[SI-LIST] Re: PSU ripple measurement
- From: Istvan Novak <istvan.novak@xxxxxxx>
- To: "Lakshmi N. Sundararajan - PTU" <lakshmi.s@xxxxxxxxxxx>
- Date: Mon, 15 Mar 2010 20:24:26 -0400
Hi Lakshmi,
This is indeed a tricky question. I personally do not agree with the
recommendation either.
Not necessarily because the added 10.1uF capacitance would change the
output loading
too much, though that can happen as well. I dont like this
recommendation because this
tells me the noise at the capacitor, but at other places it might be
more important to know.
To be fair with the vendors, they have no way of knowing the myriads of
different user
layouts, even if everyone would be using the same exact parts, which is
clearly not the
case. So from a vendor prospective this ensures some degree of
repeatability in the
testing, which is a valid argument on their side. This is usually not
an excuse when
the vendor supplies an evaluation board with test points to measure
ripple: I dont like if
the test point is across a capacitor.
Many times the recommendation is to measure ACROSS the capacitor body,
not NEAR
a capacitor. This is clearly inexcusable, because we dont have anything
connected
across the top terminal points of capacitors, so whatever we measure
there is irrelevant
for the application. The downside is that measuring across a capacitor
body will alter
the noise signature: some components may be boosted, some will be
attenuated, so we
dont get the correct answer.
My suggestion: follow the instructions and measure the output ripple the
way how it is
recommended (so that you can compare your data against the spec), but at
the same
time measure also in your actual system application at the location(s)
where it matters
for you.
Regards,
Istvan Novak
Oracle
Lakshmi N. Sundararajan - PTU wrote:
> Hi Team,
> I was wondering if any one would help me on this requirement.
>
> To measure the PSU ripple output over a given bandwidth, the PSU vendor
> specifies I need to attach a 10uF and a 0.1uF cap at the point of
> measurement.
>
> But, I find this requirement very strange.
>
> 1) Adding these caps change the loading of the PSU output. It
> modifies the real system.
>
> 2) The probe attached to the PSU output to measure ripple, can be
> set to high impedance, which will then have very low effect on the PSU.
>
>
>
> Why then this requirement has to be met to measure the PSU ripple.
>
> This requirement is not part of a single spec, but I see this mentioned
> in many prominent PSU specs too.
>
> Can anyone please shed more light on why we need to do this?
>
>
> Thanks,
>
> -LN
>
>
>
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