Question about op-amps and adc.

mosquito56

· I am using an LM358M op amp increasing a thermocouple wire to a adc0831, works fine. The strange thing is when I use a 6.5V battery and a 9V battery connected directly to the op-amp ·the adc shows the same number.
I would think a higher voltage on the opamp would create a higher voltage on the adc pin and thus a higher output. The adc is connect to a 3.3V regulator.
· Since I have no·idea what I am doing with op-amps, I thought I would throw this out.


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phil kenny

A sketch or rough schematic of the Op amp and associated circuitry would go
a long way toward helping give a good explanation for what's going on.

I'm assuming you have the Op amp wired something like the attached schematic,
your 6.5 v or 9v battery is connected as shown and that the thermocouple
is connected to the terminal I've labeled +Vin.

If this is so, then the Op amp will amplify the thermocouple signal by an amount
controlled only by R1 and R2. The amplified output won't usually be affected by
changes in the battery voltage (except as noted below).

The two resistors provide feedback that will compensate for supply voltage
fluctuations as long as the amplified voltage is less than the supply voltage.

In this case, R2/R1 = 1,000, and the circuit will amplify the input voltage by 1,000.

Suppose the thermocouple signal was 1 mv. Then the Op amp output would be
1 v.

If the battery voltage was 6.5 volts and the thermocouple signal was 7 mv,
the Op amp would try to make the output 7 v. But since that's higher than
the supply voltage the output wouldn't get there and might only reach 6 v.

However, if you now changed the battery voltage to 9 v, then with 7 mv at the
input of the Op amp, the output would reach 7 v. So making the supply voltage
bigger increased the output for this situation.

For a more complete (maybe too complete) answer check out
en.wikipedia.org/wiki/Op-amp

phil

Post Edited (phil kenny) : 2/23/2008 5:16:56 AM GMT

mosquito56

Phil, yes you nailed it exactly. I can understand it can't go above Vcc but would changing the "To battery" to 36V change the output? Maybe the diff between 6v and 9v just isn't enough.

I am having a little trouble with the vref on the adc0831 also. I·switch vref· between 3.3v and ·5 and the values don't change.

·I think this is all F.M. (Frickin Magic)

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······· "What do you mean, it doesn't have any tubes?"
······· "No such thing as a dumb question" unless it's on the internet
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phil kenny

The maximum supply voltage for the LM358 is 32v, but even connecting 32 v
to the supply pin won't raise the output voltage of the amplifier as long as
it is operating in its linear range.

Attached is a section of the adc0831 data sheet where it discusses the reference
voltage. Depending on what the maximum output you are getting from the LM358,
you may want to experiment with lowering the reference voltage, say to 1.2 v.

This will make the adc output vary from 0 to 255 counts as the input to it goes from
zero to 1.2 v.

phil

mosquito56

· I thought that an op amp output was a function of the input and the supply voltage but you seem to be telling me that's not true. I didn't know that, thanx.

Because of the gain on the op-amp. I can't even lower the adc to 3.3v. I could lower the gain of the opamp but I noticed it became more non-linear and I get nothing until the wire is very warm.

· Since my electronics training was in the early 70's my background is almost comepletely gone.· I love this new digital stuff as it's more programming then electronics.

Have a good one.

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······· "What do you mean, it doesn't have any tubes?"
······· "No such thing as a dumb question" unless it's on the internet
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