SOIC ICs?

[idlefaction]

just wondering if anyone's used SOIC ICs before?  they're surface mount packages.  i've ordered an RMS-to-DC-converter chip from analog devices (AD736), hoping i can make a no-parts no-footprint envelope detector out of it, but would appreciate any words of advice on treating them.

they have really short legs and are tiny, so i'll be taking care not to heat them much with the soldering iron, but maybe there are other things to look out for?

(and does anyone know of a no-parts 4-pole LP VCF i could couple it to? :roll: )

[Peter Snowberg]

Hi idlefaction,

I've been building systems with surface mount parts for about 10 years. I find I can build a simple circuit faster in surface mount than I can with full sized parts.

If you have a low voltage soldering iron with a very fine tip you shouldn't have problems at all.

If your SOICs are .05 pitch, just bend up every other lead and solder the unbent leads to the pads of regular old perf board. I use plated, double sided board for this. Single sided boards are a pain here and I often end up lifting a pad from them after a few part changes.

To connect the bent 'flying' pins, I use #30 wire to go from the next row of pads to the pins. Start with a few inches of stripped wire; Solder the wire into the desination pad; wrap the wire 1/2 turn around the SOIC pin; clean and wet your tip; quickly solder the wire and pin (only takes 1/4 to 1/2 second per pin); trim off the extra wire with flush cutters; there you go! The final length of wire used is a little less than .2" per pin including the through board distance.

If the pin has a resistor in series with it, you can use a 1206 package part to substitute for the wire.

For the picky details... I like ~550 for an iron temp and won't use anything other than .020 solder with a 63/37 mix and a no-cleanup-required rosin (I'm picky). Kester '245' is my favorite, but Kester '44' is great too.

For another way to go, try a "surfboards" from http://www.capitaladvanced.com/.

Great idea on using an RMS converter for envelope!

I don't know about the VCF you're looking for, but a chip like the LTC1062 combined with a VCO might do the trick.

-Peter

[Transmogrifox]

do a search for sockets in the DigiKey or Mouser catalog.  I have just recently ordered some surface mount sockets with a DIP pinout.  If you have a fine tipped soldering iron, it should be pretty easy to convert your SOIC to DIP with these sockets

[idlefaction]

peter - thanks man!  that's heartening    i'll give your flying leads method a try.  i guess this is the best excuse i've had to get around to getting a new tip for my iron  :oops:

hrm, Linear also have the LTC1563-X chips which are 4th order and cutoff is set by just a variable resistance!  thanks for the pointer.  they only go down to 256Hz tho...  maybe with some abuse they'll go lower  :wink:   does anyone know if you can get resonance out of a Bessel or Butterworth filter by routing out to in?  (guess i'll be reading those filter theory books again... where's that Jung one lol)

transmogrifox - thanks!  jaycar in nz don't seem to have those, but it's good to know they exist.  if i go into production i'll probably get a few of those  

[idlefaction]

...and now that i've thought about it, the DCO-switched frequency one is waaaay easier to votage control (thanks!!!)

[Peter Snowberg]

My pleasure.

A couple tips on that method....

- fill the holes you are placing the chip on with enough solder to make a tiny dome before placing the chip. That way you can use reflow soldering. A little bottle of liquid rosin flux with a tiny paint brush is invaluable for making really clean connection, but it's not required at all. It just makes things easier.

- if you place SMT resistors or caps on the board, fill one pad first with enough solder to create a tiny dome, put a little liquid rosin down to keep things clean and act like a little glue, set the component on the pads, reflow solder the first pad while pushing down on the part with tweezers, and then solder the second pad normally. That method makes the parts lay down perfectly flat on the surface. Use 1206 or 0805 sizes and a good pair of tweezers.

- MOST IMPORTANYLY: (at least for me:)) Don't do any coffee or caffeine for 12 hours before hand.

Thanks for the info on the LTC1563. I just peeked at the data sheet and found this on page 4:

Note 2: The minimum cutoff frequency of the LTC1563 is arbitrarily listed as 256Hz. The limit is arrived at by setting the maximum resistor value limit at 10M. The LTC1563 can be used with even larger valued resistors. When using very large values of resistance careful layout and thorough assembly practices are required. There may also be greater DC offset at high temperatures when using such large valued resistors.

with R = 10K (256KHz/Fc), ~42 Megs would get you down to 60Hz. A carefully designed current source LED driver and an H11F1 PhotoFET could be used here I'll bet. They work as variable resistors from 100 ohms to 300 Megs!

[idlefaction]

the bum thing about those filter chips is that you'd have to alter six resistances at the same time to make them variable.  the OCF one actually has a VCO in it so it's pretty much what i was looking for!  there's a note in the datasheet somewhere saying you can replace one of the capacitors in the app note schem with a smaller cap and variable resistance.

thanks for the tips on surface mount stuff - what exactly do you mean by 'reflow soldering'?

[Peter Snowberg]

Reflow soldering is simply when you make a connection with solder already placed on the surfaces. When you heat them together, surface tension takes over and the now re-molten solder re-flows to connect the parts. Since the connections are actually soldered twice, the extra flux is a big help.

-Peter