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Why are V(rms) measurements frequency dependant?
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sourcecharge:
So, I've got this great new add-on for my current measurements, the uCurrent. 

It is going to allow me to measure DC current up to 999.99 mA with a 0.05% accuracy with only 10 milliohm added impedance.

The problem is that in order to measure AC current, the VAC from the meter must be used. 

I have measured a signal from my FG and have found that the V(rms) is changing from 45hz up to 10khz.

My meters (Mastech 8040), and all of the meters I've seen so far, seem to all have the error problem that is listed in the accuracy table specifications.

They all have an error that is frequency dependent.

Why is this?

Can it be solved like a uCurrent add-on to get an accurate VAC measurement that would cancel any frequency dependence?

Is there a meter that is already out that can accurately measure VAC across a broad range of frequency up to say 1 Mhz, or even non frequency dependent?

I know that I could just use low turn on diodes to find a Vp, and reverse calculate it, but I'm not sure even it that would be non frequency dependent. 

Would this be possible as a realistic piece of equipment so that this is not necessary?
JS:
Because of frequency reaponse of the DMM chipset. When using an onboard TRMS converter inside the DMM chipset is uaual to get reasonable specs up to 1kHz and then starts to be less accurate. Better DMM have a dedicated IC to make the TRMS conversion and get good specs up to 100kHz or so. Compare BM235 and 121GW, EEVBlog's DMMs and you'll see. BM235 is using the chipset and only specified up to 800Hz, 121GW is using an external IC and specs go up to 5kHz. There are audio TRMS converters which do a good job over 20kHz depending on the level.


If you want to go higher you could use a dedicated  higher freq instrument. TRMS doesn't go much higher but you could build a higher freq measurement by just using a diode. μCurrent leaves you with a decent level to amplify a bit further and use a diode rectifier to get a value. If you use something like a voltage doubler you will be seeing something close to a peak to peak value.

JS
sourcecharge:
I get what you are saying, and it makes sense.

How expensive would it be to make a high quality DIY type Vrms meter.

Basically, it would be only for AC Vrms.

It would output VDC so that normal 4.5 digit meters could be used, and use that 2032 battery.

It sounds like a lot of programing firmware, some pricey ICs, and a custom board, but I think it would be much cheaper than a 100000 count meter.


Where do I start?
JS:
A single 2032 might not be enough, you usually want higher levels to get a bettee linearity. Start looking at a TRMS converter from a parametric search on your supplier. Once you pick one see what you need around it... With that approach no need for programming anything. What are the specs you are trying to get from it?

JS

sourcecharge:
Well, I guess I just want a Vrms meter that's gives me the correct Vrms output without having to worry about frequency.

That might be asking too much...maybe 100khz bandwidth being a flat output? That might be asking too much too.

I took a look at PMIC RMS to DC converters, and digikey does not look like they have a good selection.

The AD8436 is in there, but I think the thing to do is to rip open a really really expensive 100000 count meter and check out what it's packing.

Maybe I'll find some toober videos that show their guts.



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