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### Topic: analogreference and resolution question (STILL NEED HELP) (Read 5285 times)previous topic - next topic

#### jcarabet

#15
##### Jul 28, 2011, 09:43 pm
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

#16
##### Jul 28, 2011, 10:53 pm
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specifically the lm781cn

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

Quote
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

#17
##### Jul 29, 2011, 12:35 am
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.

im confused.

#### Grumpy_Mike

#18
##### Jul 29, 2011, 12:42 amLast 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.

Quote
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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