Maker Pro
Maker Pro

LM339 comparator input levels

D

David Novak

Jan 1, 1970
0
I find two input voltage specifications in the LM339 data sheet, Vidr
(input differential range = 36V) and Vicmr (input common mode range,
-0.3V to Vcc). I understand Vidr, but how do I interpret Vicmr? Is this
stating the voltage range of the inputs if they move together at the
same rate? In other words, the DC offset that effects both inputs equally?

Since the Vidr specification does not refer to Vcc, it appears that I
can set IN- = 1V, Vcc = 5V and let IN+ swing from 0V to 36V. Is this
correct? I am worried about letting either input exceed Vcc.

Thanks,
David
 
J

Jamie

Jan 1, 1970
0
David said:
I find two input voltage specifications in the LM339 data sheet, Vidr
(input differential range = 36V) and Vicmr (input common mode range,
-0.3V to Vcc). I understand Vidr, but how do I interpret Vicmr? Is this
stating the voltage range of the inputs if they move together at the
same rate? In other words, the DC offset that effects both inputs equally?

Since the Vidr specification does not refer to Vcc, it appears that I
can set IN- = 1V, Vcc = 5V and let IN+ swing from 0V to 36V. Is this
correct? I am worried about letting either input exceed Vcc.

Thanks,
David
yes, i think you have it.
CMR = Common mode rejection.
Simple test.
Take both inputs driven by the the same source with the unit
set up to have equal gain on both inputs.
the Rejection ratio is that of the difference that changes as you
increase the input.
the ideal Op-amp would have 0 change on the output from low to high
on both inputs.
----
and yes, you can allow it to swing to 36 volts.

and if the question ever comes up about offset voltages.
that usually means if both inputs are set to 0 for example.
in a +/- Dual rail supply, the offset is usually the maximum
voltage that should appear at the output.
but then again, i've seen batches of cheap op-amps actually
perform far lower in the offset than specified in the specs.
THe LN741 has offset inputs for bias to balance that problem, that
was nice to have to tweak up the problem how ever, you had to maintain
a steady temperature around the chips once set.
 
J

John Popelish

Jan 1, 1970
0
David said:
I find two input voltage specifications in the LM339 data sheet, Vidr
(input differential range = 36V) and Vicmr (input common mode range,
-0.3V to Vcc). I understand Vidr, but how do I interpret Vicmr? Is this
stating the voltage range of the inputs if they move together at the
same rate? In other words, the DC offset that effects both inputs equally?

Since the Vidr specification does not refer to Vcc, it appears that I
can set IN- = 1V, Vcc = 5V and let IN+ swing from 0V to 36V. Is this
correct? I am worried about letting either input exceed Vcc.

You are correct. It is fine to exceed Vcc with either
input. This device is fairly unusual in this respect.
 
E

Eeyore

Jan 1, 1970
0
Jamie said:
and yes, you can allow it to swing to 36 volts.

No you *can't* !

Don't exceed Vcc on either input.

Graham
 
E

Eeyore

Jan 1, 1970
0
John said:
You are correct. It is fine to exceed Vcc with either
input. This device is fairly unusual in this respect.

Taking a look at the data sheet you would appear to be right. In which case the
common mode input votlage spec is slightly misleading.

Graham
 
J

John Popelish

Jan 1, 1970
0
Eeyore said:
John Popelish wrote:

Taking a look at the data sheet you would appear to be right. In which case the
common mode input votlage spec is slightly misleading.

Not really, Common mode voltage range covers the comparator
effectively deciding which of the two inputs is more
positive. Differential voltage range has nothing to do with
comparison, except that it remains correct (no phase
reversal) as long as at least one of the two inputs also
remains inside the common mode voltage range.

This ability of the lateral PNP input transistors of the
LM339 and LM393 (dual) can come in very handy when the
signals are not derived from the same supply as the one
feeding the comparators (i.e. 24 volt field signals being
converted to 5 volt logic levels).
 
J

Jon

Jan 1, 1970
0
Vicmr is the absolute maximum voltage range, beyond which the device
will be damaged. The operational common mode range is given in the
"Electrical Characteristics" section of the data sheet, and is the
input common mode voltage range over which operation is guaranteed.
This range is given in terms of Vcc. Unfortunately, the LM339 data
sheet does not give the Common Mode rejection ratio (CMRR), so you can
not determine how much the input offset voltage will change over this
common mode input range.
 
D

David Novak

Jan 1, 1970
0
David said:
I find two input voltage specifications in the LM339 data sheet, Vidr
(input differential range = 36V) and Vicmr (input common mode range,
-0.3V to Vcc). I understand Vidr, but how do I interpret Vicmr? Is this
stating the voltage range of the inputs if they move together at the
same rate? In other words, the DC offset that effects both inputs equally?

Since the Vidr specification does not refer to Vcc, it appears that I
can set IN- = 1V, Vcc = 5V and let IN+ swing from 0V to 36V. Is this
correct? I am worried about letting either input exceed Vcc.

Thanks for all the help! I received the following from an ON Semi FAE.

"On page 6 of the data sheet it states (clarifies) that Vdif can indeed
be greater than the supply, as long as neither input goes negative by
more than .3 V.

The common mode range is the input voltage where the part will function
properly. This device has PNP inputs so once the input signal
approaches the positive supply rail (within 1.5 - 2.0 V)the transistors
turn off. So if you have a 5 V supply and the inputs are 0 V and 36 V,
it will work ok since the Vdif is met and one input is within the common
mode range. If the input signals are 5 and 36 V, Vdif is met but the
common mode isn't so the output will be unknown. Neither of the input
transistors are on so the output is not controlled. However, the part
won't be damaged."

I had completely missed this page in the data sheet. It is worth noting
that the previous paragraph on the same page recommends input resistors
of < 10K and a < 10mV positive feedback.

David
 
J

Jim Thompson

Jan 1, 1970
0
I find two input voltage specifications in the LM339 data sheet, Vidr
(input differential range = 36V) and Vicmr (input common mode range,
-0.3V to Vcc). I understand Vidr, but how do I interpret Vicmr? Is this
stating the voltage range of the inputs if they move together at the
same rate? In other words, the DC offset that effects both inputs equally?

Since the Vidr specification does not refer to Vcc, it appears that I
can set IN- = 1V, Vcc = 5V and let IN+ swing from 0V to 36V. Is this
correct? I am worried about letting either input exceed Vcc.

Thanks,
David

Your last paragraph is correct.

However the specification is confusing. Inputs CAN exceed VCC but,
for proper comparator decision output, ONE input must be below VCC by
about 1.5V. (The National data sheet shows this.)

...Jim Thompson
 
J

Jamie

Jan 1, 1970
0
Eeyore said:
Jamie wrote:




No you *can't* !

Don't exceed Vcc on either input.

Graham
What an asshole, Go pound sand you puke head.
 
J

Jim Thompson

Jan 1, 1970
0
What an asshole, Go pound sand you puke head.

I knew Eeyore was ignorant, but I guess I mistakenly presumed he could
at least read a data sheet?

...Jim Thompson
 
J

joseph2k

Jan 1, 1970
0
Jamie said:
What an asshole, Go pound sand you puke head.

Study the datasheet. Graham is correct on this one. It really pays to note
the test conditions for each paramater.
 
E

Eeyore

Jan 1, 1970
0
joseph2k said:
Study the datasheet. Graham is correct on this one. It really pays to note
the test conditions for each paramater.

The datasheet is slightly confusing in this respect actually. Note 8 IIRC
explains in more detail. It's really a question of being very precise about the
measurement conditions.

Graham
 
J

Jim Thompson

Jan 1, 1970
0
Study the datasheet. Graham is correct on this one. It really pays to note
the test conditions for each paramater.

Except that Graham is NOT correct.

"Study the data sheet"? Indeed!

In the positive direction, as long as at least ONE input is below
VCC-1.5 the comparator will output correct results. The other input
can be well above VCC, but can not exceed 36V.

...Jim Thompson
 
Top