50 Ohm matching with pads

[jl010i]

For high speed analog design, I assume you need 50 Ohm matching at the input so as to ensure maximum signal power transfer. How do you achieve this without using LC matching? Do you just use a resistor or do you play with the ESD or what? Any advice would be greatly appreciated...

 

[johnson2]

What do you mean with LC matching? I suggest that it mean you need to implement and adjust your TL to havehm charactristic impedance.

 

[flatulent]

You need to be more specific about what you mean by high speed. Generally, it is poor economics to pay for a transistor and then reduce the circuit gain with attenuators.

There is nothing magical about 50 ohms. If you are going to or from a transmission line then you need to terminate the line so that there are no reflections which will distort the signal. If you stay on a circuit then the impedances between stages is whatever it takes to economically meet the system performance.

 

[jl010i]

I am talking about speeds in the 1-2GHz range. I have a high frequency input signal in the 1-2GHz range and I am worried that reflections are going to kill me. Should I just up the input signal power to make sure that enough signal goes through or should I try to match the input impedance of the pad + circuit to 50Ohms in the 1-2GHz range?

 

[userdce]

in pads the power loss in the forward as well as reverse direction provides matching.

it means pad as bing 3db would reduce the power transmitted as well as power refleced so it works as the matcher as powr reflected is also low.

 

[flatulent]

Your using a large signal level and an attenuator will certainly work and has a high probability of success.

The next more clever way is to use a commercial amplifier module that is selected for S11 being very low.

If you have access to a microwave simulator that can do optimizations, design your own amplifier and optimize with S11 having a higher weighting than other parameters.

In all of the above cases, the reflection from the sending end of the line is important. What gets into your receiving end is the first pass signal and then the three pass signal (reflected from the load, going back to the source, and being reflected from the source. It is important that the amplifier and network at the source have the 50 ohm output impedance [S22 very low])

There is a second order effect where the load on the sending end amplifier not being the design value (due to the input impedance of the transmission line not being exactly 50 +j0 ohms due to the load not being exactly 50+j0 ohms) causes the linearity to decline.

Another second order effect is the S12 of the receiving amplifier. If the load on it is not exactly what it is designed for the input impedance will be different.

 

[Teddy]

Any resistor on chip will not be 50 Ohm at 1-2GHz. You woul have to talk about its impedance then.
You can try to do 50 ohm input structure and make sure that s11 is on 50 ohm impedance line at 2GHz In this case you will be able to consider your ESD structures and your input structure - all in spice what makes it nice. Include the parasitics too.

 

[sgperzoid]

yes, u'd better consider the pad capacitance and resonate it out with some inductor so as to leave the real part equal tp 50 ohm for input match

 

[Teddy]

going into those details - at the freqs you are looking at get very good model for the package/bonding - preferably for the die you are currently working on (die size/pad location). It will have big effect.

 

[flatulent]

You can buy read made surface mount attenuators with SWR of 1.05 over your frequency range which will probably produce less reflections (32 dB return loss) than the other imperfections in your circuitry construction.

**broken link removed**

 

[mitgrace]

you can get the pad model from the package company . in our experiment the pad
you must include simulation . but in the high frequency (2G~5G) , We always modify the pad. we using the SCR structure in the LNA input , and cut the origin to
1/3 size . So that the ESD is very weak . But the performace is better .