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LMT86-Q1: Output RC-Filter.

Part Number: LMT86-Q1
Other Parts Discussed in Thread: LMT86

Dear TI Support team,

I have a question regarding the RC sensor output filter selection.

In the datasheet, there is the Table 4 with the recommended series resistors values.

Could you please check it and explain what is the reason of choice of such values.

The table seems not logical for me: for the higher values of the output capacitance - the lower value of the series resistor is recommended to use!

For me, it should be vice versa: to allow output amplifier stage to drive the higher output capacitance, the higher resistance value should be used - "stronger isolation" between the output an output capacitance is needed.

Isn't it the error in the datasheet?

Thanks!

Best,

Igor

  • Hi Igor,

    Looking into your question, please expect a response soon.

    Jalen

  • HI Igor,

    These resistor values were determined to stabilize the output of the device with the respective capacitive loads in the table.  The resistor values are minimum values so higher resistor values should be ok to use.    

    Thanks,

    David

  • Hello David,

    thanks for the answer!

    Yes, I got the reason why the resistor is needed at the output.

    But still, after your answer I miss the logic of values choice. Could I kindly ask you to check again my initial question.

    What I can't get: why the higher minimal series resistor value is needed in order to drive lower capacitance? And not vice versa?

    So, according to datasheet, e.g:

    - there is no need need at series output resistor (Rs = 0 Ohm) to drive output capacitance of 1.1 nF;

    - the Rs = 3 kOhm series output resistor is needed to drive the 10 nF output capacitance;

    - the Rs = 0.8 kOhm series output resistor is needed to drive the 1 uF output capacitance.

    These example statements above are according to datasheet, however it's seems for me not correct. 

    Do I miss some thoughts behind or it's an error in datasheet?

    Happy New Year and looking forward to your answer!

    Best,

    Igor

  • Igor - 

    all the values will depend on ADC you are using and the filtering installed there - in this case, the part cannot drive a lot of current, so the series resistor value is the one that matters for the sensor, as it will limit the current draw on the LMT86, I think. As I said, the rest of it, really depends on what is being used for ADC. You might want to consult you MCU or ADC users guide or technical reference manual for what is recommended to do here for input filtering, if anything. 

     

  • Dear Josh,

    thanks for your quick answer! 

    All info you mentioned it's really relevant and valuable! I got, that filter values should also depend (probably mainly depend) from the uC, ADC and sample rate.

    But it's not the question I was concerned about :) My question was purely about the suggested series resistors with load capacitance stated in the datasheet. I miss the logic (as I mentioned before) why RC combinations specified like this - for me it's seems swapped...

    I'll be glad if you could put some light on it. Thanks!

    Best,

    Igor

  • Igor - 

    the table provides a range of values to cover cutoff frequencies (this is a low pass filter) from 199Hz to 48kHz. if the values in the table would swap, it would have cutoff  range from 180kHz (1.1nF & 800 Ohms) to 53Hz (1uF and 3k Ohms). I think the authors of this datasheet had it in mind to give a range of combinations that provided a cutoff frequency starting point for folks who did actually need some filtering in their circuit - you can see in Figure 13 on page 13 there is not a series resistor shown - if you don't need it, then you should be all set. 

  • Hi Josh,

    ok, got it. From cutoff frequency perspective it totally makes sense.

    Thank you for the great and quick support and have a great weekends!

    Best,

    Igor

  • Thanks! You have a great weekend, too!