sorry, i am a newbie from electronics, and i am not good at english.
I read so many datasheets of op-amp and see they got "high input impedance". But i want to know exactly how much is it, can someone tell me where can i read it on datasheet or i need do what to calculator it.
Thanks

 

The input impedance of an op amp may not be explicitly stated in every data sheet.
Here it is from the old 741:

That's probably typical of bipolar op amps.
One important parameter that affects impedance is the input bias current, which will cause an offset from the signal source impedance.

For the highest input impedance, you can use a CMOS type op amp, which will also has a very low input bias current.

 

Although crutschow has shown an extract from a data sheet giving the input resistance data – bear in mind that for the most part, the input impedance of an op-amp circuit will be determined by the circuit components.

In the attached inverting op-amp circuit, the effective input impedance seen by Ein will be the resistance R1 (regardless of the input impedance of the inverting input – pin 2).

The unity gain buffer circuit shown will have a very high input impedance (much greater than the input impedance on the non-inverting input - pin 3).

 

I read so many datasheets of op-amp and see they got "high input impedance". But i want to know exactly how much is it, can someone tell me where can i read it on datasheet or i need do what to calculator it.

You will never be able to determine the actual input resistance of an op amp from its specifications, which usually give only the minimum and/or typical values. Nor will you be able to calculate it. The only way of finding the input resistance of an op amp is by measuring it; also, that measurement would not be valid for other op amps of the same type, since there is a lot of variation from unit to unit.

 

• Look at the datasheet of a Cmos opamp or an opamp that has a Jfet input (TL071) that is spec'd at 10 to the 12th power that is 1o millon x 1 million ohms. 10 trillion ohms?

 

The input Z of the OpAmp itself vs how you use in circuit a completely
different analysis.

If you do closed loop, 1 pole or 2 pole comped OpAmp analysis you will find
the Zin circuit moves form basically real, to complex L (inductive) as you get
up in frequency.

Use either spice or do s plane analysis, one pole OpAmp easy, 2 pole a little
more involved. In latter case using PFE (partial fraction expansion) allows you
to get at the values of RLC in the impedance.

This can be done on output as well. Fun stuff and gives you insight into
element values of the Z.

The DC case is largely driven by input bias, in very low bias parts leakage comes
into play as well.

http://www.electronicdesign.com/analog/what-s-all-bias-current-stuff-anyhow

http://www.electronicdesign.com/test-amp-measurement/whats-all-femtoampere-stuff-anyhow


Regards, Dana.