LM393 Input Common Mode Voltage Range question

[Jon]

Anthony,
The common mode range is the range of common mode voltages for which
the CMRR specification is valid. If you are overdriveing the
comparators differential input, this parameter is less important. For
example, if the common mode input range is +/- 10V and the CMRR is
80dB, then a 10V common mode input can cause the input threshold to
vary by 0.1 mV. If you are trying to detect a 1 mV differential in the
presence of 10V Common mode, this may be significant.
Regards,,
Jon

 

[Jim Thompson]

news:[email protected]... [snip]

I think I understand exactly what you are saying. I'm trying to
understand why the definition of the parameter (in terms of an op-amp)
doesn't say that. IOW, why are the comparators special. If I
understand Jim, the paramater is only important when the inputs are
close to the switching point. For example, if one input is at 3V and
the other input rises to exceed 3V, the output will be predictable. If
the rising input continues to rise (meaning the mathematical average
voltage of the inputs now exceeds the input common mode voltage range
parameter), the output will remain predictable in that it wont suddenly
invert on you. However, you will need to have things back within the
specs before you can have another predictable transition of the output.
Right?

However, if _both_ inputs are above 3V, then all bets are off and
the state of the output won't depend on whether the + input is more
positive or more negative than the - input. In other words, if both
inputs are more positive than 3V, the comparator stops being a
comparator.

Re-stating John Field's statement...

ONE INPUT must NEVER exceed VCM, otherwise the output is
unpredictable.

Like wise there are restrictions below ground, but more complex...
with 339/393 architectures the behavior depends on WHICH input goes
below ground ;-)

...Jim Thompson

 

[Genome]

I've been looking at some op-amp material and I have a question. The
datasheet shows a Max value for the "Input Common Mode Voltage Range"
spec of Vcc-2 over the full temperature range. Ron Mancini says that
this parameter refers to the average voltage at the input pins. If I'm
using a 5V Vcc and have one input biased at 2.5V when the other input
exceeds about 3.5V aren't I exceeding the spec? IOW, it appears that
it's not kosher to drive the other input past 3.5V (3.5V + 2.5V) / 2 =
3V = Vcc-2

Am I understanding this? If so, any recommendations for a single supply
comparator with a bigger common mode input range?

Am have look at am datashit frum national semicunductor where give
representative of circuit bits inside. Input is darlyton sort ov stuff wiv
pnp transisters so point where raisen source suppli stage am 1.2 volt above
most negative input thing.

Raisen source am prob current mirror thing so need 0.6V to werk so need one
input
at least 1.8V below plusative rail for not get upset.

DNA

 

[Jim Thompson]

Am have look at am datashit frum national semicunductor where give
representative of circuit bits inside. Input is darlyton sort ov stuff wiv
pnp transisters so point where raisen source suppli stage am 1.2 volt above
most negative input thing.

Raisen source am prob current mirror thing so need 0.6V to werk so need one
input
at least 1.8V below plusative rail for not get upset.

DNA

That is correct ;-)

...Jim Thompson

 

[Spehro Pefhany]

Am have look at am datashit frum national semicunductor where give
representative of circuit bits inside. Input is darlyton sort ov stuff wiv
pnp transisters so point where raisen source suppli stage am 1.2 volt above
most negative input thing.

Raisen source am prob current mirror thing so need 0.6V to werk so need one
input
at least 1.8V below plusative rail for not get upset.

DNA

IOW, RTFM and LATFS.


Best regards,
Spehro Pefhany

 

[Genome]

IOW, RTFM and LATFS.


Best regards,
Spehro Pefhany

But you am old bast so kno how loadstone werk. Ant sound like crumbpizza
progrimmer what might understood own comfy bum but not have reverend
knowledge ov the 'sparky nail'.

DNA

 

[John Fields]

news:[email protected]... [snip]

I think I understand exactly what you are saying. I'm trying to
understand why the definition of the parameter (in terms of an op-amp)
doesn't say that. IOW, why are the comparators special. If I
understand Jim, the paramater is only important when the inputs are
close to the switching point. For example, if one input is at 3V and
the other input rises to exceed 3V, the output will be predictable. If
the rising input continues to rise (meaning the mathematical average
voltage of the inputs now exceeds the input common mode voltage range
parameter), the output will remain predictable in that it wont suddenly
invert on you. However, you will need to have things back within the
specs before you can have another predictable transition of the output.
Right?

However, if _both_ inputs are above 3V, then all bets are off and
the state of the output won't depend on whether the + input is more
positive or more negative than the - input. In other words, if both
inputs are more positive than 3V, the comparator stops being a
comparator.

Re-stating John Field's statement...

ONE INPUT must NEVER exceed VCM, otherwise the output is
unpredictable.

---
No, what I said was that at least one input must be below VCM for
the output to be predictable.

That is, if one input is below VCM and the other is above VCM, the
output will be as predictable as if both inputs are below VCM.

The ground restrictions apply as well, of course.

 

[Terry Given]

That is correct ;-)

...Jim Thompson

translation:

vat um rite

Cheers
Terry

 

[Terry Given]

IOW, RTFM and LATFS.


Best regards,
Spehro Pefhany

LATFS - nice

Cheers
Terry

 

[Phil Hobbs]

IOW, RTFM and LATFS.

Hmm. Could be "Look at the $&%%#@ schematic", or possibly more
appropriate to s.e.d, it could be "Live at the freak show."

Cheers,

Phil Hobbs