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

[Zhangkun <zhang_kun@xxxxxxxxxx>]

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
> > 
> > 
> > 
> > 
> > 
> > 
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