LM311 with out-of-spec input signal.

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.

Reply to
Sylvia Else
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If the input that is below ground can suck current out of the LM311 substra te, the behaviour of the rest of the LM311 can become unpredictable.

If you can protect the potentially negative going input with a series resis tor, and use a Schottky or Ge diode to provide the current going into that resistor, so that you aren't pulling current out of the LM311 substrate, yo u may be a be able to get away with it.

--
Bill Sloman, Sydney
Reply to
Bill Sloman

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

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Reply to
Tim Williams

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.

Reply to
Frank Miles

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

Reply to
Spehro Pefhany

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.

Reply to
Sylvia Else

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

Reply to
piglet

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

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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.

Reply to
Sylvia Else

If one input is at +2V and the other input is at 0V, the common mode input in this case is = +1 V? no?

Mark

Reply to
makolber

Now you're just being difficult.

The common mode input (at any given moment) is not the same thing as the common mode input range (where it is guaranteed to work with a 'small' differential voltage).

--Spehro Pefhany

Reply to
Spehro Pefhany

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

Reply to
piglet

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

Reply to
ehsjr

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.

Reply to
Sylvia Else

Can you use LM339, (or the dual LM393) instead? These parts behave well even with input slightly below the -ve supply

--
umop apisdn
Reply to
Jasen Betts

Probably could, actually. I was thinking it would be too slow, but it's fast enough. Thanks.

Sylvia.

Reply to
Sylvia Else

in this case is = +1 V? no?

I wasn't meaning to be difficult.

Which is the requirement for the device to work?

1) That the common mode of the 2 input signals combined remains within the specifed common mode input range?

or

2) That each individual input signal taken separatly remains within the specified common mode input range?

It appears that in the OPs case, #1 is met but not #2.

Mark

Reply to
makolber

e

t.

e specified common mode input range?

specified common mode input range?

The second.

Correct. If Sylvia takes either input within 0.3V of the negative rail the more negative base of either of the input PNPs (IN- or IN+) pushes the emit ter of the active NPN in the long-tailed pair close enough to the negative rail to starve the constant current sink (into the 250R resistor) which pro vides the current which is steered to drive the rest of the circuit. See th e circuit diagram on page 3 of the data sheet.

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I'm not interested enough to trace how this will mess up the circuit - it w ould be obvious to Jim Thompson - but it's fairly obvious that it will, som ehow.

--
Bill Sloman, Sydney
Reply to
Bill Sloman

You lose response time to small signal overdrive and also input current on the offending signal will increase. Just hang some resistors on the inputs to keep it in range: V+-1.5 on the high end and V- + 0.5 on the low end.

Reply to
bloggs.fredbloggs.fred

The LM311 is quite sluggish anyway with small overdrive- eg. sub-mV. The datasheet only shows down to a couple mV, and it's already into the glacial range of multiple microseconds. Typical gain is 200K, so it should switch with a few tens of uV.

I doubt it's much worse with the proposed inputs.

--sp

Reply to
Spehro Pefhany

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

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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Reply to
Jim Thompson

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