[SOLVED] LT6015 OpAmp and LTSpice simulation

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flote21

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Hello guys,

I am wondering why the simulation of the scheme attached is working. Should an OpAmp configured like inverter work when the input voltage is negative and the power supply is between 0 and a positive voltage?

According to the simulation is working....but I don't know what is going to happen in the real life???

Thanks in advance!

 

In real life will happen more or less what you see in the simulation. I do not understand your question... why are you surprised?
 

the input voltage is negative and the power supply is between 0 and a positive voltage
Click to expand...

You're saying the op amp is powered by a single-ended positive supply. It cannot handle a negative voltage. (A simulator is easily fooled by such a setup.)

Operating guidelines state you should only apply a voltage to the input which is within the supply rails.
 

I am wondering because other OpAmp are not working when the input voltage is out range of the power supply...For example if you do the same simulation with the LT1007, you can see that the output is totally wrong when the input voltage is -2V....Could it be realted with the rail to rail charcteristic of the OpAmps? i mean if the OpAmp is a rail to rail OpAmp, it is mandatory to suplly +/- votage in order to get a good behavior when the input voltage is negative and the configuration of the rail to rail OpAmp is like an inverter...nevertheless if the OpAmp is not rail to rail you can supply it between 0V and positive voltages and you will get always good performance when the input voltage is <0 inverter configuration and it is in the range of the power supply..



CataM said:
In real life will happen more or less what you see in the simulation. I do not understand your question... why are you surprised?
Click to expand...
 

No, a simulator is not easily fooled by such a setup. The simulator is showing a correct response.

The voltage is interest is not the resistor input voltage but the voltage at the op amp (-) input. As long as that doesn't go significantly below the negative supply (for a rail-rail op amp) you are okay.
With a negative input to the inverting amp, the output goes positive due to the negative feedback, keeping the (-) input very near 0V, which is fine.
Thus the behavior you simulated is normal for that op amp and circuit.

As the OP noted, the same simulation with a non-rail-rail op does not work, also a correct response.
 

I did the simulation with the LT6003 single positive supply OpAmp and it is also working..so the problem however with other OpAmps the simulation is not working. I really can't understand why it is happening this...is there any parameter in the datsheet which tell you when the behaviour of the OpAmp is totally bullshit when is supplied between gnd and 5v and the input is -2V in inverter configuration??
 

Not BS, the op amp is just doing what the data sheet tells you, if you look at the right parameter.
The input common-mode range of the op amp will tell you how close to the supply rails it can operate.
Rail-rail and single-supply op amps have a common-mode input range that goes to the negative supply (thus to ground if the negative supply is ground).

Standard op amps usually have an input common-mode range that goes no closer than within about 1 to 2V of the supply rails.
Obviously then they won't work with an input to ground if the negative supply is also ground.
Make sense?
 


You are correct for those circumstances. Furthermore LTSpice is more likely to act correctly than the simulator I use (Falstad's). I have seen Falstad's op amp model act simplistically, too simplistically.
 

So it means that the LT6015 should work on real life when input voltage is -2 and is supplied between gnd and +5V?? with Gain=-1 of course!

Thanks

BradtheRad said:
You are correct for those circumstances. Furthermore LTSpice is more likely to act correctly than the simulator I use (Falstad's). I have seen Falstad's op amp model act simplistically, too simplistically.
Click to expand...
 

Hi,

So it means that the LT6015 should work on real life when input voltage is -2 and is supplied between gnd and +5V?? with Gain=-1 of course!

Thanks
Click to expand...

The LT6015 does not work with input voltages of -2V.
But the shown LT6015_circuit does work with input voltages of -2V at the resistor.

Start your simulation again and measure the true OPAMP input voltages at IN- and IN+ and see that both are close to 0V.
(Not -2V)

Klaus
 

Yes Klaus when i measure in V+ and V- the voltege is 0V. So it is not negative!! Therefore I can trust on the simulator....

Thanks!


 

Hi,

Yes, I'd say the simulation is correct.
But for a reliable "true" design I recommend not to go to the very limits of the device.

Means: if it is possible...then lift both IN+ and IN- to a safe area...maybe some +100mV.

Klaus
 

LT6015 specifies minimal common mode voltage of 0, this should be read so that no extra "safe area" is required for a non-inverting amplifier. Personally I think that the datasheet specification could be clearer in this regard. For some reason Linear prefers to publish some measurements with arbitrary ranges and a lower limit of e.g. 100 mV or 1 mV, which doesn't make sense at all. I tend to avoid using components with intentional vague specification, unless they are essential for a particular design.
 

BradtheRad said:
You are correct for those circumstances. Furthermore LTSpice is more likely to act correctly than the simulator I use (Falstad's). I have seen Falstad's op amp model act simplistically, too simplistically.
Click to expand...
Yes, I was referring to Spice derived simulators which use the manufacturer's particular device Spice model for simulation.
That should generally give a reasonably good simulation to the real life circuit.
Falstad's simulator uses a simple generic op amp model I assume, and that is okay for simple circuit functional evaluation but is much less suited to serious design work, or catching subtle quirks of operation when the device is operated outside its performance limits.
 
Hi guys!

I have doing some simulation with different rail to rail LT OpAmps in inverter configuration and I am getting crazy with the results:

LT6015: Simulator hanged and I have to stop it manually. Then I measure Vout=2V when Vin=-2V.
LT1881: Vout=+3.5 when Vin=-2V (see Sim1)
LT1802: vout=+2V when Vin=-2V (see Sim2)

Any logical explanation to this issue?? I am planing to buy the OpAmp LT6015 and do a test on real life to see what is happening exactly...

Regards
 

Not surprising at all. Problem is not reading the datasheets thoroughly. LT1881 is specified as "rail-to-rail output" device, datasheet never claims R2R input operation.
 

FvM said:
Not surprising at all. Problem is not reading the datasheets thoroughly. LT1881 is specified as "rail-to-rail output" device, datasheet never claims R2R input operation.
Click to expand...
Yes, look at the Vcm "Input Voltage Range" in the LT1881 data sheet.
It's within 1V (1.2V over temp range) of the power supply rails, thus it won't work down to 0V input with the minus supply at 0V.
Flote21, you really need to learn how to read data sheets. It'll save you a lot of frustration.:bang:
 

Ahhh ok I get it why the LT1881 is not working....but then why the simulator is hanged with the LT6015?? It will work on real life?? Or there is an issue in the simulation model??
 

flote21 said:
Ahhh ok I get it why the LT1881 is not working....but then why the simulator is hanged with the LT6015?? It will work on real life?? Or there is an issue in the simulation model??
Click to expand...
I'm confused.
In your first post you stated "According to the simulation is working" but now you say it won't simulate. :-?
What's the difference between the two simulations?
 

Because it is working when you finish the simualtion manually. Normally you launch the simulation and after the specific simulation time, the simulator stops and it displays you the result. With the OpAmp LT6015, it is hanged and you have to stoped manually. Then you measure Vout and you get the right result. But I don¨t know if I should trust on this result...

crutschow said:
I'm confused.
In your first post you stated "According to the simulation is working" but now you say it won't simulate. :-?
What's the difference between the two simulations?
Click to expand...
 

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