question on GND

stef

Hi

I dont know if this is the right place but I'm developing a device for text overlay by use of a propeller. It recieve it text and commands by a lantronics Xport and sends it to the right BOB4 module. I made a test board with one BOB4 module and that is working fine. Now I need more than 1 BOB4 module on the same prop. So in my print layout I'm designing I got stuck on a question.
As you can see in the specs (see attachement) of the BOB4 the connection 28,29 is video out and 29, 30 is video in. So 29 is GND of the video. They alo mention that you need to connect 29 (what is video GND) to the GND of the power supply. Now when you have multiple BOB4 modules on the same power supply you are connecting the GND of all the camera's that you connect together. That is also the case on the matrix device where the video out is going to. Now when you connect the video gnd together on more then one place you create ground loops??
Is this not the case in this setup ?
Is somone having an other opinion? Can leave the connection to power supply GND from 29?

Thanks for any reacktion.

Stef

Sariel

Neat little module. I might have to grab one to play with at some point.

By looking at the guide, I would say that you are ok to link the video grounds together. I may be wrong on this, but you should be ok since neither the prop or the BOB4 seperates digital and analog grounds. (and someone please correct me if I am), but according to page 9 of B4AppGuide.pdf, you might want to follow the rule of using 22ga wire or better, just to be on the safe side.

Phil Pilgrim (PhiPi)

Just an FYI: The BOB-4 sells for almost $90. Parallax's Propeller Backpack, which sells for $39.99, can also do character overlays over video with open-source software downloadable from the Backpack product page.

-Phil

Martin_H

I'm a satisfied owner of the propeller backpack. So I'd like to add it is one sweet little module and packs a lot of functionality into a small form factor. So far I've only used it for image processing with my line scan camera, but character video overlays are something I plan to use as well.

Phil, to me the propeller backpack seems better than the Basic Stamp 2pe mobo. Do you know if Parallax plans to migrate to the propeller backpack from the 2pe?

Phil Pilgrim (PhiPi)

Martin,

Actually, I have a MoBoProp on the drawing board, with the same form-factor as the MoBoStamp-pe. It just needs prioritizing to get it done. One open question is whether to include the AVRs to do A-to-D and PWM, or to rely on the same sigma-delta circuitry and software PWM used by the Backpack. My original MoBoProp prototype omitted the AVRs, and doing so would keep the cost down. Plus, the Backpack seems to work nicely with the TSL1401's analog output. OTOH, the AVRs would provide cog relief and be able to take advantage of already-written firmware. The other question is whether to provide a separate 1/8" stereo connector for A/V output. The MoBoProp would not have all the A/V features of the Backpack, though -- not enough room, not enough Propeller pins.

-Phil

Toby Seckshund

The reason that some designs show the two sorts of ground is to try and separate the dirtier side of the digital chomping ups and downs and power surges from the analog side where these edges etc could be seen as background interference.

Loops are only a problem when the loop is relativly large and stuff can get induced into them.

stef

Hi Phil

Yes I saw the propeller backpack but I was a bit scared because it only mentioned NTSC video and not PAL. My region is Europe and they use PAL as standard. That is one of the reasons why I chose Bob4. Now the end user is willing to invest, so price is not an issue.

Thanks again for the sugestion.

Toby

So you think it is not a problem tying them together so Video gnd = power supply gns?



Stef

Sariel

Just an FYI: The BOB-4 sells for almost $90. Parallax's Propeller Backpack, which sells
for $39.99

I am constantly amazed at the deals this company dishes out. I should read up more on some of these toys already offered.

Toby Seckshund

Hi Stef

Given a free choice on the disign then keeping the dirty grounds sparated away from the clean grounds is allways good. Even on internal bits such as SDRAMs half of the "GNDs" are classed as clean (data) but ground is ground and so all should be used so that all the internal current flow paths are equalized (as should the VDDs). The problem of noise margins shouldn't be a digital problem as this noise would have to aproach half VDD levels but a few mV on the analoge would probably be seen or heard ( I explain these away as a retro "Feature" ;-) )

RinksCustoms

@stef - on my lunch here.. a ground loop is created when you have signal cables (like RCA) and separate power cables. When the ground power cable becomes disconected, the camera or device using multiple ground lines will try to use another ground line for power, usually the ground in the RCA connector since power ground and signal ground are tied together on the circuit board in the camera. As long as your power connections are secure and not "daisy chained" to one another, you will have good solid grounds and least noise on the power wires.

see the attached spin file for a schematic of what i'm talking about.

stef

Hi RinksCustoms

I will take your advice in use when I design the pcb layout.

stef

madrfskills

Analog Devices and Fairchild have both written some outstanding app notes on grounding and layout that you should probably read. Enclosed for your perusal.