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pad capacitance measurement.

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steadymind

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pad capacitance

Hi,

I want to measure the capacitance at the output of a CML driver terminated to a 50 Ohm resistor. I need to find both single ended and differential caps.

I know a couple of ways but the results from them are not conclusive .
So is there a method that has worked for you in the past

Thanks
 

contact pad capacitance

PAD: 88um*88um

the PESD and NESD is about 400um/0.8um

the max parasitic cap < 8pF.

commonly 3-5pF.

but if there is wire routing on the PCB, or drive other load, then such caps should also be included.
 

measure padas

industry design and validation may use two method to measure pad capacitance, one of them is simple LRC meter, which adopted a electrical bridge connection and tune all the legs to be balanced and calculate the cap (all of the tunning operation is automatic). the other way is using TDR unit, which is an expensive equipement, by sending out a very narrow pulse to a socket with/without unit plugged in, then mesure the returned signal waveform, by analying the differences of these two waveforms, the load cap can be calculated.
 

capacitance measurement ic

Yes, TDR is very good method to obtain sought capacitance, but very often it is acceptable to calculate capacitance by well know formulas. If you want to account fringe effect you need to replace Er of material with Ereff where Ereff is the effective dielectric constant. It accounts the actual part of electric filed that exists into analyzed region. Calculation of Ereff usually is not difficult, but depends on material parameters and geometry, as well as differences in Er of all different layers between analyzed metals. Good reference is B.Wadell. Transmission Line Handbook.Artech House.1991. The definion of Ereff is on page 22 and then you need to find how to calculate it for your particular geometry.

Best regards,
Rf-OM
 

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