What information does the Input Voltage Range Graph from LM358N Datasheet conveys ?

[muhibraza]

What information does the Input Voltage Range Graph from LM358N Datasheet convey ?

Hello,
I cannot understand what information does the following graph convey. Can someone explain this please ?



link to LM358N Datasheet :
https://www.alldatasheet.net/datasheet-pdf/pdf/113612/STMICROELECTRONICS/LM358N.html

 

[FvM]

In my view it's a confused way to visualize the input common mode range specification in the electrical characteristics table.

I must confess that the diagram, which can be also found with slightly different annotation in the original National datasheet, didn't bother me when I was using the part during the last 35 years or so.

 

[muhibraza]

yes but I think it may bother me... I am using it to sense very low voltages in a non inverting configuration with a gain of 67 or so...
I need to understand the graph...
Anybody there ?

 

[FvM]

As I already said, all necessary information is in the common mode range specification, which is apparently less ambiguous.

You should describe your application problem if you have difficulties to relate it to the datasheet specification.

 

[muhibraza]

I am using the LM358N to sense the voltages on a shunt resistor which would go up to 75 mV at 1 amp (full scale current)... It will usually remain even below that 75 mV value which is quite near the ground and the diagram is also a bit complicated so I cannot conclude whether the device would be able to sense an input this low or not ?

my application is as follows :

 

[FvM]

The input common mode range of LM358 includes the negative (ground) rail, in so far it has no problems to amplify the small input voltage correctly. The minimal sensed voltage will be however restricted by the output voltage range of the first amplifier. You achieve better performance if you omit the first amplifier stage and connect the shunt voltage directly to the gain stage. Another option to measure voltages near zero despite of amplifier offset voltages is to add a known small positive offset of a few mV to the shunt voltage and substract it after A/D conversion.

 

[muhibraza]

Can you please ellaborate how the output range of first amplifier would restrict the minimal sensed voltage ? It is just a voltage follower from the same package (LM358N dual op amp) which I have added so that there would be high input impedance that would not disturb the main circuit.....now in what sense would it restrict the minimum sensed voltage ?

 

[FvM]

Amplifier input voltage range includes zero, output range doesn't.

 

[muhibraza]

But the datasheet says the output can also swing to ground...
really confusing datsheet :/
:???:
see :


this one is from national semiconductor:


both manufacturers term this "output touching ground" thing as an special feature/advantage... how come the device would not comply to this feature in practical ?

 

[muhibraza]

I will appreciate if anyone could help ...
Anybody here ?

 

[FvM]

"Touching ground" doesn't talk about residual voltage in a mV range. You have to refer to datasheet detail specifications instead of reading a specification into the first datasheet page.

I must confess, that I don't exactly remember what's the typical minimal output voltage without a load is. If you look at the transistor level schematic, you'll see that the saturation voltage is determined by a 50 µA current mirror (drawn as a current source). Because it's bipolar technology, you can expect a few mV saturation voltage.

A few mV output voltage is considered as "zero" in single-supply OP terminology, but it probably isn't zero enough for a mV buffer amplifier.

 

[muhibraza]

Alright....I'll remove the follower and let you know the result then...
thanks