Question on fiber optic link circuit

[q2418130103p]

Hello,
I plan on implementing a fiber optic channel between two devices.

Based on the speed and distance requirements, I have settled on a receiver, transmitter, and interface circuit recomended by the manufacturer.

I suppose my question boils down to another ananlog vs digital ground question. In the manufacturer document AN1121 (page 5) they show the transmit circuit with the LED (HFBR-14x4z) tied to digital ground (note, the hfbr-15x7z shown in the diagram is meant for an alternate configuration, ignore it). On page 6 they show the receive circuit, with the receiver tied to ananlog ground. Then on page 8, they show that you should tie the 'RxGND' of each circuit together, however this ground was never shown in the transmitter circuit.

In another tech note release by the manfucaturer, Fiber Optic Cookbook, they show the two circuits combined, with both the transmit and receive being tied to analog ground (page 14).


Currently, I am planning on implementing the circuit as shown in the cookbook, I just wanted to see if anyone had thoughts about it, or perhaps an alternate fiber tx-rx solution (noting that 1km, dc-~10MBd is required).

Thanks,
Jay

 

[BradtheRad]

I seriously doubt they intend for you to connect a wire between the transmitter circuit and a distant receiving circuit. Isn't the fiber optic signal supposed to be the only link?

Anyway, kidding aside...

There is the circuit ground icon and there's the earth ground icon.

The circuit ground icon has obvious electrical purpose of course.

As for the earth ground, it may be a precaution recommended by the manufacturer (who hopes the device will be used in numerous ways rather than get a name as unreliable).

This is because the circuit could inadvertently be installed in a location prone to static charge. Could be outdoors, could be indoors.

It's vital to neutralize this to avoid ruining static-sensitive components.

A wire going from the circuitry, down into the soil (or to a copper pipe, etc.) is needed to bleed away static charge. Or a wire from the circuit, to the surrounding metal enclosure, then down to the soil.

Another purpose for the ground icon (circuit and earth) being drawn on a component: It can be to signify that the metal shielding around a component or circuitry should be grounded.

The shielding is not a circuit component per se, but is there as a barrier to magnetic or electromagnetic interference. To do its job it must be connected to a supply rail.

Another reason to specify a ground connection is in radio applications.

To work properly the circuit often needs an electrical connection to soil, or it won't transimt/receive at greatest efficiency.

This doesn't seem to apply in your case. It may be the manufacturer trying to anticipate all applications customers may use the device for.