What to do with Unused comparator OpAmps?

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.

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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.

Click to expand...

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

Click to expand...

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

I´m not sure if every comparator can handle

Click to expand...

I'm sure some don't. But the question was about TLV170x.

Hi KlausST,

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




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