opamp has high input current

crutschow

Joined Mar 14, 2008
23,337
Does note 3 on page 2 mean the opamp inverting & non inverting terminals can draw ~20mA?
The note says:

Note 3: The inputs are protected by ESD protection diodes to each power supply. If the input extends more than 500mV beyond the power supply, the input current should be limited to less than 10mA.

What is not clear about that?
For those conditions the input looks like a diode to either of the supply rails and will conduct a large current unless limited by some means.

Under normal operation, with the inputs less than the rails, they will draw no more than the spec'd input bias current.

Make sense?
 

LesJones

Joined Jan 8, 2017
2,319
It means that you should ensure that you do not exceed 20 mA as that is the absolute maximum rating. (Just as you should not exceed ANY of the maximum ratings.) This situation will only occur if the input goes more negative than - 500 mV with respect to the negative supply (Pin 4 on an LM358) or more positive than + 500 mV with respect to the positive supply (Pin 8 on an LM358)

Les.
 

dl324

Joined Mar 30, 2015
8,906
Does note 3 on page 2 mean the opamp inverting & non inverting terminals can draw ~20mA?
The datasheet you referenced isn't as good as some. The NatSemi datasheet shows the circuit, so you can see what happens when the inputs go below ground:
upload_2019-8-23_14-54-52.png
In this case, going 500mV below ground starts forward biasing a C-B junction.

If you consider the common mode input range, you won't try to drive the input below ground:
upload_2019-8-23_14-57-3.png
upload_2019-8-23_14-57-20.png
 
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