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 |
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
~0.05...0.1pF depending on the particular resistor and PCB technology.
~0.5pF for each 0603 pad.
Huh?
Vladimir Vassilevsky DSP and Mixed Signal Design Consultant
Did you do a good old low noise cascode?
Murata says 0.04pF pad-pad for their 0603 inductor in this doc:
I've seen higher with 0603. Hard to measure though, you'd need a HP4191 or similar but that won't go above 1GHz.
Entirely up to how they lashed them up.
-- Regards, Joerg http://www.analogconsultants.com/
Not yet. I'm trying to analyze the client's measurements versus simulation and ascertain what the strays are.
...Jim Thompson
-- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et |
Did they do a layout or a kludge on copper clad?
-- Regards, Joerg http://www.analogconsultants.com/
Professional Layout.
...Jim Thompson
-- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et |
Then the CAD should tell them everything beyond the part's capacitance.
-- Regards, Joerg http://www.analogconsultants.com/
Hi Jim,
In
I hope this will help you to extract a spice model.
Best regards,
Wim PA3DJS
Thanks! Looks useful!
...Jim Thompson
-- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et |
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
-- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et |
20kHz?
Is there not other way? If that's the load resistor of a photodiode, no chance to do a TIA?
-- Regards, Joerg http://www.analogconsultants.com/
High precision TIA. That's all I can say ;-)
...Jim Thompson
-- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et |
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?
-- Regards, Joerg http://www.analogconsultants.com/
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.
-- Reply to nico@nctdevpuntnl (punt=.) Bedrijven en winkels vindt U op www.adresboekje.nl
Cool. 1 pF at 20 KHz is about 8 megs.
Heap-o-trouble.
John
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
Interesting information there, Win. Thanks!
Do you think 50fF for end-to-end capacitance is a good number?
...Jim Thompson
-- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et |
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.
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