[SI-LIST] Re: Calculating Zin for a DSP Clock

  • From: Jesús Castañé <jesuscastanemnez@xxxxxxxxx>
  • To: si-list@xxxxxxxxxxxxx
  • Date: Mon, 24 Nov 2014 10:41:58 +0000

Hi,
Thank you all for your replies.

Ray, so if these parameters are not available what can i do?

Todd, could you explain why I can´t use that equation for this? I don't
understand why It would look like the input capacitance.

Tom, thank you for your comments. The track will be pretty long, around 300
mm and the clock will be 25 Mhz so we want to include some terminations to
assure the signal integrity.

I've never heard about calculating Zin from the physical dimension of the
package. Could I do that? It seems a bit weird to me.
I don´t understand your last comment "  Trying to compute the Zo of this
pin plus C_comp is likely not the correct equation to use.  A better model
would be the Xc of C_pkg and C_comp." Could you please explain it?

Thank you in advance.

Jesús Castañé
uk.linkedin.com/in/jesusalbertocastanemartinez/



On 21 November 2014 at 23:21, Tom Dagostino <tom@xxxxxxxxxxxxxxxxx> wrote:

> Todd pointed out that the input impedance of this will mostly look like
> the input capacitance of the pin/die in a private conversation.  This is
> true, especially at the 25 MHz frequency of operation of this crystal
> oscillator.  Trying to compute the Zo of this pin plus C_comp is likely not
> the correct equation to use.  A better model would be the Xc of C_pkg and
> C_comp.
>
> Tom Dagostino
>
> Teraspeed Labs
> 9999 SW Wilshire Street
> Suite 102
> Portland, OR 97225
>
> tom@xxxxxxxxxxxxxxxxx
> www.teraspeedlabs.com
>
> 971-279-5325 office
> 503-430-1065 cell
>
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
> On Behalf Of Tom Dagostino
> Sent: Friday, November 21, 2014 11:20 AM
> To: HUBING@xxxxxxxxxxx; si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: Calculating Zin for a DSP Clock
>
> Todd, well yes and no.  This part seems to specify the clock input of 25
> MHz or a period of 40 nsec.  I'll bet that period is much longer than the
> electrical length of the net so for 25 MHz it could be a good approximation
> of the input impedance.
>
> Tom Dagostino
>
> Teraspeed Labs
> 9999 SW Wilshire Street
> Suite 102
> Portland, OR 97225
>
> tom@xxxxxxxxxxxxxxxxx
> www.teraspeedlabs.com
>
> 971-279-5325 office
> 503-430-1065 cell
>
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
> On Behalf Of Todd Hubing
> Sent: Friday, November 21, 2014 10:29 AM
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: Calculating Zin for a DSP Clock
>
> Jesús,
> Even if you have the correct values for L_pin and C_pin, the equation you
> cite cannot be used to calculate the input impedance.
>
> Todd Hubing
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
> On Behalf Of Jesús Castañé
> Sent: Friday, November 21, 2014 4:49 AM
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Calculating Zin for a DSP Clock
>
> Hi Experts,
> I'm trying to calculate the Zin for the CLK in the ADSP21469.
>
> As far as I know I can use the next equation (Sorry for the plain text
> format)
>
> Zin= srqt [L_pin / (C_pin + C_comp ) ]
>
> I have tried to find this parameters in the IBIS model but  L_pin and
> C_pin are defined as N/A
>
>  L1 CLKIN PDXO01DG NA NA NA
>
> You can find the IBIS model in this link.
>
> http://www.analog.com/static/imported-files/ibis_models/ad2146x_cspBGA_ibs_ver.1.3.txt
>
> Why are these parameters defined as N/A? What am I missing?
>
> Thank you so much for your help. I find this list really interesting.
>
> Regards,
>
> Jesús Castañé
> uk.linkedin.com/in/jesusalbertocastanemartinez/
>
>
>    -
>
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