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?
Electronic – How to calculate open-loop input impedance of an opamp if it is not explicitly given in the datasheet
datasheetinput-impedanceoperational-amplifier
Related Solutions
Open-loop output impedance of a opamp is rarely, if ever specified. However, the maximum current source and sink capability usually is.
You should therefore go with the datasheet and consider the opamp output a current source within the maximum specified current capability. It's impedance should therefore be considered infinite, since that is the impedance of a perfect current source.
In reality, the impedance won't be infinite, but you don't know what it will be. CMOS output opamps probably look mostly resistive, but the current source model may actually be closer for bipolar output opamps.
In any case, don't try to read into the datasheet what it doesn't say. You are only guaranteed what it guarantees. Assume the current source model when designing the circuit, and everything should work. If you design the circuit right, the open-loop output impedance will be irrelevant anyway. In the closed loop case, the output impedance, like the gain, will be governed by the feedback.
The simplest way to eliminate your problem is to reduce the effective impedance of the source, and a voltage follower will do nicely
simulate this circuit – Schematic created using CircuitLab
The choice of buffer op amp is left to you, depending on the frequency response you need.
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Best Answer
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.