Video amplifier layout, decoupling capacitor on bottom through via?

Status
Not open for further replies.
J

Jester

Full Member level 6
Joined
Aug 18, 2012
Messages
377
Helped
7
Reputation
14
Reaction score
7
Trophy points
1,298
Location
.
Visit site
Activity points
4,754
I'm working on a video amplifier circuit using a LMH6703 (the SOT23-6) package, this is a fast (1.2GHz part).


The reference board for this part LMH730216 has the bulk decoupling capacitors on the top side, however the smaller 10nF decoupling capacitors are placed on the back side with the +ve side passing through a via. There is plenty of space to have the small caps (C1, C3) on the top. Any insight as to why this would be done?

See page 4 of the attached as well as my layout (in progress).

https://www.ti.com/lit/ug/snou024/snou024.pdf
 

Attachments

  • via.jpg
    234.2 KB · Views: 108

What are the relative inductance values for a via
connection, and a remote trace (pair) connection? It
may be as simple as proximity. Or that the best ground
plane happens to be on the backside, and one end of
the cap wants to be on it.
 

Referring to the image above:
- The blue (bottom side polygon) is an island of ground with a single return to system ground (to the left just off screen)
- C1 (bottom side), can easily be flipped to the top side, the blue polygon could become red (top), negating the need for any vias except the common one off screen.
- The other odd thing is the data sheet states to leave a void under the op amp (U2), and this is where NS placed C5, the cap that couples the +5V & -5V supply rails

re proximity, the cap can be in the same location (just the other side of the board).

I'm inclined to move the caps to the top side. I guess I will see how it works when the board comes back.

Thanks
 

Status
Not open for further replies.

Similar threads

Menu
Log in
Register
Cookies are required to use this site. You must accept them to continue using the site. Learn more…