FreeLists / si-list / [SI-LIST] Re: Measurement peaks of current.

[steve weir <weirsp@xxxxxxxxxx>]

Mike, another way to derive current measurements above the PS closed loop 
frequency is to calibrate transfer impedance near the power supply with a 
VNA, and then to measure  the voltage noise with a scope.  The scope 
results can be fed back into an FFT and then normalized against the 
impedance found with the VNA.  It is important to use test pads and not 
measure at a capacitor.

For frequencies below the VRM closed loop response, you can temporarily 
kill off the response of the power supply feedback to ease measurements 
down in frequency, or alternatively, sample and filter the current sense of 
the power supply if available.

Steve
At 04:29 PM 8/4/2004 -0600, msharpes@xxxxxxxxxx wrote:
>Tal -- I have had a need in the past for current measurements over time,
>particularly coincident with timing (specific commands or clock edges
>for read and write for example).  Depending on what equipment you have
>in your lab and if you just need a one time measurement or need to poll
>a circuit regularly, there are different solutions.  The best solution
>for a one time measurement that I've had good results with is a Hall
>effect sensor.  This is a ferrous toroid - we happen to have one in our
>lab made by Tek that plugs into a 'scope channel and you can display
>edge events from voltage probes along with current measurements on the
>same screen (of course you interpret the V/div as A/div for the current
>probe).  The problem with this approach though, is that you need to be
>able to place all the current carrying conductors you wish to measure
>inside the toroid.  PCBs don't lend well to this approach, but if you
>have a length of wire in your design (or can prototype into your
>design), that represents the current you are looking to measure, then
>you're set.  If you are still prototyping you could probably figure
>something out there.  Otherwise, you may need to design in a shunt for
>the purpose of measuring current.  Many companies make precision shunt
>resistors for exactly this purpose (in the 5mOhm or 10mOhm neighborhood
>perhaps). ..mike
>
>
>-----Original Message-----
>From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
>On Behalf Of Tal Segev
>Sent: Wednesday, August 04, 2004 2:34 PM
>To: si-list@xxxxxxxxxxxxx
>Subject: [SI-LIST] Measurement peaks of current.
>
>
>Hi all,=20
>
>I'm designing an embedded system with 8 DDR-SDRAM
>(DDR-1) chips, large FPGA, CPU & so on. Due to the
>amount of current gorges on the design, the result of
>power calculation for this board is about 45W, without
>the VTT and not considering the voltage regulators
>efficiency, (a beet less then a lamp). I personally
>predict that this design will use about 25W in
>average.
>
>I need to measure the current, especially the peaks of
>the current; I'm considering using a serial 1.5mOhm
>power resistor to measure the current through a scope.
>
>So (finally), my questions are (I have lot of them).
>* Is it that simple to measure the current, or am I
>naive?
>* Should I use some OP-Amps to isolate the scope from
>the power?
>* should I use active probe, or simple probe will
>content?
>* Can I avoid using a power resistor (its size is
>huge, even the SMD)?
>* Can any one suggest some other way to measure the
>current?
>
>Thanks for the patience,=20
>Tal
>
>=20
>
>
>
>         =09
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