Comparator with constant offset

[benta]

I'm working with two voltage levels where I need a comparator to switch when the difference between the two pass 100 mV.
Problem is, the common-mode voltage varies, so I can't make the offset between the two inputs using resistors.

One voltage is DC, the other is 16 kHz pulses.

Can anyone point me in the right direction?
Do comparators exist with built-in constant offset?
Or can you suggest a different way of "injecting" 100 mV into one input?

BTW, supply voltage is 12 V, the signal is somewhere in between.

Thanks,

Benta.

 

[VEBill]

"...when the difference between the two pass 100 mV..."

Is that difference in both directions? In other words, 100mV of hysteresis? If so, then easy (?).

 

[benta]

No, not hysteresis, rather pure DC offset.
But it must be the heat affecting me...
A summing amplifier before the DC input to the comparator can of course add the 100 mV offset.
As usual, this came to my mind when driving home from work (emptying your brain suddenly brings solutions, which is why it's important to take a 5...10 min break regularly).

Thanks for replying, sorry I wasted your time.

Cheers,

Benta.

 

[VEBill]

 

[itsmoked]

You can also just add the offset directly to one of the comparator inputs. Say the the positive side would normally be grounded, instead have it go to your offset voltage.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[benta]

itsmoked:
No, the problem is exactly the varying common-mode voltage. None of the inputs are ground or positive referenced.
Anyway, I've solved it now.

Cheers,

Benta

 

[jimkirk]

Glad you solved it, I'll share this anyway, it may come in handy for someone some day...

Depending on the application, another way to do this is by intentionally unbalancing the input. For example the LM311 comparator has two balance inputs which can vary the input offset over a modest range.