Cant I connect a OpAmp directly to the Atmega?

[fsoender]

I got a OpAmp connected to my Atmega16. OpAmp gives me 0-3 Volts on Atmega input A/D converter.
My problem is that the value/signal on A/D input is floating. The OpAmp gives me the volt range as expected.
I have connected a LCD to see the value. When I connect power supply with
0-3 volts it works. Cant I connect a OpAmp directly to the Atmega?

Can anyone help me?

 

[goldsmith]

Re: Operational Amplifier

Hi Fsoender

0-3 volts it works. Cant I connect a OpAmp directly to the Atmega?

Why not ? it should work without any problem .
Best Wishes
Goldsmith

 

[fsoender]

Re: Operational Amplifier

Ok, but why is the value on my LCD fluctuating with OpAmp connected, and not with power supply with same voltage?

 

[goldsmith]

Re: Operational Amplifier

Can you attach your circuit here ? how i should know that what circuit you are referring to ?

 

[fsoender]

Re: Operational Amplifier

Here is the circuit

 

[goldsmith]

Re: Operational Amplifier

LCD fluctuating with OpAmp connected, and not with power supply with same voltage?

What difference is your mean ? can you bring both differences here ? because apparently your circuit is correct .

 

[fsoender]

Re: Operational Amplifier

Ok, good that its correct. The other way I tried is to replace the OpAmp with a potmeter, and that works well.

 

[godfreyl]

Re: Operational Amplifier

Here is the circuit

What opamp are you using?
What are the resistor values in the opamp's feedback loop?
What are the opamp's positive and negative supply pins connected to?

 

[fsoender]

Re: Operational Amplifier

Im using a LM358, and its connected to a coil (to read magnetic field).
Resistor value 10K and 1K to get 100x

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I think the LCD is not responding quick enough to see the quick change from the coil, can that be the issue?

 

[goldsmith]

Re: Operational Amplifier

Resistor value 10K and 1K to get 100x

Are you sure that it should give you 100X gain ? (1+RA/RB)=1+10000/1000=11 ! not 100 !!

 

[fsoender]

Re: Operational Amplifier

Sorry I use 100K, so 101

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Is there any way to "hold" the signal from the coil longer?

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Here is the circuit

 

[godfreyl]

Re: Operational Amplifier

Im using a LM358, and its connected to a coil (to read magnetic field).

Output from the coil is AC (positive and negative). The ADC can not process negative voltage at it's input.

I think the LCD is not responding quick enough to see the quick change from the coil, can that be the issue?

Yes.

Is there any way to "hold" the signal from the coil longer?

Can't you do that in software in the microcontroller?

 

[fsoender]

Re: Operational Amplifier

Yes I can do it in the microcontroller, but was wondering if there is a hardware way aswell?
I know the Coil deliver AC power, I get only positive output from the OpAmp.