowing on the circuit is funny to see how hopeless such things are, adding a heat source deliberately makes it hysterical. All that said, potting might offer some solution to that issue.
Again, I don't believe the LM35 can be connected to the AMC6821, those pins for the temp. sensing device are a current source, the LM35 only uses a supply voltage and outputs a voltage, not a current, so a BJT would be needed.
I think, and I might be wrong, the best approach is to consider what temperature you wish to monitor, and place the BJT as close to that spot on the PCB as is physically possible, assuming you aren't measuring ambient air temperature.