[Help]How determine the width of the wire for sampling path

[BackerShu]

Hello, everyone!
It is a samping path of a SH circuit as shown in the image, and the paramater is annoated on the scheme. 50MHz is the maximum frequency of the input singal.


What I want to know is how to decide the width of the wire when layout this circuit. Is it something just based on empiricism, or some determined principles should be observed to make the security of the circuit? If it's the former, could you please tell me what is the rule of thumb? Otherwise, what is principle.

Thank you!

 

[erikl]

Re: [Help]How determine the width of the wire for sampling p

You can easily estimate the needed wire width from the calculated current (≈ 0.37mA) and the max. allowed current per wire width ( ≈ 1mA/µm for high temperature 125°C). Skin effect doesn't play any role at this frequency.

 

[BackerShu]

Hi, erikl. Thank you for your reply.
Maybe I didn't describe my question clearly. I know that the estimate current is about 0.37mA. And I calculated the current as follows.
Vc=Vin*(1/sCs)/(Rs+1/sCs)=Vin/(sRsCs+1)=0.3sin(2*pi*50)t (assume sRsCs-->0)
and then use
Ic=Cs*(dVc/dt)
to get the result 0.3768mA.
What I really want to ask is if there would be a problem with the assumption made in the calculation. In other words, if I chose the width 0.4um, you think it will be all right. Because during the start time of the sampling, that is charge the Cs with voltage from 0 to Vc, the current can be very large. Am I right, would this be a problem? If it is so, then how can I chose the proper width. If I am wrong, plz let me know. Thank you!

 

[erikl]

Re: [Help]How determine the width of the wire for sampling p

during the start time of the sampling, that is charge the Cs with voltage from 0 to Vc, the current can be very large. Am I right, would this be a problem?

Not at all. The recommended max. current density of 1mA/µm is valid for an average current value. A single short excessive peak even of several orders of magnitude in excess doesn't matter, as long as the max. average current density isn't exceeded.

BTW, the initial charge current is still limited by the switch's Req (and the output impedance of your power supply).

 

[BackerShu]

So it is the average current density that matters.

Thank you erikl!