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Is it the current you have to drive in the input to get a specific, defined voltage at the output? And don't the currents cancel each other when applied to both inputs?

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The ideal opamp has infinite input impedance. That means its inputs don't draw any current at all. There is no such thing as a ideal opamp, so all opamps draw some amount of current. This is what the input bias current spec tells you.

Any current times the impedance of whatever is driving the opamp input will cause a voltage, which is a error voltage between the real signal and what the opamp sees. This error is added directly to the input offset error of the opamp to get the effective offset error for your circuit.

For example, if the opamp input bias current spec is 1nA, then must assume a voltage error of 1mV with a 1MOhm resistor in series with that input. This is in addition to other sources of error, like the input offset voltage spec.

Different opamp technologies can vary widely. Old bipolar input opamps like the LM324 have orders of magnitude higher bias current than FET input opamps. For something like a LM324 you have to consider the input bias current spec carefully. For many modern CMOS input opamps the input bias current is so low that you can often ignore it (after a quick calculation to verify this for your case of course).

The input bias current spec is for each input. These do not necessarily cancel since the polarity can not be guaranteed. Some opamps have a input offset current spec that tells you the worst case current mismatch between the two inputs. That is common in bipolar input opamps since the current direction and magnitude is somewhat known. For example, the LM324 is specified for 100nA input bias current, but only 30nA input offset current. CMOS input opamps often don't have a offset current spec since the bias current is due to leakage and there is go guarantee what its polarity is.

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Thanks Olin. I understand that there will flow a certain current into the inputs, but they will be dependent on the voltage, won't they? I would like to know what the value in the datasheet means. –  Federico Russo Jun 12 '11 at 14:10
The bias current will be dependent on voltage somehow, but you shouldn't assume you know how. It's not as simple as a resistor to some voltage source. Assume only what the datasheet tells you. The input bias current can be either polarity and vary unpredictably as a function of voltage unless the datasheet explicitly says otherwise. –  Olin Lathrop Jun 12 '11 at 14:18
maybe I didn't express myself well, sorry about that. What I meant was: the input bias current in the datasheet may be 100nA, but it can take other values under other circumstances, like other input voltage. Why is the 100nA value published, and not another? Is there a standard setup which makes this the accepted value? Thank you –  Federico Russo Jun 12 '11 at 15:20
The datasheet tells you the worst case error, since that what you need to design to. Note that 100nA is in the MAX column of the LM324 datasheet. That means you can count on the bias current being anywhere from -100nA to +100nA. Beyond that, you can't make assumptions about what the bias current will be of any one device under any one set of conditions. Since you can't count on it, it's irrelevant to a design. –  Olin Lathrop Jun 12 '11 at 16:39
I've seen typical values mentioned in datasheets. I agree that you should design for worst case, so maximum. –  stevenvh Jun 12 '11 at 17:08

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