Advantages to dual-tap diodes

[MacGyverS2000]

Wondering what the advantages to dual-tap diodes are when it comes to placing them on comm lines (i.e., serial ports, etc.)? My first thought is limiting ringing, especially for long transmission distances, but there must be more uses.

Any downsides?


Dan
Owner

http://www.Hi-TecDesigns.com

 

[itsmoked]

Excuse me, but what in the heck are you talking about???

I have never even heard of a "dual tap diode"!

Part number please...

 

[fsmyth]

May be a reference to a back-to-back package such as
the SB series (or FMP/FML for higher voltages) with
the common pin unconnected. I have seen them used
as a sort of MOV substitute. Highly dependent on
the device and a good understanding of operating and
breakdown characteristics of the diodes, but perfectly
functional for the particular usage.
Spikes, surges, OVP, maybe, but "ringing"? Dunno.

<als>

 

[Noway2]

Without reference to a specific part number, it sounds like you are refering to diode packages where there are two diodes connected back to back with one common terminal (anode - C - anode, cathode - C - cathode, or anode - C - cathode).

I have used these types of devices for clamping where I wanted to minimize parts count and or PCB footprint. I am presently working on an ADC interface circuit and I used one dual diode SOT-23 package as opposed to using two discete diodes to clamp the ADC inputs. The packages are nice in that you can connect the line to be clamped to the common and then connect the anode and cathode to the clamping supplies.

As far as any special electrical properties, I haven't considered it but there may be, especially at very high frequencies.

 

[OutToLunch]

If you are referring to dual packaged diodes that have either their anodes or their cathodes tied together, I use the former quite a lot in dual synch buck regulators for the boot diodes that are used to create drive for the upper FET gates. Takes a little less board space for one SOT23.

 

[itsmoked]

I agree with all your great suggestions gentlemen and have used "duel diodes" many times for that last desperately needed square millimeter of space... But have you heard of them called "dual tap"? Or is this just our compatriots twist of the tongue?

 

[MacGyverS2000]

The dual-tap terminology has come from some of the datasheets on these parts. You guys have the correct idea... something like the BAV99 from Fairchild, where the anode of one is connected to the cathode of another and the common connectionis also avaiable. SOT-23 packages, especially.

I recently came across an application where they were placed on a series of TTL lines, ground at one end, Vdd the other. Reduction of ringing over long run lengths seemed to be the most obvious answer to me (basic clamping), but I was wondering if I was missing something else in the design. It seems not...


Dan
Owner

http://www.Hi-TecDesigns.com

 

[Noway2]

I recently came across an article / app note about how these dual diodes are ( the manufacturer implies) recommended method of terminating lines. The article claimed that they provide superior performance to the traditional pull up - pull down resistor and had some graphs to "prove it"

When I came across the article I wasn't looking for diodes for this purpose so I ignored it and hence I can't comment further about its claims.

 

[MacGyverS2000]

Diodes in place of pullups? I can see them being used simultaneously, but in place of? That just doesn't sound right.


Dan
Owner

http://www.Hi-TecDesigns.com

 

[coconutalley]

One application where I'd find this useful in the design of surge protectors for data line. I used diodes to reduce the capacitance of the Transorbs or TVS [clamping devices--the frequency response is poor] and also save on the number of TVS. You can protect many lines with just one TVS using diodes to isolate them.

 

[Noway2]

Here is a link to an article on the subject that provides a brief explanation of the how it works and why it (doesn't) work:

http://www.sigcon.com/Pubs/news/2_19.htm

Dan, I too agree that the idea of a diode termination doesn't sound quite right, which is part of the reason the article suprised me. I think I am going to stay with the resistive methods.

 

[Warpspeed]

These fast low capacitance clamp diodes can be especially useful where high speed data and address busses wander around through ribbon cables to connect individual circuit boards together.

If the equipment has to withstand direct electrostatic discharge onto the ribbon cable, and CMOS or other ESD susceptible logic is used, it will often fail without this direct hard clamping of logic lines to Vdd and Vss.

 

[MacGyverS2000]

I have Howard's first book "Black Magic", but haven't (so far) needed the hardcore high-speed stuff in his second. In the article you linked to, he does give a practical application for the real-world dual-tap diodes:

In slower applications, like SCSI, diode terminations are great, because (1) the signals are intentionally slowed down to meet radiated emissions requirements, so the diodes are naturally faster than the signals, (2) we can afford to build fancy clamping-voltage generators at Vcc-Vf(diode) and Gnd+Vf(diode), and (3) we can specify receivers with tight V(IH), V(IL) margins that are tolerant of the lingering residual reflections.

A current project will be using long transmission lines (30'+) for data in the 200kbps+ range, so it was opportune timeing that I came across this when I did. These little buggers are going to find their way onto any line that leaves the main box.

Dan
Owner

http://www.Hi-TecDesigns.com