[SI-LIST] Re: Calculating Zin for a DSP Clock
- From: Todd Hubing <HUBING@xxxxxxxxxxx>
- To: "jesuscastanemnez@xxxxxxxxx" <jesuscastanemnez@xxxxxxxxx>
- Date: Mon, 24 Nov 2014 23:31:06 +0000
Jesús,
Your original post seemed to indicate that you wanted to calculate the input
impedance (i.e. looking in through the package to the clock input). The clock
input (C_comp) is approximately 5 pF and has an input impedance that is
approximately 1/(2*pi*f*C_comp). If you have properly controlled your
transition times, L_pkg and C_pkg will not affect the signal integrity of your
25 MHz clock.
You don't provide enough details about your application to determine whether it
is necessary to provide a matched termination, but if you do provide a matched
termination, you should match the transmission line characteristic impedance,
not the termination input impedance. The transmission line characteristic
impedance is sqrt(L_trace/C_trace) where L_trace and C_trace are per unit
length parameters of the transmission line.
Todd
Todd Hubing
Clemson University
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Jesús Castañé
Sent: Monday, November 24, 2014 5:42 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Calculating Zin for a DSP Clock
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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