I'm using an FPGA to control short, high powered pulses (1A-40A) coming out of a supply, but need a way to detect when a pulse starts. This is all done in an extremely noisy environment, with noise levels increasing as the pulse power increases. Whatever sensing mechanism is used needs to be optically isolated from the FPGA.

I'm using a current sense isolation amp to sense the current, but there isnt a chip I was able to find fast enough (~10mhz) to detect when a pulse starts.

I want to use a Schimtt trigger hooked up the the 5mΩ current sense resistor whose window and midpoint increases as the current in a pulse increases, see graph.

I've been scratching my head for about an hour trying to figure out a circuit for this, and have nothing. Any Ideas?

Is there a better way to detect these pulses?

 

I want to use a Schimtt trigger hooked up the the 5mΩ current sense resistor whose window and midpoint increases as the current in a pulse increases, see graph.

How often do these pulses occur?
Do they repeat at the same amplitude for a number of pulses?
The way you describe it, you want to set the trigger point based upon the pulse amplitude before the pulse arrives (a future detector ).

 

That might be answered in many different ways, depending on ALL the details. As you add more details, the answers will change.

Why not save a lot of time and changing answers.........and explain exactly what you are trying to do......and what components and equipment you are trying to do it with?

We use schematics, data-sheets and diagrams.

It will also depend on the final function of your control system.

Saying I need x to control y has many answers.

 

Maybe look at using a current sense transformer with a operational amplifier? The magnetic coupling of a transformer will provide galvanic isolation, will be less likely to have bandwidth issues than a isolation amplifier and will provide significant immunity to the noise. You could even use a OP Amp with the FPGA to provide switchable gain to help deal with the dynamic range your trying to sense. If your specifically looking for edge detection your sensed signal could be put into a differentiation circuit to capture the rate of change and block dc values.

 

If the pulse is repetitive, then you could simply detect the pulse amplitude and set the Schmitt trigger point to a percentage of that amplitude.