Testing DMMs ACV using a scope calibration signal - Page 1
Forum section: electronics / testgear

author: jpb
author_rank: Super Contributor
timestamp: June 06, 2014, 09:03:29 pm
content: I've been playing with my new Keithley 2000 and checking it against my ancient uncalibrated Advantest 7451A (£170 off e-bay a year ago and badly recapped by me).I thought to use my WaveJet calibration signal to look at ACV as it is a square wave and therefore a good test of true RMS.The results are amazingly close between the scope and the two meters.The test signal is 1kHz square wave with 50% duty cycle (nominally) going between zero and 600mV. The AC rms value is therefore 300mV but if you include the dc component (AC+DC) then you get a value sqrt(2) times bigger - this is the value the scope gives.The attached pictures show the experimental setup, a screen grab of the scope, then the the Advantest set to AC+DC (the K2000 doesn't have this option I think) and then the two meters with both set to ACV .The scope measures 421.6mV rms (averaging used) which divided by sqrt(2) is 298.12 mV rms.The K2000 measures 297.889 mV rms.The AD7451A measures 298.03 mV rms or 421.8 mV (AC + DC rms)The WaveJet is very accurate for a scope, given that it is only 8 bit but I guess using 256 sample averaging combined with equivalent time sampling (to average further) gives a higher effective resolution.NB I've found on dcV the Advantest was reading about 0.01% higher than the recently calibrated K2000 so the ACV are consistent with this.

author: G0HZU
author_rank: Super Contributor
timestamp: June 07, 2014, 02:07:20 pm
content: I must admit I find it hard to stay awake when considering accuracy requirements down below about 0.5% for ACV measurements as I simply don't need this level of accuracyBut I did try comparing my old Fluke 45 against a 2015 on a similar waveform and found that they both agreed to 5 digits on the first test. Which was a surprise. However, a relatively large and clean square wave is an easy waveform in terms of amplitude and crest factor and 1kHz is a very low test frequency.If I gang both meters using coax and vary the levels then across a range of level inputs from a few mV to 1V (from a voltage source) the 2015 generally reads slightly higher. But I'm probably not doing a very well controlled test and I'm not particularly fussed about the results...egFluke 45 vs 201510.138mV vs 10.139mV103.51mV vs 103.57mV200.74mV vs 200.84mV500.18mV vs 500.34mVThese results are good in terms of agreement but I generally wouldn't use a DMM for measuring ACV mainly because I don't trust the DMM to cope with more challenging waveforms. eg looking at a noisy baseband output from a downconverter.I traditionally use a Racal rms meter for stuff like this and cross check it with a decent DSO and maybe look on an analyser as well to see what the noise looks like in relation to the wanted signal. DMMs tend to get a bit crappy if you run them at very low levels into the rms detector (eg sub 1mV) because they lose bandwidth and accuracy in my experience. So it's a fairly rare event for me to select ACV on a DMM

author: G0HZU
author_rank: Super Contributor
timestamp: June 07, 2014, 03:45:47 pm
content: I tried (separately) feeding a noise waveform out to 50kHz into both meters and unsurprisingly the Fluke 45 started to lose out here. However, the 2015 agreed very well with my Racal meter at 9.94mV rms. I monitor the DMM output from the Racal to get this resolution.The Fluke 45 showed just under 9.89mV rms but was very jumpy. I had some filtering enabled on the 2015 and the Racal meter gives a steady reading anyway.However, if I drop the level to 0.62mV rms then the Fluke loses out even more and only shows 0.47mV rms as I think it loses bandwidth with low levels like this. The 2015 and the Racal both hovered around 0.621mVrms. However, I don't think the 2015 is spec'd when the drive level is less than about 5mV.Therefore, I would expect that the 2015 would suffer a similar fate if I were to test it at around 1mV (sine) and up to 200kHz at the input. i.e. it would lose bandwidth and go slightly deaf at low levels like this at 200kHz.

author: jpb
author_rank: Super Contributor
timestamp: June 07, 2014, 04:16:07 pm
content: Quote from: G0HZU on June 07, 2014, 02:07:20 pmI don't really have a need to measure ACV, I was doing purely to "test" the meter (i.e. play with my new toy). The agreement between the two meters wasn't surprising but I was surprised how accurate the scope was given that it is working with 8bits albeit with averaging. Particularly so given that there has been a few threads on this forum from people with scopes that are way out on simple dc measurements though still within the scopes nominal 3% specs.Following your post I might try and introduce a bit of attenuation and see what happens though I don't think I can get below a mV - my scope probably wouldn't be able to measure that level anyway.I would guess that the target application for the DMM ACV (and ACV + DC) is measuring mains related signals and ripple on power supplies perhaps at levels well above 1mV.

author: jpb
author_rank: Super Contributor
timestamp: June 07, 2014, 05:39:57 pm
content: I have now repeated the experiments with a much tougher test by putting an attenuator in series with the calibration signal.The results are a very noisy curve until I smooth it with averaging (on the scope - the DMMs see the noisy signal).First I did it with dc so I could only compare it with the Advantest as the K2000 doesn't have AC+DC.The first screen grab shows the noisy signal, the second the smoothed signal.The rms value from the scope is 6.512mV the value from the Advantest is 6.92mV.Then I used ac coupling.The next screen grabs show the noisy and smoothed signals and finally the DMM values.There is good agreement between the scope and the K2000 the scope value is 9.124mV and the K2000 gets 9.094mV but the Advantest only reads 7.85mV.

author: G0HZU
author_rank: Super Contributor
timestamp: June 07, 2014, 06:28:36 pm
content: I think my old Fluke 45 and the 2015 are able to cope well with noisy audio. I was quite surprised how well the 2015 coped in the 50kHz noise tests.It also has the ability to measure Vrms via its ADC/DSP but I haven't really tried it with very small signals.I've always used a Racal true rms meter for ACV because it has a 20MHz BW and can accurately work down to tiny levels, eg 30uV. The main reason I bought the Fluke 45 was because it has a very useful dB REL function and I could connect it to the back of the Racal and get dB readings via the DC output port on the Racal. The accuracy is better this way and it's far more convenient. eg I can zero the dB scale on the 45 and quickly make (S+N)/N measurements with the combined Racal + Fluke meters.When I first got the 2015 I did a few tests on it to see how it compared to the Fluke 45 on the dB REL function and I tested them using a precision attenuator. The attenuator is a precision part rated to 1GHz and does 0 to 50dB in 1dB steps. These tests are taken direct to each meter on ACV and the Racal isn't involved. I did similar tests using DC as the source and got very similar results. I was really just testing the attenuator and logging the results but the meters do agree very wellThe meters should both be better than the attenuator at 10kHz so the tiny offsets are mainly due to the attenuator performance but here's the results I took:Tests at 10kHzLevel Fluke 45 Keithley 2015THD0 0.00 0.00 dB-10 -10.01 -10.007 dB-20 -20.07 -20.073 dB-30 -30.00 -30.010 dB-40 -39.93 -39.958 dB-1 -0.99 -0.989 dB-2 -1,98 -1.98 dB-3 -2.99 -2.994 dB-4 -3.98 -3.989 dB-5 -5.00 -5.002 dB-6 -6.02 -6.03 dB-7 -6.99 -6.994 dB-8 -7.95 -7.951 dB-9 -9.00 -9.000 dB-10 -10.02 -10.022 dB

author: G0HZU
author_rank: Super Contributor
timestamp: June 07, 2014, 07:51:34 pm
content: I did a few low level tests on the 2015 on ACV and it doesn't perform that well at higher frequencies. eg 100kHz.Also, at very low signal levels (eg 1mV) the bandwidth tails off a lot if you select the 'fast' option so don't get seduced by the lower offset it displays in this mode.I also tried testing the sensitivity on ACV using the internal DSP and some old software I wrote for the 2015 and it seems to work down to about 5mV rms and then says the signal is underrange. But I'm doing this via GPIB and may have missed something because I'm also doing lots of other measurement commands at the same time via GPIB. The meter + SW stopped working below drive levels of 5mV as the meter reported underrange.One of the neat things you can do with the 2015 is to look at harmonic distortion and I tried two different AF sources. One was from my trusty old Marconi 2024 sig gen AF output and the other was from an HP/Agilent ESG 4000D sig gen via the LF output. See the analyser plots below.You can see they are both extremely clean sources but the Marconi is slightly better according to the 2015. But it looks like you can't go below about 5mV drive level in this mode.

author: jpb
author_rank: Super Contributor
timestamp: June 07, 2014, 08:02:28 pm
content: Quote from: G0HZU on June 07, 2014, 07:51:34 pmThe specs for both high frequency end (100kHz - 300kHz) and the low end below 10Hz are significantly worse than in the optimal range of 10Hz to 20kHz.On the K2000 the specs for 50kHz-100kHz are 0.60+0.08 % but at 100kHz-300kHz it leaps to 4 + 0.5%.I guess the chip/circuits are optimised for the audio range.