This topic does not really fit this forum but if someone knows a good EE-forum let me know. Anyhow I think its an interesting question.
TI recommends for its TP235 temperature sensor to NOT avoid stubs. Any thoughts what the reason could be? I’m kind of curious as I have never seen this recommendation before.
BR Dom
Is this just a temperature sensor with an analog output?
For low frequency stuff recommended PCB guidelines are different from the (usual) high frequency digital stuff.
For examples, via’s apparently have a bit of inductance, which slows down high speed signals. For ADC’s and DAC’s you sometimes see inductors in the power rails to keep away unwanted noise from the power planes.
Also, stubs are a nuisance when the’re longer than about a quarter of the wavelength of the fastes frequency component in your signal (higher harmonics usually have lower energy, so no absolutes here). For an (almost) DC sensor, stubs therefore do not matter (in this context).
Stubs can pick up noise, as they work as an antenna, but that will be mostly shorted by the decoupling cap.
Temperature sensors also may have special requirements. You can for example use the PCB tracks to couple the temperature of the sensor to something else.
Yes its just an analog output. Your suggestion makes sense but shouldn’t it make any difference whether I place the capacitor before or after the stub, in this DC design?
The recommendation almost sounds like “when you do not place the stuff as shown in the picture, it wont work correctly” but maybe I’m just imagining this. I’m pretty sure in practice it wouldn’t make much difference at least when one does not require the highest possible resolution or something like that.
I have not looked at the datasheet, but my first guess would be that the person who wrote, wrote it in the same style as he was used to write other stuff. Just habit. But this is just a guess.
The layout is not for this part. Look at the package pin out.it won’t work in that layout.
I agree with the suggestion you place the capacitor as close as possibility the the power input.
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