Help!How to choose capacitor and resistor for RC circuit?

[garygothic]

Hi. I need help from experts. I'm doing a project on reading analogue voltage humidity sensor (**broken link removed**) using PIC16F917 (datasheet) and then showing the converted value on a LCD. I'm using the built in ADC in the PIC for the value conversion. The problem is I cannot get the accurate reading from the PIC. A quick check up on the Humidity sensor datasheet,I found that the sensor's maximum settling time is 70ms. Meanwhile the AD acquisition time (page 183) Tacq with 10kHz source of impedance and 50degreesC temperature is 4.67us. So I suspect the causes of the inaccurate reading are because of high possible transient amplitude and slow settling time. I have read somewhere that I need to build an RC circuit in between the output voltage,Vout, of humidity sensor and input of the ADC in order to rectify this problems. Now, my problem is I do not know how to build one and the theories behind it. I just know the basic design of RC circuit: **broken link removed**.
What value of resistor and capacitance should I used in my RC circuit?

 

[MikeMl]

The Pic input impedance should be 10K or less, so try a series 10K, with the Pic ADC pin shunted to ground with a 100nF. That will give a settling time of about 3msec. If you need longer, increase the cap to 1uF.

 

[garygothic]

May I know how did you get the value of settling time of 3ms from the resistance and capacitor value?What is the formula?

 

[MikeMl]

May I know how did you get the value of settling time of 3ms from the resistance and capacitor value?What is the formula?

2.2*R*C, I used three time-constants: 3*1*10^4*100*10^-9 = 300*10^-5 = 3ms

 

[MikeMl]

May I know how did you get the value of settling time of 3ms from the resistance and capacitor value?What is the formula?

I used three time-constants: 3*1*10^4*100*10^-9 = 300*10^-5 = 3ms.

Maybe **broken link removed**?

 

[garygothic]

But the AD acquisition time,Tacq,is 4.67us. I'm supposed to make a settling time below 4.67us right?Let say I take 4 time-constants and 10K source impedance,so the value for capacitor is 0.1nF which will equal the Tacq time.

 

[MikeMl]

The important thing is not the acquisition time, but the sampling rate. e.g., ff you were only doing a conversion once per second, the sample has the whole second to settle, even though it might only take 3us to actually do the conversion.

 

[garygothic]

Meaning the longer sampling rate the better AD conversion value?

 

[angophora]

it might be electrical noise on the adc power supply, try averaging 128 readings.