Author Topic: new DM3068: high zero readings in AC current, shunt resistance too high  (Read 880 times)

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Offline AndyT

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Hi,
I am just testing out my new DM3068 and found (at least in my opinion) some issues.

With no leads connected and a quiet RF backgound I have the following readings in AC current mode (20 Hz):
(readings are about the same with a short connection between the input connectors)
range     reading
200 uA:  100 nA
2 mA:    338 nA
20 mA:      5 uA
200 mA:  33 uA
2 A:       650 uA
10 A:     4.3 mA

My dealer says: "this is well within specs"

When I start the selftest after being in AC current mode it fails with errors 609 and 615.
When running the selftest after being in any othe mode it passes the test.
My dealer says:  "RIGOL says, the selftest does not work and will be removed in a firmware update."
My firmware: 01.01.00.01.10.00

I measured the input resistance in DC and AC current modes:
range            resistance
<2 mA          103.3 Ohm  (datasheet: 100 Ohm)
>=20 mA         3.3 Ohm   (datasheet 1 Ohm)
My dealer says: "you are measuring the shunt + internal wiring + the fuse resistance"

Can anyone confirm my measurements? Any comments? Is this ok or should I return the instrument?

Thanks!
Andreas
 

Offline 2N3055

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Re: new DM3068: high zero readings in AC current, shunt resistance too high
« Reply #1 on: February 20, 2018, 09:13:41 am »
From Rigol "DM3068 performance verification guide"

Range    Input     Zero offset max /1Year
200.000uA    open    ±30nA
2.00000mA    open    ±60nA
20.0000mA    open    ±3uA
200.000mA    open    ±6uA
2.00000A    open   ±400uA
10.0000A    open    ±1mA

So no I would say offsets you are seeing are way out of specs.. Shunt resistance might just be fine... They are right, wires and fuse are included when you are measuring it from outside.....

Regards,

Sinisa
« Last Edit: February 20, 2018, 09:15:20 am by 2N3055 »
 

Offline EVS

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Re: new DM3068: high zero readings in AC current, shunt resistance too high
« Reply #2 on: February 21, 2018, 12:28:36 am »
>2N3055
It's performance guide for _DC_ Current only

>AndyT
Hi, Andy.
My DM3068 ACI open input (FW 01.01.00.01.10.00):
 
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Offline 2N3055

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Re: new DM3068: high zero readings in AC current, shunt resistance too high
« Reply #3 on: February 21, 2018, 12:48:04 am »
>2N3055
It's performance guide for _DC_ Current only
:palm:
Yes sorry, my bad...
 

Offline AndyT

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Re: new DM3068: high zero readings in AC current, shunt resistance too high
« Reply #4 on: February 21, 2018, 12:51:57 am »
Hi EVS,
thanks for the pictures!

My dealer is refering to the ACI accuracy specifications of the meter.
Since the measured value is 0 the percentage of range value applies.
So at 200 uA a value of 120 nA on the display would be within specs.
Or at 10 A even 8 mA are ok.
But your values show me that this is not good at all.
Other 6 1/2-digit multimeters have similar accuraties. I wonder, if they have also such high open input readings.

regards
Andreas
« Last Edit: February 21, 2018, 12:54:15 am by AndyT »
 

Offline Pinkus

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Re: new DM3068: high zero readings in AC current, shunt resistance too high
« Reply #5 on: February 21, 2018, 12:58:39 am »
The dealer shall open another box and try this one out. If it is better, they shall you send this unit in exchange.
If they do not want to help you, just send your unit back (in case you do not own it for longer than 14 days - "Fernabsatzgesetz") and buy another one elsewhere.

 

Offline TurboTom

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Re: new DM3068: high zero readings in AC current, shunt resistance too high
« Reply #6 on: February 22, 2018, 01:04:51 am »
Andreas -

I only now had the opportunity to cross-check with my DM3068 and the figures are in the same ballpark as yours, maybe slightly lower but not substantially. The measured resistance across the terminals is also quite similar to yours. I rather question the readings that @EVS published -- plain zero in those low AC ranges appears rather strange to me, even in the DC current ranges there are slight excursions (noise) once in a while, and I guess that's completely normal. If the meter is reading zero, I'ld assume the output of the RMS rectifier is biased negative which would negate the opportunity to measure very low AC currents. I'ld rather have some noise background reading and use the "relative" function instead. Yet, a superposition of RMS noise and RMS signal in the same level range can also lead to some erroneous readings if doing just arithmetic subtraction. Don't know how the DM3068 is proceeding here, probably had to do some further testing...

So let's put it like this -- either your DM3068 is okay or we're already two with a faulty instrument...  ;)

Cheers,
Thomas
 
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Offline Macbeth

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Re: new DM3068: high zero readings in AC current, shunt resistance too high
« Reply #7 on: February 22, 2018, 03:19:03 am »
Other 6 1/2-digit multimeters have similar accuraties. I wonder, if they have also such high open input readings.

My 3.5 year old 5 1/2 digit DM3058, which has worse specifications, open ACI readings:

Range       Reading
20mA       00.000,0mA
200mA     000.000mA
2A            0.000,00A
10A          00.000,0A

ETA: I recall wraper had awful zero readings on his DM3068 and argued with Rigol who used the "it's in specification" line.

He got a replacement after months of wrangling.

If you are within your 14 day cool off period you can send it back (at your postage cost) for a full refund.
« Last Edit: February 22, 2018, 03:34:15 am by Macbeth »
 
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Offline Performa01

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Re: new DM3068: high zero readings in AC current, shunt resistance too high
« Reply #8 on: February 22, 2018, 11:47:23 am »
Hi,
I am just testing out my new DM3068 and found (at least in my opinion) some issues.

With no leads connected and a quiet RF backgound I have the following readings in AC current mode (20 Hz):
(readings are about the same with a short connection between the input connectors)
range     reading
200 uA:  100 nA
2 mA:    338 nA
20 mA:      5 uA
200 mA:  33 uA
2 A:       650 uA
10 A:     4.3 mA


TRMS Converter have an offset by design, it does not affect accuracy and the REL function must not be used to get rid of it.

How can that be?

Just have a look at the specifications, e.g. the footnote for ACI :

Quote
[8] Specifications are for sinewave input >5% of range. For inputs within 1% to
5% of range, add 0.1% of range additional error
. Specifications are typical
values for 200uA and 2mA, 2A and 10A ranges when frequency >1kHz.

So your DMM just cannot measure a zero AC current.
You'll find a similar footnote in the specifiactions for any TRMS multimeter.

I've just checked one of my best bench DMM, Keithley 2001:

200µA  : 500nA
    2mA : 5.15µA
  20mA : 51µA
200mA : 510µA
    2A    : 5.18mA

Should I start worrying now? ;)
 
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Offline AndyT

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Re: new DM3068: high zero readings in AC current, shunt resistance too high
« Reply #9 on: February 22, 2018, 09:42:28 pm »
Hi Performa01,

thanks for checking with your 2001, that´s very interesting. I have not used by myself such high class multimeters, so I have no comparison. Yes I know about the offset of TRMS-DC-converters, but I was hoping that they use a kind of auto-offset trim from time to time in such an instrument. But I find the postings of EVS and Macbeth´s DM3058 also very interesting.
REL-ing is of corse no option (was off on EVS´s pictures). They seem to have even a zero offset (what I would not expect).

What bothers me most, is the high input resistance of 3.3 Ohm @ 200 mA. I selected this instrument for its advertised 1 Ohm shunt.
Anyway, I just returned it to the dealer for full refundimg. The search for a good multimeters goes on....

regards
Andreas
 

Offline Performa01

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Re: new DM3068: high zero readings in AC current, shunt resistance too high
« Reply #10 on: February 22, 2018, 11:30:04 pm »
thanks for checking with your 2001, that´s very interesting. I have not used by myself such high class multimeters, so I have no comparison. Yes I know about the offset of TRMS-DC-converters, but I was hoping that they use a kind of auto-offset trim from time to time in such an instrument. But I find the postings of EVS and Macbeth´s DM3058 also very interesting.

I have seen this on cheap multimeters (e.g. UNI-T) at times, even for DC; they just suppress low readings and make them artificially zero. This looks good, but it's a fake. For TRMS AC it is legitimate though, as the range below at least 1% of the range is unusable anyway. For the Keithley 2001, ACI rms measurement is even only specified for 10-100% of the range.

My Picotest M3500A does the same thing, it shows zero below 1% of the AC range (and it’s a quite good meter). The A-brands like Keithley 2015 THD, Keithley 2001, Fluke 8840A and 8842A do not.


Quote
REL-ing is of corse no option (was off on EVS´s pictures). They seem to have even a zero offset (what I would not expect).

I thought someone else (not you) mentioned this, so I just wanted to make it clear that this is a big no-no.


Quote
What bothers me most, is the high input resistance of 3.3 Ohm @ 200 mA. I selected this instrument for its advertised 1 Ohm shunt.
Anyway, I just returned it to the dealer for full refundimg. The search for a good multimeters goes on....

Current measurement is always a problematic issue. With high shunt resistors, the burden voltage is high too and introduces systematic measurement errors, plus the self-heating of the shunt causes additional errors at higher currents. With low shunt resistors, we cannot get high accuracy right from the start, because of the low voltage drop. This is why serious bench multimeters the burden voltage will either not be low or they have a miserable accuracy as was common a few decades ago, where many bench meters only had a single shunt resistor and only very few current measurement ranges, only one of them with decent accuracy.

On the Keithley 2001 I can measure ~2.4 ohms in the 200mA range.
 

Offline EVS

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Re: new DM3068: high zero readings in AC current, shunt resistance too high
« Reply #11 on: February 23, 2018, 08:00:59 am »
Small test - measurement of the sensitivity limit for 200mA ACI range. For a current of less than about 0.025mA - exactly zero (my DM3068).
I think it's not matter what the device shows for a current less than 2mA (1% of 200mA range).
 


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