One of the LMV324IDT op amps as a comparator

[dgutierrez]

Posted on November 03, 2015 at 17:13

We have an extra op amp in the quad package that we'd like to use as a comparator to save cost.  Any recommendations?  I'm a little concerned about stability and oscillations.

[Nickname14663_O]

Posted on November 03, 2015 at 17:23

Hello,

You're right that you can perform a comparator fonction with an opamp.

If you are concerned about oscillations, it may mean that you have an input signal that is a little bit slow, thus it will lead to different toggling due to noise on the input signal.

In order to avoid it, I would recommend to add an external hysteresis based on two resistors.

We developed on application note that highlights the main points on comparators.

You can have a look at the section 4.2, page 17, that deals with the external hysteresis.

This application note is available here :

http://www.st.com/st-web-ui/static/active/en/resource/technical/document/application_note/DM00050759.pdf

Regards,

Sylvain

[dgutierrez]

Posted on November 03, 2015 at 17:39

Yep, we'll definitely be using hysteresis in the manner you described.  I'm more referring to using an op amp as a comparator instead of a dedicated comparator IC, and any recommendations that ST might have on avoiding misbehavior.  The possibility for misbehavior is described in section 1 ''Comparator substitution by an op amp'' of the app note you provided.

Thanks

[Nickname14663_O]

Posted on November 04, 2015 at 08:18

Hello,

I understand your point, but the main thing you should take into account is the external hystereis.

You definitely have the possibility to use an op-amp as a comparator (wherease it is not possible to use a comparator as an opamp).

The trade-off is that for a similar current consumption between an op-amp and a comparator, you will have a better speed (propagation delay and slew-rate) with a comparator.

But if the slew-rate of the opamp is enough in your application you can use the unsued channel as a comparator to save space and money.

Moreover, regarding the possiblity of misbehavior, the external hysteris will avoid oscillations and will provide you with a robust design.

Regards,

Sylvain

[dgutierrez]

Posted on November 04, 2015 at 16:22

Got it, thanks for the feedback (no pun intended).

[Nickname14663_O]

Posted on November 05, 2015 at 16:07

Hello,

Note that the LMV324 has a Vid limitation. It means that if there is an input voltage difference between the inputs bigger than 1V, there will be a current that will go thru the internal diodes.

Thus it is recommended to add a resistor in-series with the input in order to limit the current that is going thru the back to back diodes.

To highlight it, you can find here below a simplified schematic of the op-amp (the ESD diodes are included inside the opamp)

If it is an issue for your design, it is also possible to use the TSV524 that has no Vid limitation with similar performances (a little bit faster with a lower consumption).

Regards,

Sylvain