What would probably be the likely cause of the output bouncing on the
lower 4 bits, bits 0,1,2,3, but has good stability on the upper bits.
I'm using a junk box Pic 16f807 with one input line to AN0 which is
being fed from the output of a LM324 op amp wired as a non-inverting
dc gain.  The signal to the op amp is coming directly from a
thermocouple.

If it's do to noise why does it only bounce on the lower bits?

Pj

Pj wrote:
> What would probably be the likely cause of the output bouncing on the
> lower 4 bits, bits 0,1,2,3, but has good stability on the upper bits.
> I'm using a junk box Pic 16f807 with one input line to AN0 which is
> being fed from the output of a LM324 op amp wired as a non-inverting
> dc gain.  The signal to the op amp is coming directly from a
> thermocouple.
> 
> If it's do to noise why does it only bounce on the lower bits?
> 
> Pj

Lower bits are the smaller voltage detection bits, right...

On Fri, 22 Feb 2008 07:21:52 -0800 (PST), Pj <f...@yahoo.com>
wrote:

>What would probably be the likely cause of the output bouncing on the
>lower 4 bits, bits 0,1,2,3, but has good stability on the upper bits.
>I'm using a junk box Pic 16f807 with one input line to AN0 which is
>being fed from the output of a LM324 op amp wired as a non-inverting
>dc gain.  The signal to the op amp is coming directly from a
>thermocouple.
>
>If it's do to noise why does it only bounce on the lower bits?
>
>Pj


It's analog noise from the opamp; digitized, the lower bits wobble.

An LM324 is a terrible opamp for this. But expect noise anyhow, since
thermocouples make tiny voltages.

Use a better opamp, and take lots of samples and average.

John

John Larkin wrote:

> On Fri, 22 Feb 2008 07:21:52 -0800 (PST), Pj <f...@yahoo.com>
> wrote:
> 
>> What would probably be the likely cause of the output bouncing on the
>> lower 4 bits, bits 0,1,2,3, but has good stability on the upper bits.
>> I'm using a junk box Pic 16f807 with one input line to AN0 which is
>> being fed from the output of a LM324 op amp wired as a non-inverting
>> dc gain.  The signal to the op amp is coming directly from a
>> thermocouple.
>>
>> If it's do to noise why does it only bounce on the lower bits?

> It's analog noise from the opamp; digitized, the lower bits wobble.
> 
> An LM324 is a terrible opamp for this. But expect noise anyhow, since
> thermocouples make tiny voltages.
> 
> Use a better opamp, and take lots of samples and average.

Or use a better ADC. For instance, the AD7792 has an integrated 
amplifier, and 50/60 Hz filters. With some careful layout, it will get 
you 16 stable bits.

On Feb 22, 3:21=EF=BF=BDpm, Pj <frysn...@yahoo.com> wrote:
> What would probably be the likely cause of the output bouncing on the
> lower 4 bits, bits 0,1,2,3, but has good stability on the upper bits.
> I'm using a junk box Pic 16f807 with one input line to AN0 which is
> being fed from the output of a LM324 op amp wired as a non-inverting
> dc gain. =EF=BF=BDThe signal to the op amp is coming directly from a
> thermocouple.
>
> If it's do to noise why does it only bounce on the lower bits?
>
> Pj

A low pass filter to reduce the bandwidth will reduce the noise, you
only need dc to 0.5Hz at most.

On Fri, 22 Feb 2008 18:49:44 +0100, Arlet Ottens <usenet+5...@c-scape.nl>
wrote:

>John Larkin wrote:
>
>> On Fri, 22 Feb 2008 07:21:52 -0800 (PST), Pj <f...@yahoo.com>
>> wrote:
>> 
>>> What would probably be the likely cause of the output bouncing on the
>>> lower 4 bits, bits 0,1,2,3, but has good stability on the upper bits.
>>> I'm using a junk box Pic 16f807 with one input line to AN0 which is
>>> being fed from the output of a LM324 op amp wired as a non-inverting
>>> dc gain.  The signal to the op amp is coming directly from a
>>> thermocouple.
>>>
>>> If it's do to noise why does it only bounce on the lower bits?
>
>> It's analog noise from the opamp; digitized, the lower bits wobble.
>> 
>> An LM324 is a terrible opamp for this. But expect noise anyhow, since
>> thermocouples make tiny voltages.
>> 
>> Use a better opamp, and take lots of samples and average.
>
>Or use a better ADC. For instance, the AD7792 has an integrated 
>amplifier, and 50/60 Hz filters. With some careful layout, it will get 
>you 16 stable bits.

Maybe he only needs 8 or 9 bits... not everything has to be a frick'n
gold plated design y'know. 
Best regards, 
Spehro Pefhany
-- 
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