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Re: Analog signal conditioning
by PicDude
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I am not the expert with things analog, but for automotive apps (lots of noise
sources), I find good layout (separating analog/digital sections) and good
bypass caps very close to the PIC make a significant difference. Then,
depending on signal being measured, I generally average ~20-50 samples, then
IIR filter that. And I get pretty good results.
Some of these are also ground-referenced variable resistances with a fixed
resistor to Vdd to create a voltage divider. I generally use a 50% voltage
divider to Vref+, in order to use more of the PIC's A/D range, but in one
specific application where the varying-resistance sender is far from the PIC,
I moved the 50% Vref voltage divider near the sender. The theory here is
that the Vref will have the Vdd and Vss line, and hence should have the same
noise as the input signal, and it's ratiometric. But I can't say that I've
actually compared this with a similar version where the 50% Vref resistors
are near the PIC.
Cheers,
-Neil.
On Thursday 01 May 2008 11:01, John Ferrell wrote:
> Analog signal conditioning is an overwhelming thing to Google!
>
> Here is the problem: I need to measure a remote resistance of 0 to 500 ohms
> with the AD input on a Pic 16F877A. what ever I do, it has a lot of jitter
> on the reading. A super cheap Harbor Freight Ohmmeter works fine.
>
> Details: The Ohmmeter is using a 0.22 ma current for measurement. My
> circuit has tried to series the top of the pot with 4.3k down to 680 ohms.
> I have tried up to 0.22 UF capacitor in parallel with the port to no avail.
> The ohmmeter works all the way to the circuit board so I cannot see how I
> could have a wiring error. My development board is powered by the USB port
> and the program works with the Board's 5K pot across VCC.
>
> For the curious: The resistance I am trying to read with the PIC is at the
> end of about 80 feet of wire in my Ham Radio rotator. I took a lightning
> strike several years ago and have since dedicated a meter and toggle switch
> for a control box. I am just now looking for a better solution...
>
> I am not looking forward to exploring Ohm meters!
>
>
>
> John Ferrell W8CCW
>
> "All that is necessary for the triumph of evil is for good men to do
> nothing." -- Edmund Burke http://DixieNC.US--
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sources), I find good layout (separating analog/digital sections) and good
bypass caps very close to the PIC make a significant difference. Then,
depending on signal being measured, I generally average ~20-50 samples, then
IIR filter that. And I get pretty good results.
Some of these are also ground-referenced variable resistances with a fixed
resistor to Vdd to create a voltage divider. I generally use a 50% voltage
divider to Vref+, in order to use more of the PIC's A/D range, but in one
specific application where the varying-resistance sender is far from the PIC,
I moved the 50% Vref voltage divider near the sender. The theory here is
that the Vref will have the Vdd and Vss line, and hence should have the same
noise as the input signal, and it's ratiometric. But I can't say that I've
actually compared this with a similar version where the 50% Vref resistors
are near the PIC.
Cheers,
-Neil.
On Thursday 01 May 2008 11:01, John Ferrell wrote:
> Analog signal conditioning is an overwhelming thing to Google!
>
> Here is the problem: I need to measure a remote resistance of 0 to 500 ohms
> with the AD input on a Pic 16F877A. what ever I do, it has a lot of jitter
> on the reading. A super cheap Harbor Freight Ohmmeter works fine.
>
> Details: The Ohmmeter is using a 0.22 ma current for measurement. My
> circuit has tried to series the top of the pot with 4.3k down to 680 ohms.
> I have tried up to 0.22 UF capacitor in parallel with the port to no avail.
> The ohmmeter works all the way to the circuit board so I cannot see how I
> could have a wiring error. My development board is powered by the USB port
> and the program works with the Board's 5K pot across VCC.
>
> For the curious: The resistance I am trying to read with the PIC is at the
> end of about 80 feet of wire in my Ham Radio rotator. I took a lightning
> strike several years ago and have since dedicated a meter and toggle switch
> for a control box. I am just now looking for a better solution...
>
> I am not looking forward to exploring Ohm meters!
>
>
>
> John Ferrell W8CCW
>
> "All that is necessary for the triumph of evil is for good men to do
> nothing." -- Edmund Burke http://DixieNC.US
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
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