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overmindx
Joined: 06 Oct 2008
Posts: 43
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 Posted: Thu Feb 12, 2015 12:13 am |
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hi,
Thank you for your answer. It seems i need to make some modification on the hardware. Just one question, with the resistor values that you recommended, will that output 4.046k which is still larger that the recommended maximum impedance of 2.5k? Thanks. |
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Ttelmah
Joined: 11 Mar 2010
Posts: 19535
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| Posted: Thu Feb 12, 2015 3:09 am |
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| The impedance is not just the value of the pull down resistor. I assumed the input impedance at the source end was nearly zero. If it isn't, then you'd have to work out new values. |
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ckielstra
Joined: 18 Mar 2004
Posts: 3680
Location: The Netherlands
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| Posted: Thu Feb 12, 2015 4:16 pm |
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@Ttelmah
I was also wondering about the resistor values...
My electronics knowledge is getting rusty, but assuming a zero impedance source I get an equivalent parallel resistance of 4046 ohm, just like Overmindx calculated. |
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Ttelmah
Joined: 11 Mar 2010
Posts: 19535
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| Posted: Fri Feb 13, 2015 2:27 am |
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Apologies. The recommended Rs on this chip, is 10KR. When I calculated the resistors I aimed for half this. 2.5KR is for the more modern PIC24's where it is needed for their faster sampling. I was calculating for this on a circuit I'm working on at the moment and typed it in by mistake.
Same comments still apply though the original circuit is way outside the required level. |
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ckielstra
Joined: 18 Mar 2004
Posts: 3680
Location: The Netherlands
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| Posted: Fri Feb 13, 2015 7:37 am |
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| Thanks for the explanation. Makes sense now. |
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overmindx
Joined: 06 Oct 2008
Posts: 43
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| Posted: Sun Feb 15, 2015 9:01 am |
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hi,
actually according to the datasheet of 18f87k22, the maximum recommended impedance for analog sources is 2.5k. |
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