[SI-LIST] Re: Current sense resistors and power integrity
- From: "Mikhail Matusov" <matusov@xxxxxxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Fri, 22 Aug 2008 17:41:24 -0400
Thanks everyone for your help and suggestions. This is actually a production
board, so the only real choice I have is to replace the resistors with a
piece of metal or something of that sort. They are by the way 2512 Panasonic
ERJM1W 1% tolerance type. Inductance is claimed to be less than 1 nH. I had
a mysterious failure on this board, which I could only explain by something
being wrong with the power. I was about to rent a VNA to try to measure the
PDN impedance when I discovered that the core voltage was actually set at
the low threshold of the part's spec, and after the resistor it was a few mV
below the minimum spec. I increased the voltage and everything works fine
now, but I am still curious to know whether I should get rid of the
resistors... Perhaps, they at least should have been in the regulator's
loop...
Steve, when you are saying to place the resistor back at the VRM, do you
mean at the input of the VRM?
Thanks,
/Mikhail
----- Original Message -----
From: "steve weir" <weirsi@xxxxxxxxxx>
To: "Mikhail Matusov" <matusov@xxxxxxxxxxxx>
Cc: <si-list@xxxxxxxxxxxxx>
Sent: Friday, August 22, 2008 4:05 PM
Subject: Re: [SI-LIST] Current sense resistors and power integrity
> Mikhail, what you need to do is maintain a low enough impedance on the
> load side. The resistor / etch drawbridge or other method will introduce
> a substantial inductance that you will need to compensate for with a
> suitably low inductance capacitance on the other side, that also does not
> resonate with the inductance in your signal frequency range.
>
> What I would recommend you do if you can is place your current sense back
> at the VRM, and then measure with your chip of interest at quiescent
> versus active. The quiescent on most FPGAs and DSPs can be made pretty
> small. This way you don't need extra parts in your bypass network, and
> you won't starve your parts.
>
> Steve.
> Mikhail Matusov wrote:
>> Hi all,
>>
>> Is that a really bad idea to insert a small current sensing resistor,
>> e.g. 3 mOhm, followed by a bulk capacitor in each power rail of a let's
>> say a DSP or a FPGA chip to be able to measure its power consumption?
>>
>>
>> Thanks,
>> =======================
>> Mikhail Matusov
>> Senior Hardware Design Engineer
>> Square Peg Communications
>> Tel.: +1 (613) 271-0044 ext.231
>> Fax: +1 (613) 271-3007
>> http://www.squarepeg.ca
>>
>>
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>
>
> --
> Steve Weir
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