Pic temp sensor

[Pj]

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

 

[Fred Bloggs]

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...

 

[John Larkin]

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

 

[Arlet Ottens]

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.

 

[[email protected]]

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

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

 

[Spehro Pefhany]

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