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op amp cmos conversion - offset setting

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anarkie

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op amp cmos conversion

Hello? I have a simple question regarding op amp cmos conversion.
I use NI Multisim and the schematic diagram of its datasheet doesn't produce what I expected. It is for a cmos comparator.

The output must be logic 0. I double checked the schematic and I guess it is correct. But I don't get what was wrong in there.

This shows a right output
c7dbe9db73.jpg


But this one produces logic 1. nV or pV is considered as a ground, it must produce logic 0.
10a93e60fe.jpg



Thanks,
 

Re: op amp cmos conversion

anarkie said:
... The output must be logic 0.
You're right, if you use an ideal comparator (your first picture). A real comparator (your second picture), however, may have an input offset which has the same or even larger offset voltage than your input voltage difference of -2mV. If your input voltage difference is less than the comparator's input offset voltage, its output won't switch to the expected state.
 

Re: op amp cmos conversion

erikl said:
anarkie said:
... The output must be logic 0.
You're right, if you use an ideal comparator (your first picture). A real comparator (your second picture), however, may have an input offset which has the same or even larger offset voltage than your input voltage difference of -2mV. If your input voltage difference is less than the comparator's input offset voltage, its output won't switch to the expected state.

Wow, thanks for the tip. I try to fix this offset setting.
 

Re: op amp cmos conversion

erikl said:
anarkie said:
... The output must be logic 0.
You're right, if you use an ideal comparator (your first picture). A real comparator (your second picture), however, may have an input offset which has the same or even larger offset voltage than your input voltage difference of -2mV. If your input voltage difference is less than the comparator's input offset voltage, its output won't switch to the expected state.

But I wonder if I have to change configuration in corresponding offset pins in the 2nd pic or add some voltage manipulation/ configuration circuit on both input pins.

Thanks,:)
 

Re: op amp cmos conversion

anarkie said:
I wonder if I have to change configuration in corresponding offset pins in the 2nd pic or add some voltage manipulation/ configuration circuit on both input pins.
Actually the input offset voltage of a comparator always defines a dead band within that range you cannot expect a correct output behaviour. Either
  • your input levels are safely beyond or below this range, or
  • you use a comparator with a sufficiently low input offset voltage (available from many semiconductor manufacturers), or
  • you guarantee such a low input offset voltage by design, with well-known methods of input offset cancellation, e.g. switched-capacitor (SC) circuitry (same as the semi companies use)
 

Maybe you should begin by characterizing the input transfer
function and see whether it is broken, or just somewhat
bent. By probing around you can determine where any
differential error appears, whether you have useful gain
and so on.

That sure doesn't look like CMOS to me. Do you believe
you even have the right models and schematic / netlist?
 

erikl said:
anarkie said:
I wonder if I have to change configuration in corresponding offset pins in the 2nd pic or add some voltage manipulation/ configuration circuit on both input pins.
Actually the input offset voltage of a comparator always defines a dead band within that range you cannot expect a correct output behaviour. Either
  • your input levels are safely beyond or below this range, or
  • you use a comparator with a sufficiently low input offset voltage (available from many semiconductor manufacturers), or
  • you guarantee such a low input offset voltage by design, with well-known methods of input offset cancellation, e.g. switched-capacitor (SC) circuitry (same as the semi companies use)

Thanks for reply!!! I am reading one of the best CMOS op amp article found in IEEE and Smith & Sedra book. Maybe looking forward to manufactures' datasheets providing good schematics and small offset voltages.

Added after 1 minutes:

dick_freebird said:
That sure doesn't look like CMOS to me. Do you believe
you even have the right models and schematic / netlist?

No, it is just a schematic. But it can be converted into a CMOS base in Tanner tools L-Edit any time.
 

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