LM311 with out-of-spec input signal.

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I need to detect when a signal rises above a threshold. The signal  
starts at ground. I want to use a LM311, with its -ve supply voltage at  
ground (to avoid having to generate a -ve supply) which means that the  
signal is sometimes below the specified minimum (but never below ground).

Can I assume that a LM311 will give the correct output provided the  
threshold on the other input is above the specified minimum, even though  
the signal is not?

Looking at the schematic for the LM311 suggests to me that this is OK,  
but are there any gotchas?

Sylvia.

Re: LM311 with out-of-spec input signal.
Input common mode range does not include V- rail (only 0.5V + V-), so you  
need an offset for the signal, or a different part like LM393 that does  
include -0.3V from V-.

Tim

--  
Seven Transistor Labs
Electrical Engineering Consultation
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Re: LM311 with out-of-spec input signal.
On Mon, 27 Apr 2015 13:27:33 +1000, Sylvia Else wrote:

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The data sheet says it works for a single 5V supply, and has example  
circuits that also avoid the negative supply.  By "below the specified  
minimum" do you mean you're violating the input common mode range?  That  
would likely be a problem.

Re: LM311 with out-of-spec input signal.
On 27/04/2015 1:53 PM, Frank Miles wrote:
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This seems remarkably difficult to express properly in words...

Suppose that both the + and - inputs are within the common mode range,  
with the + input higher than the - input. So the output is high. Now let  
the + input gradually fall. As it falls below the - input, the output  
goes low. Now, let the + input continue to fall. I need the output to  
stay low, even when the + input falls to zero, which is outside the  
common mode range.

Similarly, in reverse, as the + input rises from zero, going into the  
common mode range, and eventually rising above the unchanging - input.

I notice in the Texas Instruments datasheet

http://docs-asia.electrocomponents.com/webdocs/001c/0900766b8001cf48.pdf

Figure 15, TTL interface

does appear to envisage operating with one input sometimes having a  
level equal to the -ve power input (both being ground).

Sylvia.


Re: LM311 with out-of-spec input signal.
On 27/04/2015 12:43, Sylvia Else wrote:
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You are correct. This will happen just as you want so long as the -  
(inverting) input is above the 300mV CM lower limit.

By inspecting the internal circuit given in the datasheet you can see  
that things only get risky when *both* inputs are below 300mV when the  
second stage npns get starved of base current.

piglet


Re: LM311 with out-of-spec input signal.
On Mon, 27 Apr 2015 13:27:33 +1000, the renowned Sylvia Else

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Hi, S.  

I think it will work fine as long as one input is within the CM range
and both are above GND, but that said, I'd test it. Response time
might be a bit different, but probably not disastrously so.    

--Spehro Pefhany




Re: LM311 with out-of-spec input signal.
On 27/04/2015 2:25 PM, Spehro Pefhany wrote:
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Yes, I hadn't thought about the response time. With the input pulled  
down, the input transistor becomes saturated, which begs the question of  
how long it might take to unsaturate :(

Sylvia.



Re: LM311 with out-of-spec input signal.
On 27/04/2015 05:57, Sylvia Else wrote:
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The input pnps will not saturate, when input is at 0V it is same  
potential as their collectors - so a bit like a diode but not saturated.  
The real problem is the second stage npns are starved of drive - hence  
the cm restriction to 300mV above 0V. However if at least one input is  
above 300mV then I think normal comparator action will be OK.

piglet



Re: LM311 with out-of-spec input signal.
On 4/26/2015 11:27 PM, Sylvia Else wrote:
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Can you do this? (one diode to make a virtual ground with the
chip negative supply one diode drop below it)


  V+ ----------------+
                     |
  Gnd ---+         |\|
         |       --| \
         |         |  \
      [diode]      |  /
         |       --| /
         |         |/|
         |           |
  V- ----+-----------+

Ed

Re: LM311 with out-of-spec input signal.
On 28/04/2015 2:17 PM, ehsjr wrote:
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I'd have several amps going through the diode, which makes the "diode  
drop" rather device dependent, so I'd end up having to make a manual  
adjustment to the threshold.

I could replace the diode with a voltage regulator (which would probably  
need a heat sink) - but then I might as well use the cheaper charge pump  
solution to provide a negative rail to the comparator.

Sylvia.

Re: LM311 with out-of-spec input signal.
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Can you use LM339, (or the dual LM393) instead?
These parts behave well even with input slightly  
below the -ve supply

--  
umop apisdn


Re: LM311 with out-of-spec input signal.
On 28/04/2015 4:58 PM, Jasen Betts wrote:
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Probably could, actually. I was thinking it would be too slow, but it's  
fast enough. Thanks.

Sylvia.


Re: LM311 with out-of-spec input signal.
On Mon, 27 Apr 2015 13:27:33 +1000, Sylvia Else

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If one input is above the minimum, but within the specified common
mode range, no untoward current diversions occur, so you're OK.

However, the "rising" input signal may not exceed the positive common
mode range or there will be a possible phase flip.
        
                                        ...Jim Thompson
--  
| James E.Thompson                                 |    mens     |
| Analog Innovations                               |     et      |
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