Signal offset increases as volt/div decreases and signal changes with offset

[muawiya]

Hello,

This is on a Lecroy 64XI-A scope, though I've had this happen on some tek scopes as well.

I am probing the same signal and only changing the volt/div & noticed that when I decreased the volt/div that the signal offset would increase. Can't think of an explanation and was wondering if someone can explain this (see first 2 attachments)

I also can't explain the last 2 attachments where changing the offset (-800mv to -1.58V) changed what the scope was plotting.

[srb1954]

It looks like you are grossly overloading the scope amplifiers when you decrease the scale factor such that the waveform considerably exceeds the screen height.

This overloading would cause the internal stages of the the amplifiers to clip possibly changing normal bias conditions and causing apparent signal offsets.

[TimFox]

Traditional oscilloscopes have a "balance" pot on each input (typically a screwdriver adjust for modern units).  If the balance is not correct, then even without a signal applied the offset will change as you change the V/div switch.  The adjustment procedure is simple:  just adjust the pot until the offset remains constant as you run the V/div switch through its range.  Does your unit have visible "balance" pots?  They would affect the amplifiers ahead of the digitization.

[muawiya]

Not seeing a balance pot but applying a signal and going through the ranges, everything checks out

[muawiya]

In the attachments I am probing a signal that toggles from 80V to 0V @ 2MHz, I replicated the scenario with a power supply and am unable to replicate the same behavior, well I can't toggle the power supply at 2MHz.

I applied 80V at 1V/div and I can bring down the offset to -79V and the signal appears at the right place, I also toggled the supply low and captured the falling edge the behavior is correct (no apparent offset), repeated with 5V/div as well and it appears correct as well.

[muawiya]

The comment about overloading the scope explains what happened.

I replicated the scenario with a signal toggling between 40V & 0V and didn't see any distortion, toggling between 80V & 0V repeats what I attached.

On the Lecroy 64XI-A scope at 0 offset the max trigger level is 4.1V and min trigger level is -4.1V, as the offset is decreased by x amount from 0, the max/min trigger level both increase by x amount and at a higher max trigger level the scope is able to be overloaded  more and explains why the signal shows up when the offset is brought down.