# LM139AD Comparator Negative Inputs

I am wondering if the LM139AD comparator from TI can tolerate negative voltages at its inputs. I see that it can operate from a dual supply and that a supply rail can be negative? Is the same true for its inputs?

I have linked the datasheet here: https://www.ti.com/lit/ds/symlink/lm339.pdf?HQS=dis-dk-null-digikeymode-dsf-pf-null-wwe&ts=1645409876374

• It can tolerate input voltages down to its negative supply voltage (GND). It's up to you what voltage you connect that pin to. Feb 21, 2022 at 21:45
• The table shown gives the answer to your question. The answer is no. Neither input can be more than -0.3V below the negative rail. Feb 21, 2022 at 21:46
• @TonyStewartEE75 so it is referencing the negative rail? In which case if my negative rail was -18 V, it would be ok to have down to -18.3 V at the input? Feb 21, 2022 at 21:49
• Table specifies absolute maximum ratings - circuit won't be destroyed when you apply -.3V at the input, but it do not have to meet its specification. Few tables below you can find 'Electrical characteristics' section. In that you can find that input common mode voltage must be greater than 0 Feb 21, 2022 at 21:51
• @ColeFehr Yes. All voltage values, except differential voltages, are with respect to network ground. E.g. V- = -18V and V+ =+18V is perfectly fine (dual supply). Feb 21, 2022 at 21:57

## 2 Answers

The comparator has no idea what is negative, because it doesn’t know what ground is. So the question doesn’t make much sense. If you were the comparator, how would you tell that a voltage is negative relative to some ground? You couldn’t. The comparator has two power input pins labeled V- and V+. The first must be negative relative to the second, but there’s no requirement for any particular relation to any other voltage elsewhere in your system. As long as the comparator’s inputs and outputs are within the bounds of the potentials put on V- and V+, the comparator will not be damaged.

The comparator’s input voltage range is between the supply rails, i.e. from V- to V+. That’s all. If you power the comparator from +/-12V, you can supply the inputs with anything from -12V to +12V, but the meaning of what’s above or below ground is only between you and the design. The comparator simply doesn’t have a dedicated ground reference and nothing in its simple circuit has any reference to ground.

1. The maximum supply voltage is 36 V on Vcc relative to the negative supply.
2. The maximum differential input voltage is ±36 V. That means that you can have
• V+ = 36 V while V- = 0 V
• V+ = 0 V while V- = 36 V
3. The minimum input voltage can be extended to -0.3 V but the maximum is Vcc.
• Would recommend that you rather refer to Recommended Operating Conditions and Electrical Characteristics tables in your answer instead of the values from the Absolute Maximum Ratings table the OP posted in the question. As written some might think that your answer implies normal device operation at those levels... Feb 21, 2022 at 23:06