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From: Jeff Layman <Jeff@invalid.invalid>
Newsgroups: sci.electronics.design
Subject: Re: Wideband ammeter
Date: Sun, 23 Mar 2025 08:31:19 +0000
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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?

-- 
Jeff