Ravinder Active probe brought a lot of noise into the measurement result. If the signal is measured, the noise does not matter. If the noise in power is measured, the noise matter. I do not know the effects of the differential probe. Best Regards Zhangkun 2004.8.5 ----- Original Message ----- From: <Ravinder.Ajmani@xxxxxxxxxxxxxx> To: <zhang_kun@xxxxxxxxxx> Cc: <si-list@xxxxxxxxxxxxx> Sent: Thursday, August 05, 2004 9:42 AM Subject: [SI-LIST] Re: Measurement peaks of current. > Zhangkun, > Will you please elaborate why can't we directly use an active differential > probe across the resistor, instead of connecting it to an Op Amp. > Regards, Ravinder > Server PCB Development > Hitachi Global Storage Technologies > > Email: Ravinder.Ajmani@xxxxxxxxxxxxxx > > > > > Zhangkun <zhang_kun@xxxxxxxxxx> > Sent by: si-list-bounce@xxxxxxxxxxxxx > 08/04/2004 05:55 PM > Please respond to zhang_kun > > > To: tal_segev@xxxxxxxxx, si-list@xxxxxxxxxxxxx > cc: > From: si-list-bounce@xxxxxxxxxxxxx > Subject: [SI-LIST] Re: Measurement peaks of current. > > > > > Tal > > In the CPCI, there is one kind of method to measure the current in the > power. One resistor of 5m ohm is used to get the current and the voltage > between the two terminal of the resistor are input to one IC. In the IC, > there is one op-amp. This method is similar to yours. > > My suggestion is that the trace, which connects the resistor and the IC > input pin, should be differential pair. Active probe should not be used. > In our situation, it is difficult to get the exact current in the power > net. > > Hope this will help > > Best Regards > Zhangkun > ----- Original Message ----- > From: "Tal Segev" <tal_segev@xxxxxxxxx> > To: <si-list@xxxxxxxxxxxxx> > Sent: Thursday, August 05, 2004 4:34 AM > Subject: [SI-LIST] Measurement peaks of current. > > > > Hi all, > > > > 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, > > Tal > > > > > > > > > > > > > > __________________________________ > > Do you Yahoo!? > > Yahoo! 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