Products > Test Equipment

Scope Current Probe Deskew Fixture PCB Project

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Ribster:

--- Quote from: steve1515 on August 23, 2020, 10:10:24 pm ---_Wim_ and Ribster, would you be able to post schematics of your projects?

--- End quote ---

Steve,

I've added the schematic to the design.
It's not all that complicated. I've reused your NE555 circuit with some fixed resistors. I'm just opting for a fixed frequency and fastest rise time possible.
The project is done in Altium Designer. If you need the project, PM me.


--- Quote from: Jay_Diddy_B on August 23, 2020, 10:19:55 pm ---

--- End quote ---

I don't know if you can see it or not, but i via stiched the whole ground plane. Look at the attached schematic.
If you look at page 3 of my pdf, you see the top copper in red, and all those green dots. Those are stitching via's.
So i think my ground inductance will be okay, since the bottom copper layer is almost uninterrupted.
I could add some more capacitance on the other side if required. I could experiment with that once i get them built up.


--- Quote from: Jay_Diddy_B on August 23, 2020, 10:19:55 pm ---I would suggest looking at the BSS316N MOSFET. This is in a more friendly SOT-23 package.

If I compare gate charge:

BSS316N



--- End quote ---

Well that's a nice find.
I've added the part to my parts library. I'll order some samples of them together with my parts order for this.
It should be straight forward to replace the GaN mosfet with this one, and compare!

@All:
What do you think about the solution proposed with the molex connectors ?
Those are plugin connectors, so you can add a wire or a coil, but its unpluggable, so it's handy to handle.
I'm just a bit worried about the distance between the connectors (net distance), but you can have the loop coming forward.
I don't see the use of making the pcb bigger for that reason. And i quite like the odd shape.

This is the male PCB side
https://www.digikey.be/product-detail/en/0395026002/WM25530-ND/2735307?utm_campaign=buynow&WT.z_cid=ref_octopart_dkc_buynow&utm_medium=aggregator&curr=eur&site=us&utm_source=octopart

This is the female plug in side
https://www.digikey.be/product-detail/en/molex/0395005002/WM23176-ND/2735117

_Wim_:

--- Quote from: Jay_Diddy_B on August 23, 2020, 06:55:29 pm ---Do not oversize the MOSFET this is will slow it down.

You are looking for low gate charge.

The RDS(on) could be 1 \$\Omega\$.

--- End quote ---

I personally would avoid an RDS(on) of 1 ohm. This means about 20 mA keeps flowing through the coil during the "off" period (with the resistances I used), and as some current probes have it difficult to nicely return to zero after a steep peak, I find it important that the off period is as close to zero current as possible. This then also tells you something about your probe. I see this more as a "current probe test fixture" than as a "deskew-only fixture".

So I would advocate for a low RDS on also, but I agree also to take a small FET with low input capacitance, although a little higher input capacitance can be covered by putting the clock driver really close.

BTW, I started first without the clock driver, but got rise time similar to the results from steve1515.

_Wim_:

--- Quote from: Ribster on August 24, 2020, 08:04:15 am ---@All:
What do you think about the solution proposed with the molex connectors ?
Those are plugin connectors, so you can add a wire or a coil, but its unpluggable, so it's handy to handle.
I'm just a bit worried about the distance between the connectors (net distance), but you can have the loop coming forward.
I don't see the use of making the pcb bigger for that reason. And i quite like the odd shape.

--- End quote ---

What is the remaining net distance? This we could compare to the width of some of the "typical" probes. I will start with mine:

- Fluke i50A:  17mm
- Pearson 411: 26 mm
- Singer CP315: 32 mm
- ILA SMZ802: 27 mm

(yes, I had a TEA-current-probe-problem a while ago  :-[ )

But anyway, your current pcb footprint allows for the use screw terminals also, so I think the design is flexible.

Elasia:

--- Quote from: Ribster on August 24, 2020, 08:04:15 am ---
--- Quote from: steve1515 on August 23, 2020, 10:10:24 pm ---_Wim_ and Ribster, would you be able to post schematics of your projects?

--- End quote ---

Steve,

I've added the schematic to the design.
It's not all that complicated. I've reused your NE555 circuit with some fixed resistors. I'm just opting for a fixed frequency and fastest rise time possible.
The project is done in Altium Designer. If you need the project, PM me.


--- End quote ---

That looks pretty good, i'll have to remember to hit you up once its done being hashed on here less you eventually attach the ad files ;)

Ribster:
So i've gotten around to assembling them.
I need to still solder on the mosfets. Then i'll test the BSS316N and the GaN mosfet.

Will report back when testing is done ;)

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