End-to-End Capacitance of SMD Resistors

[Winfield]

Do you think 50fF for end-to-end capacitance is a good number?

It's probably much higher for 0603 parts. I made a bunch
of measurements for caps on a PCB a year ago, but I seem
to have misplaced the folder. The problem with my fancy
impedance-measuring instruments and their special fixtures
is the ZERO cal step, which can cancel out legitimate
portions of the measurement.

A better way to take accurate measurements is in-situ, on
a PCB, rather than with the test fixture. Toward that end
my PCB engineer has created a test board for all types of
components and I plan to order soon and take measurements
with it next month. We need them for a section in AoE 3rd
edition, on component properties.

 

[John Larkin]

That value's far too high. John, how'd you measure it,
exactly?

Dropped the resistor across the smd fixture of the AADE meter and
pushed it down with the corner of a q-tip. The fixture is really just
a pcb with a slit in a trace. The only delta in the situation was the
presence of the resistor itself; I checked that the location of my
hand, and the q-tip, weren't what was being measured.

John

 

[John Devereux]

Dropped the resistor across the smd fixture of the AADE meter and
pushed it down with the corner of a q-tip. The fixture is really just
a pcb with a slit in a trace. The only delta in the situation was the
presence of the resistor itself; I checked that the location of my
hand, and the q-tip, weren't what was being measured.

Are you sure the meter can distinguish the capacitive impedance from
the resistive impedance?

 

[Joerg]

Dropped the resistor across the smd fixture of the AADE meter and
pushed it down with the corner of a q-tip. The fixture is really just
a pcb with a slit in a trace. The only delta in the situation was the
presence of the resistor itself; I checked that the location of my
hand, and the q-tip, weren't what was being measured.

A Q-Tip has a plastic pin and its dielectric constant isn't favorable.
Got zinged by that once. I prefer bone-dry toothpicks. Just don't use
them for picking teeth afterwards if you touched leaded parts too much.

Got to check out that Aade meter. Missed the tax curfew, dang. So far I
am using a HP4191 but that thing is so freaking bulky and heavy. It does
measure to 1GHz though.

 

[Joerg]

That value's far too high. John, how'd you measure it,
exactly?


50M and 0.16pF gives -3dB at 20kHz, but if 0.05pF is
assumed for a single part, that's a 60kHz bandwidth
you may enjoy. If you use five resistors in series,
you'd like to say that's 0.05pF/5 = 0.01pF for the
50M, and therefore -3dB at 330kHz.

,- -||- +- -||- +- -||- +- -||- +- -||- ,
---+-/\/\--+-/\/\--+-/\/\--+-/\/\--+-/\/\--+---

But watch out, you'll need to construct that five-
resistor stack carefully, because stray capacitance
from the wiring and resistor bodies, etc., forming a
T network, will damage the TIA's frequency response.

,- -||- +- -||- +- -||- +- -||- +- -||- ,
---+-/\/\--+-/\/\--+-/\/\--+-/\/\--+-/\/\--+---
_|_ _|_ _|_ _|_
--- Cs --- --- ---
| | | |
--+-------+-------+-------+-- gnd

We might be talking about Cs in the 0.01pF territory.

That's why I usually use a single resistor, and fix
it for bandwidth using the R-C-R trick I've mentioned
several times before here on s.e.d.

Cf ,-- adjust so R1 = Rf Cf/C1,
,- -||- , R1 / the new apparent Cf is
---+-/\/\--+---+--/\/\----- given by Cf' = C1 R2/Rf
Rf | C1 R2
'--||--/\/\-- gnd

(In production it's possible Cf may be sufficiently
predictable to use a fix value for R1.) In practice
it's easy to get an effective Cf = 0.0025pF or better.
But one has to realize that in doing so he's depending
on the use of just one small Rf resistor and is relying
on its single-capacitance model. Frankly, that would
be difficult to do with multiple resistors in strings.

One more non-trivial issue, how to test these beasts.

This is where an almost forgotten tool comes in handy: The dipmeter.
Hang a well known inductor in parallel and dip the resonance. Then whip
out the old HP calculator or slide rule ...

 

[John Larkin]

A Q-Tip has a plastic pin and its dielectric constant isn't favorable.
Got zinged by that once. I prefer bone-dry toothpicks. Just don't use
them for picking teeth afterwards if you touched leaded parts too much.

I used an "industrial" single-ended wooden q-tip, and pushed the
resistor with the sharp corner edge of the non-cotton end. If one end
makes contact, I get about 0.15 pF; push harder to make both ends
contact and it jumps to about 0.32.

I suppose I could TDR a resistor, too.

Got to check out that Aade meter. Missed the tax curfew, dang. So far I
am using a HP4191 but that thing is so freaking bulky and heavy. It does
measure to 1GHz though.

The AADE thing is great, especially at the price. But only for
small-valued Ls and Cs.

John

 

[John Larkin]

Are you sure the meter can distinguish the capacitive impedance from
the resistive impedance?

Unlikely, knowing how the AADE thing works. But I'll try some higher
values to make sure.

John

 

[mng]

A better way to take accurate measurements is in-situ, on
a PCB, rather than with the test fixture. Toward that end
my PCB engineer has created a test board for all types of
components and I plan to order soon and take measurements
with it next month. We need them for a section in AoE 3rd
edition, on component properties.

Spent a few weeks trying to do that for a class. We had brand-new VNAs
to use, but I got the impression that no one had experience with board-
level RF work. In the end we couldn't reliably model the parasitics in
simulation and, for lack of time and effort, resorted to tweaking.

I'd be interested in what the folks here think about the lab
procedure, which is at the end of the filter lab document:
http://rfic.eecs.berkeley.edu/~niknejad/ee142/labs.html
The test fixtures we used are on page 30. Using port extension (which
I admit I don't totally understand), we measured the S-params of a
zero-ohm 0603. My rough pi model from that measurement gives 0.4pF for
each "leg" to ground and 0.8pF || 1.3nH across. This is hardly exact;
I found out sometime later that ADS has a S2P -> spice model utility,
but I didn't try it and now my account's been deactivated. I still
have the S2P (and others) if anyone is interested.

Thanks,
Mike

 

[Winfield]

I used an "industrial" single-ended wooden q-tip, and pushed the
resistor with the sharp corner edge of the non-cotton end. If one end
makes contact, I get about 0.15 pF; push harder to make both ends
contact and it jumps to about 0.32.

Wait, you earlier wrote 0.32pF (see above), now you
say 0.15pF, could you please spell this out for us?

 

[Joel Koltner]

Got to check out that Aade meter. Missed the tax curfew, dang. So far I am
using a HP4191 but that thing is so freaking bulky and heavy. It does
measure to 1GHz though.

The AADE meter is a toy compared to an HP 4191. While a useful tool --
I've had one for a handful of years now -- a somewhat more sophisticated (if
still not close to a 4191) is this: http://www.m3electronix.com/lcr.html
....which has been mentioned a few times on SED previously as well.
Significant advantages over the AADE meter include the ability to use various
fixed test frequencies (whereas the AADE meter uses a variable frequency as
part of its impedance determination) and the use of a 4-wire connection, which
allows for better "zeroing" of parasitics than the AADE's simple "delta" mode.

One downside of the M^3 meter there is that it only has a DC power input jack
whewreas the AADE is battery-powered. With some creative shoehorning one
might be able to get a bunch of "AA"s into it, though. Both the M^3 and AADE
meters are almost entirely through-hole components -- they could both be about
1/4-1/2 their respective sizes if they used SMT parts everywhere. Apparently
the reason they're sticking with through-hole is due to both being available
as kits, and a lot of people still seem put-off by SMT parts, even though
they're actually user to hand-solder in most cases IMO.

---Joel

 

[John Larkin]

Wait, you earlier wrote 0.32pF (see above), now you
say 0.15pF, could you please spell this out for us?

I couldn't say for sure, without a full e-m simulation, but it appears
that some capacitors may demonstrate more capacitance when the
measuring instrument connects to both ends.

John

 

[[email protected]]

I have a client using SMD 0603 resistors in his lash-up.

It's a pretty critical frequency response optical link.

So I'm looking for the end-to-end capacitance to include in my
simulations.

Typical pad-to-ground stray information would also be helpful.

Thanks in advance for any information!

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice480)460-2350 | |
| E-mail Address at Website Fax480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |

America: Land of the Free, Because of the Brave

Some of the HP (Agilent) network analyzers have surface mount
component test fixtures and software to create LCR models. It
wouldn't be the first time that someone asked Agilent to loan a unit
for evaluation, ran their tests, then said they weren't interested in
the box.

 

[Joerg]

Spent a few weeks trying to do that for a class. We had brand-new VNAs
to use, but I got the impression that no one had experience with board-
level RF work. In the end we couldn't reliably model the parasitics in
simulation and, for lack of time and effort, resorted to tweaking.

I'd be interested in what the folks here think about the lab
procedure, which is at the end of the filter lab document:
http://rfic.eecs.berkeley.edu/~niknejad/ee142/labs.html
The test fixtures we used are on page 30. Using port extension (which
I admit I don't totally understand), we measured the S-params of a
zero-ohm 0603. My rough pi model from that measurement gives 0.4pF for
each "leg" to ground and 0.8pF || 1.3nH across. This is hardly exact;
I found out sometime later that ADS has a S2P -> spice model utility,
but I didn't try it and now my account's been deactivated. I still
have the S2P (and others) if anyone is interested.

I like the term "de-embedded"

I have seen mixed results with port extensions and other compensation
mechanisms of analyzers. At least I'd use the good stuff such as rigid
coax. But in order to estimate the stray capacitance of a package I
always use resonance with a known inductor. Mostly a known LC circuit to
which the DUT gets paralleled, watching the change in resonance. That
takes a whole lot of compensation tricks out of the equation. In my
younger days I even had a "broom stick oscillator", a circuit that would
oscillate upon finding an ever so light resonance. But after a move
across an ocean it and the schematic was gone, probably tossed with
mountains of packaging paper.

I know this suggestion sounds pre-historic: Teach your students how to
use of a dip meter. No idea where to buy them these days but they are
cheap and can be built. Really practical for finding resonances,
especially in circuits that others have designed but not documented too
well.

Anyhow, I applaud you guys for providing such hands-on training for
future engineers. Seems not many universities are doing such a good job
of it these days.

 

[Paul Hovnanian P.E.]

Then after it's all designed to circumvent that 1pF the resistor
manufacturer does a "minor" design change or the process drifts a bit
and ... whaddabang.

I wonder why it has to be 50M in an optics link. In my last one it was a
few kohms for the current-to-voltage conversion, followed by more amps,
and we could hear the grass grow.

A photomultiplier perhaps?

 

[Joerg]

The AADE meter is a toy compared to an HP 4191. While a useful tool --

Yeah, but the 4191 is so freaking heavy. Last time I had to repair it
(it's out of support, of course ...) I paid for that with a fair amount
of back pain. Before that repair I had split four cords of wood - no
back pain then.

I've had one for a handful of years now -- a somewhat more sophisticated (if
still not close to a 4191) is this: http://www.m3electronix.com/lcr.html
...which has been mentioned a few times on SED previously as well.
Significant advantages over the AADE meter include the ability to use various
fixed test frequencies (whereas the AADE meter uses a variable frequency as
part of its impedance determination) and the use of a 4-wire connection, which
allows for better "zeroing" of parasitics than the AADE's simple "delta" mode.

One downside of the M^3 meter there is that it only has a DC power input jack
whewreas the AADE is battery-powered. With some creative shoehorning one
might be able to get a bunch of "AA"s into it, though. Both the M^3 and AADE
meters are almost entirely through-hole components -- they could both be about
1/4-1/2 their respective sizes if they used SMT parts everywhere. Apparently
the reason they're sticking with through-hole is due to both being available
as kits, and a lot of people still seem put-off by SMT parts, even though
they're actually user to hand-solder in most cases IMO.

The M^3 looks nice but with 15.625kHz being the highest frequency it
doesn't seem to offer much of an advantage. For intermediate frequency
work this seems to be a good deal:

http://radio.tentec.com/Amateur/vna

Goes to about 100MHz and can also do one-port measurements. Not bad at
all for $655. But no audio and such.

 

[Joerg]

A photomultiplier perhaps?

We might never know. If it's cutting edge stuff Jim won't be able to
say. And if it weren't cutting edge stuff they wouldn't be paying the
big bucks.

 

[Jim Thompson]

Paul Hovnanian P.E. wrote: [snip]

A photomultiplier perhaps?

We might never know. If it's cutting edge stuff Jim won't be able to
say. And if it weren't cutting edge stuff they wouldn't be paying the
big bucks.

Big SEG ;-)

...Jim Thompson

 

[Jim Thompson]

I like the term "de-embedded"

[snip]

Isn't that like "disembodied" ?

...Jim Thompson

 

[Joerg]

I like the term "de-embedded"

[snip]

Isn't that like "disembodied" ?

Nope. De-embedded is what happens if the girlfriend don't like you no
more. Disembodied is what happens when the extended family finds out.

 

[Winfield]

I love my AADE, but IMHO one each 4191A, 4192A, 4193A, 4194A,
and 4195A is a necessity. With HP's test fixtures, plus ones
that we can fabricate ourselves, the world can be our oyster.

Yeah, but the 4191 is so freaking heavy. Last time I had to repair
it (it's out of support, of course ...) I paid for that with a fair
amount of back pain. Before that repair I had split four cords of
wood - no back pain then.

Yes, they're heavy, but so is your upper body (or at least
mine is). Handling an axe or sledgehammer and little pieces
of wood is nothing. Lifting the hp4191's 53 pounds should be
a snap, if you expect to properly handle your own upper-body
weight in various awkward situations. When I realized I was
beginning to have some trouble, I started spending serious
time at the gym with the weights and machines. Recommended.
I can lift my 4191A on and off the table and to the floor and
carry it around, no problem. Did it yesterday. Go for it.