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I would like to know input impedance of opamps I'm using in the circuit, so that I can choose input stage resistors optimally. However, opamp datasheets usually don't give the imput impedance value explicitly.
(Examples: LMV321, OPA330)
How do I calculate input impedance in case it is not given in the datasheet?

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  • \$\begingroup\$ The only time this sort of thing is relevant is when you're doing ultra-high-impedance measuring, in which case the input bias current is the relevant number, not the impedance. \$\endgroup\$ – Connor Wolf Nov 9 '13 at 12:44
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Generally, input resistance for commercial CMOS opamps are high enough (in the 100M to G Ohms range at DC) that optimizing the circuit for it are futile, considering the optimal resistors will have mismatch themselves. This is caused by the differential input stage for the opamp being a (or two in the case for rail to rail opamps) MOS differential pair generally. As for the input capacitance for frequency domain behavior, you can see the value listed to be 2pF single ended and 4pF differential. Usually, this capacitance will not have any observable effect since the gain bandwidth of the amplifier is much lower.

For CMOS opamps, to calculate the input impedance, you can simply consider the input capacitance parallel to a very, very high resistance value, which is almost equivalent to just considering the capacitance.

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  • \$\begingroup\$ This is bad advice. Some common opamps, like the LM324 for example, have significant input bias and offset currents. You can't just blindly use 1 MOhm or even 100 kOhm source impedance. Always CHECK THE PARAMETERS IN THE DATASHEET. Anything less is bad engineering. \$\endgroup\$ – Olin Lathrop Nov 9 '13 at 12:57
  • \$\begingroup\$ I agree, that's why I've limited my advice to CMOS opamps only, which the rest of my answer referred to anyway. Bipolar and JFET input opamps will require an input bias current that will greatly reduce the resistive component of the input impedance. \$\endgroup\$ – deadude Nov 9 '13 at 13:01
  • \$\begingroup\$ I think this advice should carry a warning label. The LMV321 has a monstrous input bias current of 500nA(max) on a 5V rail and if this is a CMOS amp then the dominant feature of the input is this bias - a 100k resistor will develop a dc error of 50mV. \$\endgroup\$ – Andy aka Nov 9 '13 at 13:36
  • \$\begingroup\$ Digging through the datasheet of LMV321 (page 16) reveals that it is a bipolar input opamp, so for that opamp, you can ignore just about anything I said about CMOS opamps. There is no way a CMOS opamp will draw nA's of current through it's inputs (gates) unless you have a specificly designed one in nm CMOS. \$\endgroup\$ – deadude Nov 9 '13 at 18:06
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Opamp datasheets will usually give you input bias and offset current specs, not resistance. A resistance doesn't model what happens that well, especially for FET input opamps. It more useful to know the worst case current that could go into or come out of each input, and how mismatched it can be between inputs.

You can still use this to calculate maximum input impedance. The input bias current times that impedance will be the error voltage the opamp will see. This is in addition to the input offset voltage specified in the datasheet.

For modern FET-input opamps, the input bias and offset currents are so small that they usually don't matter unless your source impedance is in the multiple MΩ range.

However, always check. Some opamps, like the very common LM324 for example, have significant input bias and offset currents that you have to think about carefully. Generally you are safe with 100 kΩ or less source impedance, but such rules of thumb are no substitute for reading the datasheet and making sure. It only takes a few seconds to multiply the input bias current by your source impedance and compare the result with the input offset voltage.

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  • \$\begingroup\$ Does you second paragraph imply that \$ Z_{in} = V_{os} / I_{bias} \$; where \$Z_{in}\$ is input impedance, \$V_{os}\$ is input ofset voltage and \$I_{bias}\$ is input bias current? \$\endgroup\$ – hkBattousai Nov 9 '13 at 13:10
  • \$\begingroup\$ @khBa: No ----- \$\endgroup\$ – Olin Lathrop Nov 9 '13 at 13:12

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