Hysteresis in window comparator?

[cupoftea]

Hi,
Can you think of the best way to put hysteresis into this window comparator?
LTC1841 is open collector.

(attached is PDF schem and LTspice sim)

 

[KlausST]

Hi,

the usual way: "positive feedback".
--> A resistor from output to non_inverting input.

If you want the hysteresis individually then you need to separate the outputs. Maybe with a (schottky) diode each and individual pullups.

BTW: Why the PDF? Why not directly upload the picture to edaboard?

Klaus

--- Updated Feb 16, 2022 ---

Update:

Sorry - that was dumb of me not to read the datasheet before answering.
--> The LT1841 has built in hysteresis feature.

Klaus

 

[cupoftea]

Thanks, ill be sure to upload jpeg in future.

 

[KlausST]

Hi,

I just reviewed your schematic.

In my (funny) opinion: A circuit without a capacitor is no circuit ;-)

So in case you encounter any instabilites, then maybe you just need to decouple the power supply and the DC levels.
Maybe even the measurement signal path with a suitable C.
All Cs should be referenced to the same common GND point. Or better use a solid GND plane. (No copper pour)

Klaus

--- Updated Feb 16, 2022 ---

Thanks, ill be sure to upload jpeg in future.

* JPG for photos. Quality of 30% .. and in most cases 100kBytes is sufficient.
* PNG for screenshots of schematics, diagrmas, text and so on.

Klaus

 

[BradtheRad]

From what I understand, hysteresis is a method to make one comparator act like two as in your window comparator.

When the input volt level causes a change of state, afterward the input must meet a different new volt level in order to again change state.

The two trigger levels are automatically determined by the proportion of positive feedback tapped from the voltage at the output.
It's not necessarily easy to adjust hysteresis so that you obtain a desired range of operation with one device.

Sometimes it's easier to build a window comparator using two devices, and set trigger levels wherever you wish.

 

[Easy peasy]

there is no hysteresis in the schematic as far as I can see ( AFAICS )

 

[KlausST]

Hi,

Yes, because the schematic is incomplete. It doesn't show how the IC's hysteresis feature is set up.

The datasheet shows window comparator schematics. Also they use the IC_internal REF, which usually is more accurate, precise and more low noise than a supply voltage. I recommend to use the REF, unless the application really needs to refer to the VCC level.

Klaus

 

[FvM]

The LT1841 has built in hysteresis feature

According to my understanding of the datasheet, LTC1841 has no built-in hysteresis. The datasheet is somewhat confusing in this regard, the title lines give the impression that the features apply to all LTC184x types, but they don't. You can however simply add a posistive feedback resistor to the schematic appended in post #1.

 

[KlausST]

Hi,

Oh, my bad. Only the xx2 and xx3 have built in hysteresis.

Thanks for clarifying this.

Klaus