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LDC1101: Setting the Time Constants in the LDC1101

Bilal Akra
Prodigy 10 points
Part Number: LDC1101

'Hello , 

i am getting some problems using the LDC1101. i set up my sensor as Following 0.75 to 6 Kohm tying to measure coathicknes on (Fe, Alu and copper etc.. ) where the smallest Rp ist equale to 1.5 Kohm and the lagerest one on copper ist round about 3.5 Kohm. 

 and  what i saw ist the values in the air of Rp max are constant but not a max value. and the Vlaues on Aluminium are constant as Well . 

 what you see in the picture below is the measurement done while holding the probe in the air . that mean no contakt to any area. 

Whenever i  change the Taus (timeconstant 2)  the LDC will measure properly.  what i did that i selected a bigger value of R given by the fomula in the data sheet. (calculated Tau is 15 µS) and a  set tau too 85 µSecond.. my question ist

1. setting a  bigger timeconstant 2 that that  given by the formula how will this affect my system.  note that i am  using  a probe which hast a Q of 6.

2. is there is somewhere any explanations about the time constants what do  these time constants mean and what ist the Realation of R and C.. .

3. how does the whole Rp measuring system work 

thanks a lot . 

  • 0
    TI__Mastermind 23810 points
    Bilal,

    I'll look into this a little more and get back to you.

    Please note that you will get the best results if you are trying to identify repeatable targets. Any variation in distance, tilt, temperature, and sometimes thickness of the target will look like an Rp variation. If you are trying to identify the material and thickness without consistent targets, it will be a challenging deign.
  • 0 in reply to Clancy Soehren
    TI__Mastermind 23810 points
    Bilal,

    For time constant 2, how did you measure the minimum Rp? Did you use the method followed in this app note?

    www.ti.com/.../snoa936.pdf

    Also, what is the C_SENSOR? Is it possible that there is additional parasitic capacitance that is impacting your calculation?

    I cannot provide details about how the time constant functions, but the advice in the datasheet should be accurate. My best guess is that there is a problem with your calculations somewhere. If double checking your min RP and C_SENSOR does not help, then let me know and I will keep working with you.