LM311 with out-of-spec input signal.

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.

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 Website:

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.

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

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.

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

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

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.

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

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

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

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

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.

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

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

Sylvia.

in this case is = +1 V? no?

I wasn't meaning to be difficult.

Which is the requirement for the device to work?

or

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

Mark

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.

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.

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.

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

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