[SI-LIST] Re: Current Sense Resistor Measurement
- From: "Tom Dagostino" <tom@xxxxxxxxxxxxx>
- To: "'Curt McNamara'" <curt.mcnamara@xxxxxxxxxxx>, "'ERIK KUNDRO'" <kundro85@xxxxxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Wed, 30 Nov 2011 14:56:16 -0800
Let's see, 12V across 50 Ohms is almost 3 watts. I don't think there is a
scope that will handle that kind of input power. If you want to use a scope
with a 50 Ohm termination at its input I'd put a 450 Ohm or 950 Ohm resistor
in series with the coax to get a 10X or 20X attenuation of the input signal.
Tom Dagostino
Teraspeed Labs
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Suite 102
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USA
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tom@xxxxxxxxxxxxx
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Teraspeed Consulting Group LLC
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-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Curt McNamara
Sent: Wednesday, November 30, 2011 12:31 PM
To: ERIK KUNDRO; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Current Sense Resistor Measurement
It is possible (likely?) that using 50 ohm coax has resulted in a 50 ohm
termination in the o'scope, which has loaded the circuit.
Using regular scope probes will help. As others note, the grounds need to be
the same between the scope and your test system.
Many of us are using the INA209 from TI, and they have an application board
that could work for you:
www.ti.com/lit/ug/sbou055a/sbou055a.pdf
Curt
Curt McNamara, M. Eng., P.E. // Principal Electrical Engineer
LOGICPD
Logic Product Development
411 Washington Ave. N. Suite 400
Minneapolis, MN 55401
T // 612.436.5178
F // 612.672.9489
curt.mcnamara@xxxxxxxxxxx
www.logicpd.com
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-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of ERIK KUNDRO
Sent: Tuesday, November 29, 2011 9:04 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Current Sense Resistor Measurement
I am trying to measure the current going into a DC-DC converter by removing
a series fuse and placing a small value resistor in its place. I want to
make the measurement using an oscilloscope so I can get a plot of the actual
current waveform, not just an rms number like with a DMM (I need info like
peak current and the current waveform shape).
I'm measuring in system, so I'm in a tight space and single ended type
probes are too large. The input to the DC-DC is powered by 12V, but all the
differential probes I see do not have a common mode spec anywhere near 12V
(most seem to have a max common mode of about 3V). So it doesn't look like I
can use a differential probe to measure across the resistor since it sits on
a 12V line.
I tried using two coax cables, each with one end cut and soldered from power
to ground on my PCB on both sides of the resistor. Then I used the scopes
math function to subtract one channel from another to obtain a current
waveform. But for some reason the coax cables seem to be causing some
ringing or resonance like oscillation that is greatly distorting the
measurement.
I am running out of ideas here... anyone make a measurement like this
before? Any thoughts or suggestions? I'm getting desperate here. Seems like
it should be an easy measurement to make, but I can't seem to be able to do
it.
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