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To pad or not to pad (q. about using pad vs. straight surface)

V

Vitaliy

Jan 1, 1970
0
Hello,
Could someone please explain me the difference between making a pad or
having a straight surface. A picture is worth a thousand words, so I am
attaching a picture with the pad and without the pad.
My application of the pad: ground of analog circuit. So, I am placing
the
capacitor on power rail (between power rail and gnd) to filter the
noise
from power supply and bond wires. My current to power up opamp and for
diode biasing is in milliamps and my voltage is +-5V.

Does it make a difference what I use (as in terms of noise, etc.), if I
have a few of these in the circuit? Should I use instead of pads just
the
straight surface for the ground surface? I know that a pad has some
parasitic capacitance, but then wouldn't the surface have some as well?
I
will be soldering on my own, so I won't be confused at hich components
should go where.

Currently have:
--power---cap-----
--line------pad-----
||||||
||||||
cap
||||||
||||||
cap
|||---gnd---|||
||| pad |||
||| | |||
|||||||||||||||||||||
gnd
surface

Thinking about:
--power---cap-----
--line------pad-----
||||||
||||||
cap
||||||
||||||
|||||||gnd|||||||||
||||||surface|||
|||||||||||||||||||||||


I care because I need to convert very small current (in uA) to volts.
Everything is working fine as it is, but I'm looking to reduce noise as
much as possible to avoid oscillations for higher gain/bw.

Thanks,
Vitaliy
 
J

Joerg

Jan 1, 1970
0
Hello Vitaliy,

Could someone please explain me the difference between making a pad or
having a straight surface. A picture is worth a thousand words, so I am
attaching a picture with the pad and without the pad.


Sorry, but I do not find that picture below too clear. Actually rather
confusing.

My application of the pad: ground of analog circuit. So, I am placing
the
capacitor on power rail (between power rail and gnd) to filter the
noise
from power supply and bond wires. My current to power up opamp and for
diode biasing is in milliamps and my voltage is +-5V.

Does it make a difference what I use (as in terms of noise, etc.), if I
have a few of these in the circuit? Should I use instead of pads just
the
straight surface for the ground surface? I know that a pad has some
parasitic capacitance, but then wouldn't the surface have some as well?


The old rule is that there be the least amount of parasitic inductance
from the ground plane to one side of a decoupling cap, and from the
supply rail to the other side. If with "pad" you mean the SMT solder
lands then the connections must be as short as possible while
maintaining thermal relief rules.

I
will be soldering on my own, so I won't be confused at hich components
should go where.

Currently have:
--power---cap-----
--line------pad-----
||||||
||||||
cap
||||||
||||||
cap
|||---gnd---|||
||| pad |||
||| | |||
|||||||||||||||||||||
gnd
surface

Thinking about:
--power---cap-----
--line------pad-----
||||||
||||||
cap
||||||
||||||
|||||||gnd|||||||||
||||||surface|||
|||||||||||||||||||||||



I care because I need to convert very small current (in uA) to volts.
Everything is working fine as it is, but I'm looking to reduce noise as
much as possible to avoid oscillations for higher gain/bw.

Well, we all do :)

But if you already have it running you should be able to gain some extra
margin by short bypasses, clever placement and shielding.
 
B

Brad Velander

Jan 1, 1970
0
Vitaliy,
With what you have explained I would offer the following.

Since you are putting a decoupling/filtering cap on the signal/power, of
what worry is the parasitic capacitance? The parasitic capacitance is
undoubtedly much smaller than the capacitor you are using, particularly on
the power rails. More capacitance is better in that case, that is also why
closely spaced gnd/vcc planes work very well for the higher frequency
decoupling, not quite the same as your case but just presented the concept
for your consideration.

As for the assembly of such capacitor placement, as long as you can
solder then anything is fair game. The only concern would come in in the
form of reliability. Soldering the end cap of an SMT cap on top of a trace
would not be as reliable as using a pad where the solder can form a full
meniscus off the end of the cap and thus supply a highly reliable joint.
This assumes that your trace is not as wide as the pad would have been
length wise, otherwise there is no point in this whole discussion if the pad
was smaller than the trace to begin with because adding the pad would have
made absolutely no difference to the circuit whatsoever.

Connections to large blocks of copper are generally always better for
general circuit performance. Ever wonder why all of those sample/evaluation
boards from chip manufacturers are typically done removing only isolation
channels between non-connected components/signals? Some of those circuits
are difficult to get operating to the full spec'd performance if you use
more typical traces to connect the circuitry.

The largest problem for oscillations in a circuit like you are
describing is typically dealt with through good part placement and thus good
routing. Keeping inputs away from outputs, keeping feedback loops very short
and direct, etc.. Since your currents are so low, you might consider guard
tracks between the input signal(s) and other nearby copper. After making you
board you should make sure it is well dried (bake it in an oven for several
hours at up to 100C), then conformal coat it so that contaminants can't
interfere with your small currents. Although these days uAmps aren't that
small, they could still give you problems doing this yourself using (I
assume) hobbyist techniques/materials/processes.
 
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