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What to do with Unused comparator OpAmps?

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flote21

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Hello guys!

I am wondering about how I should connect unused Comparator OpAmps. According to the page number 32 of the next litterature (Bruce Trump):

http://granasat.ugr.es/wp-content/uploads/2018/01/slyt701.pdf

Unused general purpose OpAmps should be connected as ground followers...

But how should i connect dedicated unused comparator OpAmp like the TLV1704AIPW?

Thanks in advance!
 

Hi,

I'd set the inputs with resistive dividers to obtain a high impedance output and in that way not have the device sinking current for nothing/wasting energy pointlessly.

Nice comparator with no phase reversal, by the way, thanks for bringing it to my attention.

- - - Updated - - -

Hi,

Just saw that the TI pdf is about op amps, not comparators - while it's a delightful conversation topic (where does an op amp end and a comparator truly begin, is a comparator just an uncompensated op amp?, etc.) you are not using an op amp I think.

Look at the picture on page 32: op amp voltage follower (internally with some push-pull output stage or other). Now look at any old comparator schematic or typical application and spot the big difference: pull-up resistor (on an open collector output). Op amps are analog and linear input and output (hopefully) whereas the comparator is an analog linear input with a "cheapo" NPN inverter digital output.

All that waffle to explain why I think the ground voltage follower is not a good idea for an unused comparator.
 
Hi,

I am wondering about how I should connect unused Comparator OpAmps.
There us no "comparator Opamp".
It's either a comparator or an Opamp. You should treat them differently.

An Opamp output is not designed to touch the supply rails, thus bias the noniverting input to supply/2 and connect output to inverting input.
A comparator output is not designed to stay somewhere inbetween supply rail, thus use a three resistor string between the supply rails an connect the intermediate nodes to the inputs. Don't feedback the output.

Klaus
 
I doubt that TLV170x quiescent current will change much with input voltage. Biasing the inputs to +/- power supply should work as well.
 
Great! everyhing is clear now. I would tie with a series 10k resistor the Vin(+) to Vcc and the Vin(-) to -Vcc and the output floating. Greetings
 

Hi,

Great! everyhing is clear now. I would tie with a series 10k resistor the Vin(+) to Vcc and the Vin(-) to -Vcc and the output floating. Greetings
I´m not sure if every comparator can handle
* V_in+ tied to VCC
* and V_in- tied to -Vcc
Thus the "three resistor solution" is a solution that will work for every comparator:
10k - 100R - 10k

Usually on should avoid "floating" signals, even floating outputs.
The risk for a floating comparator output causing problems is low...
Comparators with push-pull-output don´t suffer from floating output.

For Open_collector / open_drain output comparator is recommend:
* make "V_in+ < V_in-" then the output is LOW, not floating.
* for "V_in+ > V_in-" a pullup or pulldown at the ouptut prevents from floating..

Klaus
 

Hi KlausST,

when you mean three resistor solution is this kind of solution:

solution.png


thanks
 

Hi,

Yes, if +15V and GND are the supplies for the comparator.
For open drain/ open emitter I recommend to interchange the inputs to get a low driven output.

Klaus
 

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