Comparator IC with negative input voltage

Thread Starter

mos_6502

Joined Dec 11, 2017
28
Hi to all,

I search for a comparator IC that can support a negative voltage as input (-3.3v as minimum voltage and +3.3v as maxmimum input voltage).
Possibly, it should be in a TSOP package.

I used the MC33079 for this purpose, but this IC are not in TSOP package.
Also, I used the TLC3704 but I noticed that this comparator do not support for negative input voltage.

Can you help me?
 

WBahn

Joined Mar 31, 2012
25,068
The common mode input voltage range for TLC3704 is GND to VDD-1V. If you connect GND to -3.3V, that give an input voltage range of -3.3V to 2.3V.
Though it would also mean that the output would be down around -3.3 V. If you need the output to be around 0 V, then you need to do something to shift or clip it.
 

WBahn

Joined Mar 31, 2012
25,068
But is there not a comparator that accepts in input also a negative voltage?
Negative relative to what?

The comparator, like any IC, has to work relative to the voltage on one of its pins. The vast majority of ICs use pad protection circuits that clamp input voltages to the range of the power rails supplying it.
 

Thread Starter

mos_6502

Joined Dec 11, 2017
28
Negative relative to what?
I clearly wrote that the input voltage should be in the range of -3.3 to +3.3 volts.

The comparator, like any IC, has to work relative to the voltage on one of its pins. The vast majority of ICs use pad protection circuits that clamp input voltages to the range of the power rails supplying it.
My question is Always the same: is there not a comparator that accepts in input also a negative voltage?
 

dl324

Joined Mar 30, 2015
9,577
My question is Always the same: is there not a comparator that accepts in input also a negative voltage?
That's an unreasonable expectation.

TI seems to have dumbed down their datasheets and don't include circuit diagrams. For the device you want to use, at least they show the input protection circuitry so you can see why you can't go more then 0.3V below GND:
upload_2019-6-25_7-55-8.png
If you take the input below -0.3V, D3 is forward biased.

Here's a schematic for LM393 that shows why the input can't be more than a diode drop below the GND pin potential:
upload_2019-6-25_7-57-45.png
The datasheet specifies the maximum negative input voltage to be -0.3V. But common mode input voltage range only goes down to ground.
 

WBahn

Joined Mar 31, 2012
25,068
I clearly wrote that the input voltage should be in the range of -3.3 to +3.3 volts.



My question is Always the same: is there not a comparator that accepts in input also a negative voltage?
Again, negative relative to what?

Voltages are ALWAYS relative to some reference potential -- it's part of the definition of what voltage means.

If your comparator negative supply is at whatever you are calling 0 V, then you will be hard pressed to find a comparator whose input should be taken more than a diode drop below the negative rail (and even then it won't work properly, you just won't damage it). Look at the data sheet for you MC33079, it's the same way. The inputs should not be taken to voltages outside the power rails and the device won't work properly unless you have something a bit more than 1 V of margin (so if your negative rail is -5 V, it may not work properly if the inputs are below about -4 V).
 

Thread Starter

mos_6502

Joined Dec 11, 2017
28
Again, negative relative to what?

Voltages are ALWAYS relative to some reference potential -- it's part of the definition of what voltage means.
In my country ALL tensions are related to GND.

[…]Look at the data sheet for you MC33079, it's the same way[…]
I'm sorry, I don't understand well.
In the Mc33079 datasheet, on page 3 I read that the input voltage can be a maximum of +/- 15V.Note 1 says that it should not exceed the supply voltage. I've never done this. Beyond my interpretation, however I made a circuit with MC33079 and it works well.
 

Thread Starter

mos_6502

Joined Dec 11, 2017
28
So how is a chip supposed to know what the GND is in your country?
I bet it's the same thing in your country too.

Seriously, I don't understand what you mean and where you want to go.

If you mean that the Chip does not have a connection to GND, this is true, but as I told you, I did not find it difficult to carry it out with an operational as the 33079.
 

Bordodynov

Joined May 20, 2015
2,469
You can use four additional resistors with the same rating (e.g. 100kOhm or 1MOhm). Only two pairs for each input. In a particular pair, one resistor (top) must be connected to a positive power supply and the other end to the comparator input. The signal is transmitted through a second resistor. In case of symmetrical supply, the resistors are the same.
 

ronv

Joined Nov 12, 2008
3,770
The specification is written for the input voltage in relationship to Vcc-.
If you Vcc- is ground you should not have an input at -3.3 volts. It should not be more negative than -.3 volts.
If your Vcc- is -3.3 volts you would be fine. This is why people are asking what the negative supply voltage is.
They do make IC's with built in protection such as this one.
https://www.mouser.com/datasheet/2/609/ADA4177-1_4177-2_4177-4-607393.pdf
But often just a clamp diode on the input will work. Read this:
https://www.mpdigest.com/2016/02/22/op-amp-input-over-voltage-protection-clamping-vs-integrated/
 
Top