Window comparator outputs

[Kisen]

Hi

Below is a schematic of a window comparator.
I followed this schematic from one i found online.

As i understood it, when the input voltage was within the window, both outputs from IC4/IC5 would be high. Leaving the window on either side results in that opamp output going low.

In this circuit, i get the opposite behaviour. IC4/IC5 are output low, until leaving the window when one goes high.
This results in the NAND gate not being appropriate. An OR gate will be needed here instead.

My confusion is why, based on this schematic, do the outputs remain low while inside the window?

Thanks in advance for your help.

 

[danadak]

Here is example and explanation using AND on output, so NAND just the inversion.

Window Comparator | Analog-integrated-circuits || Electronics Tutorial

Window Comparator | Analog integrated circuits - Electronics Tutorial

www.electronics-tutorial.net

Same circuit, AND, just using output "wired AND". So a NAND just the inverted
output of this circuit.

https://www.ti.com/lit/an/sboa221a/sboa221a.pdf?ts=1646478477311&ref_url=https%253A%252F%252Fduckduckgo.com%252F

Regards, Dana.

 

[eTech]

Hi

Below is a schematic of a window comparator.
I followed this schematic from one i found online.

As i understood it, when the input voltage was within the window, both outputs from IC4/IC5 would be high. Leaving the window on either side results in that opamp output going low.

In this circuit, i get the opposite behaviour. IC4/IC5 are output low, until leaving the window when one goes high.
This results in the NAND gate not being appropriate. An OR gate will be needed here instead.

My confusion is why, based on this schematic, do the outputs remain low while inside the window?

Thanks in advance for your help.

View attachment 136031

Do yourself a favor and use real comparators, like LM339, instead of opamps.
Then you won't need a gate.

 

[crutschow]

What behavior do you want?

Then you won't need a gate.

The reason for that is most common comparators have an open-collector output, so can be wired together with one pull-up resistor to give an AND output function.