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Topic: analogreference and resolution question (STILL NEED HELP) (Read 1 time) previous topic - next topic

jcarabet

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

Grumpy_Mike

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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.

jcarabet

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.

Grumpy_Mike

#18
Jul 29, 2011, 12:42 am Last Edit: Jul 29, 2011, 12:46 am by Grumpy_Mike Reason: 1
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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.

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