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Author Topic: analogreference and resolution question (STILL NEED HELP)  (Read 1392 times)
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So i decided to go with an op amp. specifically the lm781cn from radioshack.

What i have done so far is:

V+ to the +5V pin
V- to ground
inverting input to output with 100K resistor, then 1K to ground
non inverting is the signal

Is there anything wrong with this setup or am i good to go?

Thanks
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specifically the lm781cn
Are you sure, when I search for this I get either an LED or voltage regulator.

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inverting input to output with 100K resistor, then 1K to ground
No, put the 1K to a signal reference ground. This can be made with two 1K resistors connected like a potential divider giving 2.5V, this is the point where you put the end of your 1K resistor. Also put two 0.1uF caps across each of the two potential divider resistors.
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oh sorry it was 741.

And i dont really understand what you are saying. divide what voltage? I understand what a voltage divider is. but why?

As of what i have now im getting a output signal of 4.5 volts with no input. what is going on?

Then if i disconnect the V- from ground the output drops to 3 Volts.

help please?

im confused.
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divide what voltage?
The supply voltage.

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but why?
To give a signal ground.

Op amps require a positive and negative power supply. You haven't got one, so to trick it you make a false signal ground at +2.5V with a potential divider and use some capacitors to lower the impedance.

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oh sorry it was 741.
Look at the data sheet, it won't work with just a 5V supply. The data sheet says it needs a +/- 12V supply. That's 24V in total. You are only giving it +/- 2.5V.

An op amp has a gain of several thousand, when you say no input do you mean the two inputs connected together, that is the only way to have no input on an op amp.
« Last Edit: July 28, 2011, 05:46:38 pm by Grumpy_Mike » Logged

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