lm358 common mode range

[neddie]

Hi to all.
Anyone have experience with measuring a differential voltage in the supply line of a LM358. The datasheet shows it's input range going to the
+ and - supply rails , but it does not seem to work there. Simulation in Orcad and LTSpice shows the cct working , but in practice
no go. Is this a limitation of the 358 , that the model's don't reflect. Ive attached the basic cct.
Measuring a volt drop across a 1 ohm resistor in the supply line. If I move the lm358's supply above the original supply line by about 1Volt
then the circuit works fine. If it's the same or lower , then it stops working. Simulation does not show this behavior at all.
Cheers
Neddie

 

[Audioguru]

The datasheet for the LM358 shows an input common-mode voltage range of 0V to the positive supply voltage minus 1.5V.
So it cannot measure the positive power supply current.

 

[IanP]

Hi to all.
Anyone have experience with measuring a differential voltage in the supply line of a LM358. The datasheet shows it's input range going to the
+ and - supply rails , but it does not seem to work there. Simulation in Orcad and LTSpice shows the cct working , but in practice
no go. Is this a limitation of the 358 , that the model's don't reflect. Ive attached the basic cct.
Measuring a volt drop across a 1 ohm resistor in the supply line. If I move the lm358's supply above the original supply line by about 1Volt
then the circuit works fine. If it's the same or lower , then it stops working. Simulation does not show this behavior at all.
Cheers
Neddie

Have a look at the internal diagram of the LM358 amplifier. There are (2+2) PNP transistors that are referenced with regard to the positive rail. To work in the linear region you have to drop the input voltages below Vcc- 2x Ube - U(I=6uA) =Vcc- 1.4-1.6V ..
:wink:
IanP

 

[neddie]

Thanks all.
Why do the simulations show them working perfectly well at and above the supply rail?
According to the datasheet (attached) I have the differential goes to the supply rail , or am I misunderstanding the datasheet.
5'th point on the left hand side.
Cheers
Neddie

 

[Audioguru]

Why do the simulations show them working perfectly well at and above the supply rail?

A simulator needs an accurate model of the IC that is used. But most simulators and most students do not read details in the datasheets.

According to the datasheet (attached) I have the differential goes to the supply rail, or am I misunderstanding the datasheet.

The maximum allowed input voltage is 32V which is also the maximum allowed supply voltage. A higher voltage will cause the input to blow up.
The input common mode voltage range is the voltages that the inputs work properly. It is from 0V to 1.5V less than the supply voltage.

 

[neddie]

Thanks.
The question then becomes how are the models so inaccurate :0( .
Both Cadence and LTSpice model the same behavior.
Obviously the lm358 model is incorrect in both cases , or they share the same model.
Does not leave one with much confidence when using this chip in simulations.
Cheers
Neddie

 

[Audioguru]

I have the latest version of LTspice and it has all the modern LT opamps but it does not have a model for the very old LM358.
The LTspice forum at Yahoo Groups probably has an accurate model.
National Semi (now part of Texas Instruments) might have an accurate model.

 

[neddie]

I got the model from National :0( , before it became TI.