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LM311 Question

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saiello

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Hi All,
I'm using the LM311 voltage comparator in my circuit. Everything works great but I came across a slight anomaly. +ve supply to the 311 is 5V, -ve supply 0V. The +ve ref voltage is selected with a potentiometer ranging within 0V-5V and the -ve input voltage automatically ranges from 0V-5V. All standard stuff. From reading the datasheets and the numerous circuits on the net utilising the 311, I was under the impression that the input and ref voltages can be anything between the -ve and +ve supply voltages and the 311 will operate as specified. What I found was that when I set the ref voltage to below about 3.9V, the 311 will happily switch it's output when the input rises above 3.9V. When the ref voltage is set above this, to say 4.0V, any amount of input voltage will not switch the output. I cannot find any information in the datasheet or on the internet about this. Is this normal or have I blown the 311?

Thanks!
 
The common mode voltage should be specified on the datasheet. It does not include +VDD but does include -VEE. I don't think the LM311 will be hurt by exceeding the common mode range as long as you don't exceed the supply voltage.
 
Hi,
Thanks for the response! ;o) I can't find any reference to common mode voltages in any of the datasheets. Just to clarify, if you take the higher input voltage and subtract the difference between the higher and lower input voltages will that give you the common mode voltage? If there is an upper limit to the CMV, does this then mean that if both of the input signals are above this then the comparator will not behave as designed? If this is the case, then this is what has confused me as it is clearly specified that either of the input signal voltages can swing from -ve to +ve supply, i.e. 0V-5V, and with lack of mention of a common mode voltage, this implies that the comparator will switch with inputs signals right up to the supply voltage. Is an upper CMV limit that is well below the supply voltage ( 3.9V in my case with the 311? ) normal with all comparators, and if so, why is there no mention of this in the datasheets?

Thanks again!
 
There is a graph in the LM311 datasheet from National Semiconductor called Common Mode Limits on page 8, left column, middle of the page. What it is showing you is the range relative to the supply where at least one of the inputs must reside. This input would normally be a reference voltage. It is a function of temperature and is 0.2 to 0.3 V above the negative rail and about 1.1 to 1.2 volts below the positive rail. The problem is you have to know what common mode limits means before the information jumps out at you. It also helps to know how a comparator is built with a differential amplifier input stage. Differential amplifiers don't work to well when both inputs are close to the rails because there is no bias current flowing through the tail.

**broken link removed**
 
Last edited:
Aha, I see it! ;o) This ties in with the result I was getting, i.e. -1.1V below the 5V supply at room temperature. I'm just a beginner as you've probably gathered but I'm learning! Another mystery solved..! ;o)

Thanks!
 
Comparators can be used in either an AC or DC environment. In an AC environment they want both inputs to be in the common mode range. This is usually not a problem since the supplies are normally bipolar and allow substantial headroom. In a DC environment, with a single supply, you can fudge a bit and place the reference inside the common mode range while the other input can go to either rail. It's not really kosher, but it does seem to work. I learned this the hard way on the LM339/LM393.
 
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