End-to-End Capacitance of SMD Resistors

[Jim Thompson]

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

 

[Al]

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

Dunno, but here's a thought.

Perhaps the configuration of the mounting pads will have more of an
effect on the capacitance than the spacing of the end caps themselves.

Does your simulation include the effects of the pads and the metal traces?

Al

 

[Vladimir Vassilevsky]

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.

~0.05...0.1pF depending on the particular resistor and PCB technology.

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

~0.5pF for each 0603 pad.

Thanks in advance for any information!

Huh?


Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com

 

[Joerg]

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

It's a pretty critical frequency response optical link.

Did you do a good old low noise cascode?

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

Murata says 0.04pF pad-pad for their 0603 inductor in this doc:
http://www.highfrequencyelectronics.com/Archives/Jun06/HFE0606_Tutorial.pdf

I've seen higher with 0603. Hard to measure though, you'd need a HP4191
or similar but that won't go above 1GHz.

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

Entirely up to how they lashed them up.

 

[Jim Thompson]

Dunno, but here's a thought.

Perhaps the configuration of the mounting pads will have more of an
effect on the capacitance than the spacing of the end caps themselves.

Does your simulation include the effects of the pads and the metal traces?

Al

Not yet. I'm trying to analyze the client's measurements versus
simulation and ascertain what the strays are.

...Jim Thompson

 

[Joerg]

Not yet. I'm trying to analyze the client's measurements versus
simulation and ascertain what the strays are.

Did they do a layout or a kludge on copper clad?

 

[Jim Thompson]

Did they do a layout or a kludge on copper clad?

Professional Layout.

...Jim Thompson

 

[Joerg]

Professional Layout.

Then the CAD should tell them everything beyond the part's capacitance.

 

[Wimpie]

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

Hi Jim,

In www.ece.ucdavis.edu/vcl/asap/asap_v1/docs/RC0603JR-070RL.pdf you
can find a yageo (phicom) document that gives impedance values versus
frequency (up to GHz range) for several resistance values and SMT
shapes. It is not just capacitance that counts.

I hope this will help you to extract a spice model.

Best regards,

Wim
PA3DJS
www.tetech.nl

 

[Jim Thompson]

Hi Jim,

In www.ece.ucdavis.edu/vcl/asap/asap_v1/docs/RC0603JR-070RL.pdf you
can find a yageo (phicom) document that gives impedance values versus
frequency (up to GHz range) for several resistance values and SMT
shapes. It is not just capacitance that counts.

I hope this will help you to extract a spice model.

Best regards,

Wim
PA3DJS
www.tetech.nl

Thanks! Looks useful!

...Jim Thompson

 

[John Larkin]

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

I measured a 1M 0603 resistor at 0.32 pF standalone. PCB pads will
increase that number.

We use 0603's to several GHz. Figure that, for a kohm-GHz product of 1
or so, it will look pretty resistive.

What's the resistor value, and the frequency?

John

 

[Jim Thompson]

I measured a 1M 0603 resistor at 0.32 pF standalone. PCB pads will
increase that number.

We use 0603's to several GHz. Figure that, for a kohm-GHz product of 1
or so, it will look pretty resistive.

What's the resistor value, and the frequency?

John

5 x 10Meg in series, at 20KHz ;-)

...Jim Thompson

 

[Joerg]

5 x 10Meg in series, at 20KHz ;-)

20kHz?

<yawn>

Is there not other way? If that's the load resistor of a photodiode, no
chance to do a TIA?

 

[Jim Thompson]

20kHz?

<yawn>

Is there not other way? If that's the load resistor of a photodiode, no
chance to do a TIA?

High precision TIA. That's all I can say ;-)

...Jim Thompson

 

[Joerg]

High precision TIA. That's all I can say ;-)

Assuming the resistor stack is in the feedback you might have to
compensate at the PD node. Sounds silly but a varicap diode there plus a
nifty 20+ kilohertz carrier or pulse scheme might be needed so you can
automatically compensate for the feedback capacitance. Factory
adjustments are usually poo-pooed upon these days (at least I never use
them). Sure you need all this gain right away?

 

[Nico Coesel]

5 x 10Meg in series, at 20KHz ;-)

Recipe for disaster You could add capacitive compensation (like an
oscilloscope probe). But this would require some post-manufacturing
adjustments / calibration depending on the required frequency
linearity. If the frequency range is limited, you might get away with
a capacitive divider (if it is a divider you're trying to make) that
is dominated by its capacitors rather than its resistors at the
frequency range of interest.

 

[John Larkin]

5 x 10Meg in series, at 20KHz ;-)

...Jim Thompson

Cool. 1 pF at 20 KHz is about 8 megs.

Heap-o-trouble.

John

 

[Joerg]

Cool. 1 pF at 20 KHz is about 8 megs.

Heap-o-trouble.

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.

 

[Winfield]

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

5 x 10Meg in series, at 20KHz ;-)

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.
If a signal-generator test is desired, one shouldn't
depend on simply using a high-value resistor to create
the TIA's test current, because that resistor also has
self capacitance, which could make the amplifier look
better than it really is, etc. Instead, you can make
a corrected resistor this way,

R1 Rs, high-value
---/\/\---+---/\/\/-----
_|_
--- C <-- I like to use adjustable capacitors
|
gnd

Now, how to calibrate the "corrected resistor"? For that
I use a perfect "calibrated" amplifier, which in turn has
been adjusted with a lower value corrected Rs, which ...

This scene is pretty trivial in the 50M region, but it
can get rather tricky for Rf = 1G-ohm and up. For such
values and high frequencies, one soon realizes they're
really using capacitors rather than resistors, as the
feedback and gain-determining elements.

 

[Jim Thompson]

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.
If a signal-generator test is desired, one shouldn't
depend on simply using a high-value resistor to create
the TIA's test current, because that resistor also has
self capacitance, which could make the amplifier look
better than it really is, etc. Instead, you can make
a corrected resistor this way,

R1 Rs, high-value
---/\/\---+---/\/\/-----
_|_
--- C <-- I like to use adjustable capacitors
|
gnd

Now, how to calibrate the "corrected resistor"? For that
I use a perfect "calibrated" amplifier, which in turn has
been adjusted with a lower value corrected Rs, which ...

This scene is pretty trivial in the 50M region, but it
can get rather tricky for Rf = 1G-ohm and up. For such
values and high frequencies, one soon realizes they're
really using capacitors rather than resistors, as the
feedback and gain-determining elements.

Interesting information there, Win. Thanks!

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

...Jim Thompson