LMT86-Q1: RC selection

John,

Sorry for the delay. I reached out to the engineer who designed the TI Designs to get more insight into what the factors were in determining the RC filter. Keep in mind that these TI Designs are from a few years back, so there is a limit to the detailed analysis I can provide.

1) I saw that here was another e2e post where a similar question was asked for TIDA-0074 here. So I will defer to that thread for an answer to this. 

In both TIDA-00901/TIDA-00281 designs the temp sensors were used to monitor the drive FETs for the off-board loads.  The thermal time constants of the FET stages and surrounding board would be in tens of milliseconds typically, so slow response (relatively) is fine.

Both designs also use the C2000 launchpad and there are various tradeoffs that can be associated with different filter caps (the C value you are referencing).

When looking at the sample time found in the microcontroller datasheet, it is important to note that the voltage on Cfilter and the Csample cap will depend on the ratio of two caps.  Recall that when the ADC samples, the input is expected to charge the sample capacitor.  The “instantaneous” voltage is just the new voltage created by the charge of two caps being added together.  For that reason, having a large Cfilt cap will ensure that the sample cap charges to nearly the Cfilter voltage.  If the Cfilter is closer to the same magnitude as the sample cap, the voltage will be lower. 

The reason I provided the additional information for the LMT86-Q1, is that this needs to be considered when choosing your RC filter. If your capacitive load, mostly indicative by your Cfilter (considering Cfilter will likely be magnitudes larger than Csample of the ADC), is larger than 1100 pF you will need to add a series resistor to insure the output of the LMT86-Q1 is stable. This I explained in my above post. Now, if this series resistance value is too high, the ADC may not be able fully charge the sample cap in between samples. This will result in a DC error. 

To answer specifically to the design considerations for the TI Designs, below is the feedback I received:

2) For TIDA-00901 that value would certainly be large enough that the sample and hold capacitance of the ADC would not affect the value.  I don’t recall if the lack of an RC filter was an oversight, or whether there was any tradeoff in the design considerations.

3) For the TIDA-00281, there actually is an RC filter specified.

49.9 Ohms and 2200 pF, giving a time constant of about 0.1 microsecond, which is ridiculously fast for the temperature measurements.  That would only knock down any high-frequency noise, so I would want to change those to much slower values if I were to re-visit the design.

Jalen