Review: £10 multimeters - Now includes the new RS-14!

[Murphyv310]

I certainly agree Mark.
My little Fluke 10 is what I use on my valve stuff. It's a meter I've owned for 12 years and was bought then from a used test equipment supplier in Kilwinning Ayrshire of all places.
The Chinese meters will be used in the main on low voltage stuff etc, so far though I'm impressed with the purchase and a big thanks to you for the heads up and time you've taken to evaluate them.

[ppppenguin]

The input travels via a 10M resistance straight to to the IC. Inside the IC, that signal goes straight to an amplifier, but at the input to the amplifier are a bunch of FET switches that are used to connect external resistors to form a potential divider with the 10M, according to the required range. In mV, none of these resistors are engaged, hence the high input impedance. The fact there are two millivolt ranges for both the AN8002 and AN8008 is because of the ability to switch the input amplifier between x1 and x10.

A high value resistor like that in series with the input must give horrible AC frequency response unless they fit compensation caps, a bit like in a scope probe. If the chip has a 10pF input capacitance and the reisistor is 10M the 3dB point will be under 2kHz.

[Murphyv310]

Hi.
I concur with Jeffrey. The spec though on AC volts and milli volts is 40-1000Hz,  feeding 15v p-p at 1566kHz gives a fluctuating reading obviously as does my Fluke 10 but surprise surprise an old iso-tech  IDM96 read 11v.
On a 440Hz tone the AN8008 was spot on as was the Fluke.
At 1000Hz both were slightly down and at 2000Hz the Fluke was wildly low with the 8008 reading about 3db down .

[ppppenguin]

So as a basic rule, unless you know otherwise, don't trust a DMM on AC over a few hundred Hz.

Trevor, pesumably when you wrote 1566kHz you meant 1566Hz, not just over 1.5MHz.

[Mark Hennessy]

Some multimeters do indeed have caps in parallel with the 10M, but a lot don't. Many multimeters - including some really quite expensive ones like the Fluke 179 - are only good to low-kHz...

Obviously, it has to cope with the input voltages, and so it ends up being a reasonably expensive part. You often find a trimmer in there too, which also has to take the voltage, and someone has to set up on the production line (and it's an additional calibration step).

IIRC, you have an original Fluke 87? The schematics for that are here: http://assets.fluke.com/manuals/83_85_87smeng0500.pdf - note C37 and C20.

The DTM0660 chipset used by Aneng and countless others implements the true-RMS conversion digitally, and as such has a digital low-pass filter included as part of that. The -3dB point of that filter is 3kHz. The spec is 400Hz or 1kHz.

An AC bandwidth stretching beyond 10kHz or so is considered to be a "premium" feature, and is typically only found on the more expensive meters. The best cheap one I can think of off the top of my head is the Fluke 101, which I measured as -3dB at 11kHz, which implies about 1.5pF of capacitance to ground after the 10M input resistance. Though the spec of that one is only 500Hz.

Bottom line: you have to know your instruments.

[Murphyv310]

No Jeffrey. I was feeding the pantry transmitter just for a laugh. Was kind of gobsmacked with the Iso Tech. I looked for a spec on the tinternet but drew a blank.

Perhaps some irregularity or pick up giving a false reading, I doubt it could read RF. It was though giving good accuracy at 5kHz.

[Murphyv310]

Mark.
The spec is 40 to 1000Hz not "or" which may cause confusion.

[ppppenguin]

My 87 (an original 87, now more than 20 years old) is specified to 2% accuracy up to 20kHz. This must be at least in part due to the compensation caps noted by Mark.

Thanks for the link to the service manual. I downloaded it a long time ago. Full marks to Fluke for making their historic serrvice data easy to find.

[Mark Hennessy]

Mark.
The spec is 40 to 1000Hz not "or" which may cause confusion.

Hi Trev,

I did mean 400Hz or 1kHz - it depends on which meter the DTM0660/DM1106EN chipset is in. It's 400Hz for the AN8002, and 1kHz for the AN8008 and AN860B+. I'm not sure why they differ, given that the IC itself is the defining element, and in my tests, they all comfortably met their spec at 1kHz and all have exactly the same -3dB point of 3kHz. Specifications are generally pretty conservative though (except for CAT ratings )

Hope that clears things up.

Mark

[Murphyv310]

Ah I understand.
Not got the AN8002 yet to have a perusal at the specs.
Thanks again Mark.