Analog signal conditioning

[John Ferrell-2]

No!

John Ferrell    W8CCW

"All that is necessary for the triumph of evil is for good men to do
nothing." -- Edmund Burke
http://DixieNC.US

----- Original Message -----
From: "Richard Prosser" <rhprosser@...>
To: "Microcontroller discussion list - Public." <piclist@...>
Sent: Thursday, May 08, 2008 10:13 PM
Subject: Re: [PIC]Analog signal conditioning

> One of the legs to the pot isn't already connected to the mast or
> another cable is it??
>
> RP
>
> 2008/5/9 Jinx <joecolquitt@...>:
>>> As near as I can tell the noise is only present when the board with
>>> the AD is attached
>>
>> Re-reading earlier posts (so apologies for asking about something
>> you'd already mentioned)
>>
>> "It appears the noise is originating on the board. The AD specs call
>> for 1K max input impedance. A unity gain op amp will probably fix
>> the problem for my needs"
>>
>> I'm not aware of an AD conversing creating noise. The impedance
>> of your pot is not markedly different from the recommended. Being
>> slightly higher you can use a longer sampling time to offset this and
>> still get good results
>>
>> and
>>
>> "RA3 (AD pin) runs all over the board to the various sockets and
>> devices. However, the onboard pot does not exhibit the jitter"
>>
>> So this perhaps doesn't appear to be a problem with the remote
>> pot at all, rather that the signal path from the connector to the AD
>> pin is picking up noise. Maybe EMF from a crystal ?
>>
>> If you substitute the board pot with the remote (or put the board
>> pot on long wires) does this make a difference ? How about a
>> pot with shortish wires into the connector ?
>>
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[John Ferrell-2]

Some prototyping supplies came in today that allowed me to easily (and
quickly) redo the circuit under test. I first tried the sensor (500 ohm pot)
with a 1k series resistor. The wiper goes directly to the AD input. The
signal read was + and - about .010 around a center value. I added a 1000uF
capacitor from the AD input to ground and now only rarely does the least
significant digit waver. The total value read is from 0.014 to 1.069 volts.
The span is more than adequate for the required accuracy and I believe the
power through the sensor pot is harmless for the device.

I am concluding that there were several factors involved in the problem
including poor workmanship in the prototype fixture.

John Ferrell    W8CCW

"All that is necessary for the triumph of evil is for good men to do
nothing." -- Edmund Burke
http://DixieNC.US

----- Original Message -----
From: "Jinx" <joecolquitt@...>
To: "Microcontroller discussion list - Public." <piclist@...>
Sent: Thursday, May 08, 2008 8:03 PM
Subject: Re: [PIC]Analog signal conditioning

>> As near as I can tell the noise is only present when the board with
>> the AD is attached
>
> Re-reading earlier posts (so apologies for asking about something
> you'd already mentioned)
>
> "It appears the noise is originating on the board. The AD specs call
> for 1K max input impedance. A unity gain op amp will probably fix
> the problem for my needs"
>
> I'm not aware of an AD conversing creating noise. The impedance
> of your pot is not markedly different from the recommended. Being
> slightly higher you can use a longer sampling time to offset this and
> still get good results
>
> and
>
> "RA3 (AD pin) runs all over the board to the various sockets and
> devices. However, the onboard pot does not exhibit the jitter"
>
> So this perhaps doesn't appear to be a problem with the remote
> pot at all, rather that the signal path from the connector to the AD
> pin is picking up noise. Maybe EMF from a crystal ?
>
> If you substitute the board pot with the remote (or put the board
> pot on long wires) does this make a difference ? How about a
> pot with shortish wires into the connector ?
>
> --
> 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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[Jinx-4]

> I added a 1000uF capacitor from the AD input to ground and
> now only rarely does the least significant digit waver

That's good, sounds like you won. Be careful with such a large
cap though. A sudden discharge could take the AD pin out. A
series resistor and Schottky (anode to ground, cathode to pin)
will help protect it

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