On Analog Devices' MAT12 data sheet, there is a note (see on the top right, note #2) saying that the substrate, which is connected to the case, is usually connected to the most negative circuit potential. Since there is no pin connected to the substrate (or to the case), how do I connect the substrate to the most negative circuit potential?
\$\begingroup\$
\$\endgroup\$
3
-
\$\begingroup\$ See Figure 4.2.1 of this web page: archive.cnx.org/contents/d555241b-0aa2-4ba6-8406-14ceaf2fe90b@1/… where the substrate is shown. You can float it but you don't want it to become forward biased, so the most negative potential is where it should go if connected. \$\endgroup\$– jonkOct 16, 2016 at 19:06
-
\$\begingroup\$ If you have two separate questions, you should post two actual questions. Otherwise, which answer should you accept if someone posts an eagle library? \$\endgroup\$– pipeOct 16, 2016 at 20:20
-
1\$\begingroup\$ Ok, deleted the second question. I will post it separately. \$\endgroup\$– EnricoOct 16, 2016 at 20:41
Add a comment
|
1 Answer
\$\begingroup\$
\$\endgroup\$
5
From the datasheet:
- Substrate is connected to case on TO-78 package.
Now, here is a TO-78:
As you can see, the case is actually metal.
If you necessarily want to connect the substrate, the recommended way is to do it using a heat sink. I'm not sure if you actually gain anything by doing that. It could be feasible to solder a small wire or pin to the tab, but I've never done it, and I would be afraid that it could damage the matching. As Tom points out in the comments:
basically if you use a metal heat sink with no insulation, don't connect that heat sink to anything but the most negative circuit potential
I suspect that the only reason they mention that you can connect the substrate is because you sometimes have to, due to the heat sink. Not because they think it is warranted. If it gave any performance benefits, I'm sure they would point it out.
-
\$\begingroup\$ Ok, but in practice how do I perform the connection? \$\endgroup\$– EnricoOct 16, 2016 at 19:08
-
1\$\begingroup\$ @Enrico Either through whatever heat sink is used, or not at all ("But can be floated."). But basically if you use a metal heat sink with no insulation, don't connect that heat sink to anything but the most negative circuit potential. \$\endgroup\$ Oct 16, 2016 at 19:17
-
\$\begingroup\$ @Tom: I am not planning to introduce any heat sink - I was thinking about soldering the case to ground (which is the most negative potential in my circuit). Is it better to do this in some way or to let the case float? \$\endgroup\$– EnricoOct 16, 2016 at 19:28
-
\$\begingroup\$ @Enrico probably easier to leave it floating then. \$\endgroup\$ Oct 16, 2016 at 19:34
-
\$\begingroup\$ I've used a bunch of them and I always left the case floating. Never had a problem. \$\endgroup\$ Oct 16, 2016 at 19:36