Thanks to PCBway for providing the new PCB prototypes. They look nice. 

The EVOIII board works like a charm but it has a few things I wanted to change to make it more user friendly. The most important change is the removal of the VFD poti and the JST wire bridge.

This is one of the confusing things on the EVOIII board. Even if the poti is only adjusted once it makes it less plug&play friendly so I decided it is time to remove it and use a static voltage divider network instead and replace the JST wire bridge with some SMD jumpers.

This removes the need for users to mess around with any of this initial setup stuff. Just wire it up and press start as I like to say. 

The large 5v regulator used to power the optocoupler on the gate driver side was replaced with a smaller one. 

Those are the major changes but there are also some smaller adjustments that I had in mind for a long time but did not found time to get into until now.

The sensing circuit itself did not change. I did add some round corners to the traces for fun but the schematics remains the same as on the EVOIII board.

But there was always one thing that I wanted to test. The circuit uses two LM358 OpAmps to control the HCNR201 linear optocoupler. I always thought using a better Amp there could make the circuit faster. Not to get anything wrong. The EVOIII sensing delivers a fast and rock solid feedback but I have to wait for parts to arrive before I can finish the board and that means boredom and boredom equals time for fun.

In the past the first versions of the circuit used OPA2350UA amps that where replaced with single channel OPA350UA for the EVOIII. 

Those OPA2350 and OPA350 amps are really good. Fast, low noise, rail to rail. Compared to the LM358 there is a whole world between them. But that doesn't mean they will operate better in the given circuit.

Both Amps share the same pinout and replacing the LM358 is a matter of unsoldering it and soldering the OPA2350UA on there. Each channel (vfd/cfd) has one of them so all changes are done to both channels.

Since the OPA is rail to rail it can swing from 0 to 5v while the LM358 does not fully swing to the supply rail. 1.2-1.5v below the supply is what I remember.

This means the current limiting resistor on the HCNR LED input needs a replacement too. Well. I tested it and it doesn't seem to ever reach any critical voltage levels but just to be safe the resistor was upgraded from 200 to 330 Ohm.

This already is enough to use the OPA. But there are a view other things that I looked at. There is a little capacitor on the amps output to the inverting input. It is 100pf. Changed that one to 47pf hoping for faster signal edges.

And another thing that changed is the voltage divider after the low pass stage. It scales the 0-5v feedback down to 0-1.6v using a 10k / 4k7 voltage divider.

Pretty high values. More resistance equals more noise and slower reactions. Based on the math it is no problem to use lower values so those dividers changed to 1k / 470 Ohm instead. Lower resistance, less noise, faster reactions.

In theory. 

I was not able yet to test the board in a real scenario. The signal itself does not look much different if I supply a constant voltage to the circuit. Still a solid feedback. But it "feels" like the rising and falling edges are a tiny little faster. But without testing it in a real cut this could be fantasy too. Signal jitter does no show any difference in the test rig.

The test rig is just a poti connected to the 5v output and the output wire soldered to the vfd pad to provide a variable 0-5v feedback.

I am happy about every nanosecond and once the board is ready it will receive some in depth testing of the changes. 

The changes done to the sensing circuit can be done to any existing EVOIII board too by replacing the affected parts.

R22, R23 changed to 1 kOhm 1% (was 10k)
R26, R27 changed to 470 Ohm 1% (was 4k7)
C31, C32 changed to 47pf (was 100pf)
R28, R29 changed to 249 Ohm 1% (was 200)
IC3, IC4 changed to OPA2350UA (was LM358)

But it is not yet known if those changes are worth the soldering.