[SI-LIST] Re: Bandwidth Problem
- From: steve weir <weirsi@xxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Tue, 25 Mar 2014 02:29:20 -0700
Propagation delay specifications will get you into the ballpark. Pay
attention to specified test conditions. Beyond that a simulation model
will reveal if a given part can meet your needs.
Steve.
On 3/25/2014 2:21 AM, SoneR wrote:
> Hi All,
> I am trying to decide a voltage translator IC and I have some questions
> that I believe I can get answer only from an SI specialist.
>
> In most of the related products, neither frequency vs gain curves nor
> maximum rise time values are provided in the datasheets. So I cannot be
> sure about the bandwidth even if it is described.
>
> They generally give propogation delays for high to low and low to high
> cases for different load capacitances empirically. However, tpd and BW
> should be mutually exclusive.
> (Yes, +trace length ==> +tpd ==> +loss ==> higher frequency harmonics
> affected more ==>+RT ==> -BW but I think this is irrelevant for our case)
>
> There is another parameter, input capacitance, and I believe it is a better
> way to reach BW. (change in RT = 0.5*Z*C)
> Let say, required (single) data rate is 100Mbps and 2nd harmonic is enough
> ==> BW=200[MHz] ==> RT=0.35/200 = 1.75[ns].
> As 0.35 is more than adequate, 2[ns] looks satisfactory for the rise time.
> Then, change in RT, let say, 10% is good ==> 0.2[ns] change is the maximum
> value.
> For an 50 ohm interconnect, Cin(max)=8pF is calculated.
>
> To summarize,
> (1)Am I right with ignoring all tpd values to get BW?
> (2)Am I right in the input capacitance based method?
> (3)If the answer for (2) is NO, what do I miss? How do I get maximum data
> rate?
> (For example, what is the maximum data rate I can transfer over:
> http://www.ti.com/lit/ds/symlink/sn74avcb164245-ep.pdf for arbitrary Va,Vb
> values?)
>
> I would be glad if someone can help me.
>
> Regards,
>
>
--
Steve Weir
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