# A/D impedance PIC connected to RC circuit

Discussion in 'Microcontrollers, Programming and IoT' started by Rajinder, Jun 17, 2019.

1. ### Rajinder

419
7
Jan 30, 2016
Hi all,
I have a RC circuit in parallel 330K and 1uF.
This is going to be connected to an A/D input of a PIC. The PIC is a 16LF1936, which says the impedance is 10K source.

1. How do i calculate the impedance of the RC circuit? I think it is Z=squareroot of RxR + Xc x XC.
XC is the reactance of the capacitor. 1/2x3.142xfreqxcapacitor.
2. For capacitive reactance, what should the frequency be? Would it be at DC i.e. 0Hz. If that is the case the RC impedance is only 390K. Is that correct?
3. If the source impedance of the PIC is 10K, then the RC impedance needs to be greater than this or the signal is reduced.

Does all this seem correct?

2. ### BobK

7,671
1,681
Jan 5, 2010
Show us how it is connected.

Bob

3. ### hevans1944Hop - AC8NS

4,301
2,019
Jun 21, 2012
As @BobK (quietly) said: SHOW US HOW IT IS CONNECTED! (shouting and emphasis mine because original poster doesn't appear to be paying attention)

And to answer your question #4: No, nothing about your post appears to be correct. Capacitive reactance is not defined at DC, which by definition is zero frequency, because division by zero is not allowed! So, your "calculations" are not only wrong but irrelevant.

Statement #3 is totally wrong: the source impedance to the PIC's ADC should be as small as possible, certainly less the 10 kΩ, to avoid attenuating the signal. The source impedance along with the ADC input impedance form a voltage divider, with the attenuated divider output appearing as the input to the ADC. When driving a PIC ADC (or any other ADC), typically a low-impedance source such as an operational amplifier serves this purpose. Also, whatever attenuation of the input signal does occur can be compensated by a small gain in the op-amp circuit, or "calibrated out" of the digital conversion from the ADC.

So, please show us your RC circuit and how you plan to connect it to the PIC A/D converter input.

4. ### Harald KappModeratorModerator

9,540
1,967
Nov 17, 2011
#1: See e.g. here.
#2: Your question doesn't make sense: the reactance of a capacitor is frequency dependent, therefore you need to consider teh range of relevant frequencies. Your circuit will have a different impedance at each frequency, including DC.
#3: Much greater, see Hop's explanation in post #3
#4: No.

5. ### Rajinder

419
7
Jan 30, 2016
Hi, sorry but i cant seem to.upload files. Keep getting errors. I will upload tonight from my PC.
Thanks

6. ### hevans1944Hop - AC8NS

4,301
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Jun 21, 2012
I had the same problem the other day. Kept getting an error message saying it couldn't upload my image file. My eventual solution was to load the image in Microsoft Paint, size it to 1024 x 768 pixels and then save the re-sized image under a different file name. The new image file uploaded just fine and I didn't notice the decrease in image resolution. The software for uploading an image on this forum doesn't tell you why it failed.

7. ### Harald KappModeratorModerator

9,540
1,967
Nov 17, 2011
Despite was is often said: size matters. I don't know where the limit is here, but reducing file size usually helps.