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From: Cursitor Doom <cd@notformail.com>
Newsgroups: sci.electronics.design
Subject: Re: Wideband ammeter
Date: Sun, 23 Mar 2025 09:06:08 +0000
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On Sun, 23 Mar 2025 08:31:19 +0000, Jeff Layman <Jeff@invalid.invalid>
wrote:

>On 23/03/2025 05:47, john larkin wrote:
>> On Sun, 23 Mar 2025 03:59:46 -0000 (UTC), Sergey Kubushyn
>> <ksi@koi8.net> wrote:
>> 
>>> Bill Sloman <bill.sloman@ieee.org> wrote:
>>>> On 23/03/2025 12:49 pm, john larkin wrote:
>>>>>
>>>>> How about thermal imaging a surface-mount resistor?
>>>>
>>>> Why bother? Measuring the voltage drop across the same device is easier,
>>>> and just as fast, if not faster.
>>>
>>> That depends on what you actually want to measure. And "wideband" makes it
>>> even more difficult.
>>>
>>> Wideband current shunts made for AC-DC transfer are all of very special
>>> costruction and cost arm and leg. If you want to measure the voltage drop
>>> over those resistors, without making AC-DC transfer, you're up to another
>>> challenge, measuring the AC voltage. Should start from the definition, what
>>> IS the AC voltage? What the actual number your measurement shows means and
>>> so on.
>>>
>>> Look at e.g. not all that precise but much better than most LT1088 chip,
>>> long obsolete. There is another one, proprietary and much better precision
>>> inside e.g. Fluke 5790A Standard (which is a misnomer -- it is actually an
>>> AC and DC voltmeter, 10x more precise that the venerable HP/Agilent/Keysight
>>> 3458A).
>>>
>>> ---
>>> ******************************************************************
>>> *  KSI@home    KOI8 Net  < >  The impossible we do immediately.  *
>>> *  Las Vegas   NV, USA   < >  Miracles require 24-hour notice.   *
>>> ******************************************************************
>> 
>> Many otherwise great DVMs have an AC bandwidth that doesn't even
>> handle the audio range.
>> 
>> The resistor has great common-mode rejection too.
>
>There is a skin effect in a surface-mount resistor (see 
><https://www.vishay.com/docs/60107/freqresp.pdf>). If an AC current 
>flows nearer the surface of the resistor, would that make it "hotter" 
>the higher the frequency? In other words, would the ammeter also be a 
>frequency meter?

What's the big deal here? Just use a oscilloscope with a current
probe.