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Amplifier Q's

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petesmc

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Hi,

Note: Using LM741

I have a DC input at about ranging from -0.1 to 0.1V with a permanent fixed offset of about 0.04V. I would like to remove this offset, AND amplify the voltage ACCURATELY 10 times.

So I designed a simple, difference amplifier to remove the 0.04V offset, and then a 10 gain amplifier. However, in simulations each of the output voltages from each system is slighly off (assuming due to input offset of op-amp). I've tried to compensate for the offset using pots at the offset nulling pins but has no effect in the simulation so i'm not sure whether it will in real life.

Does anyone have any recommendation on how I can fix this, or possibly some more accuracte op-amps I should use for this purpose? I would like to have the voltage output no more than 1 or 2 mV out so I need some fairly high accuracy components.

Thanks
-Peter
 

Agent 009

New Member
...so I need some fairly high accuracy components.
Hehe, It's so damn obvious if you're working with +/-.1 volts. But could you give out your design's schematics, so I clearly could -maybe- spot the problem? (What are you using as resistors: I mean tolerance...)
 

pebe

Member
petesmc said:
Does anyone have any recommendation on how I can fix this, or possibly some more accuracte op-amps I should use for this purpose? I would like to have the voltage output no more than 1 or 2 mV out so I need some fairly high accuracy components.

Thanks
-Peter
You could use a chopper stabilised op amp like the ICL 7650CPD. It self-adjusts the input offset to typically 0.7microvolt - yes, that's microvolts! Its input temp coefficient is even less.

It is an instrument amplifier and is a little more expensive than a 741. but if you want results - it's good. There are several others in this category. You can find them with Google.

Use 1% resistors to get max accuracy.
 

Agent 009

New Member
Try using resistors with no band, or a gold ,or silver band, which have 0/10/5 % tolerance respectively... Resistors are the primary power loss in circuits, especially acurate-important ciruits.
 

pebe

Member
Agent 009 said:
Try using resistors with no band, or a gold ,or silver band, which have 0/10/5 % tolerance respectively... Resistors are the primary power loss in circuits, especially acurate-important ciruits.
No resistor has a 0% tolerance. If there is no band it is probably an old 10% or 20% tolerance.

In fact, I can't think of anything that has ever been manufactured to a 0% tolerance!
 
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