0
\$\begingroup\$

This is probably a basic question which i'm stuck with:

Why should there be offset at the input signals?

=> We have offset voltage of 1V on both the signals, when i remove the offset i get very weird result (result are given below)!

enter image description here

  1. Weird result (without offset) enter image description here

  2. normal result (with offset) enter image description here

\$\endgroup\$
4
  • \$\begingroup\$ I think you're going to have to clarify what you mean by "very weird results". \$\endgroup\$
    – brhans
    Nov 28, 2016 at 20:06
  • 1
    \$\begingroup\$ Also, either add an -ve supply or read the datasheet very carefully abouth the common mode input voltage range - it may not include the -ve rail. \$\endgroup\$
    – user16324
    Nov 28, 2016 at 20:10
  • \$\begingroup\$ @brhans just uploaded the results ..... :) \$\endgroup\$
    – Hilton Khadka
    Nov 28, 2016 at 20:13
  • \$\begingroup\$ For 5Vdc reduce both V1,V2 to (5-1.5)=3.5Vpp max if both are biased at (3.9-0.3)/2= 1.8Vdc otherwise if biased at 2.5V max swing= 2.5Vpp MAX. approx depending on tolerances \$\endgroup\$
    – Tony Stewart EE75
    Nov 28, 2016 at 20:56

1 Answer 1

Reset to default
4
\$\begingroup\$

I don't know what weird results you are getting, it could be that you didn't check the input common range.

enter image description here

The input common mode voltage (average voltage of the inputs) has to be about 0.2V above the negative rail.

As pointed out by Carloc, a detailed description of the behavior of the comparator over the input voltage range can be found in the datasheet. I've added Carloc's screenshot below

enter image description here

\$\endgroup\$
4
  • \$\begingroup\$ just uploaded the pictures :D ... have a look please \$\endgroup\$
    – Hilton Khadka
    Nov 28, 2016 at 20:13
  • \$\begingroup\$ It looks like your signal is not only outside the common mode range but also below the negative rail. That does not work, the results you are getting are ok. You have to fix the input signal, i.e. add an offset. \$\endgroup\$
    – Mario
    Nov 28, 2016 at 20:17
  • 2
    \$\begingroup\$ Although you definition of common mode voltage as average is generally okay, it does not apply in this case and in any such opamps or comparators specifications. Both V+ and V- inputs have to be within those limits as clarified here \$\endgroup\$
    – carloc
    Nov 29, 2016 at 8:10
  • \$\begingroup\$ Good point, I'll update my answer! \$\endgroup\$
    – Mario
    Nov 29, 2016 at 16:33

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.