If you don't mind, please run that test for me on your 189 and let me know if you see the same thing. This one shows revision 015. I find it hard to believe this meter is not plagued with the same problem but this one sure seems to look good.
I conducted the tests at 50 Hz since that is my local mains frequency and the 189 is also set to that. I could redo the tests with the 189 set to 60 Hz if that would make any difference.
In the 500 mV AC range (the lowest ACV range), it reads 500 mV
RMS with 2.8 VDC offset (the maximum my function generator will do at this amplitude) without issue (within 0.1% from the value without offset, and that could easily be my function gen).
In the 50 mV DC range (manual ranging), a 45 mV DC signal with 50 Hz AC superimposed on it:
Generator mVRMS (50 Hz) | F189 reading (mV DC) |
50 | 45.20 |
100 | 45.19 |
200 | 45.41 |
500 | 45.91 |
1000 | 46.76 |
2000 | 48.61 (last digit unstable) |
3000 | 48.32 (last digit unstable) |
4000 | 46.00 (last digit unstable) |
5000 | 37.5 (last digits unstable) |
6000 | 28.7 (last digits unstable) |
7000 | 23.7 (last digits unstable) |
If I upranged to 500 mV with 7 V
RMS AC, then it would show 51.7 mV, which could very well be correct. So I would say it seems to be performing worse than its NMRR spec of > 90 dB, which would have allowed only 0.2 mV DC change at 7 V
RMS AC. Changing the frequency to 49 Hz or 51 Hz did not change the reading at all, suggesting to me that the AC signal is not reaching the ADC (which should have a much worse NMRR at 49 Hz than at 50 Hz), but affecting the analog path. If you let it autorange, it would keep switching between 500 mV and 5000 mV above 2 V
RMS, so you do have to wrestle it to get it to display misleading values.
This is relying on the offset of the function gen to be correct at high amplitudes. It would probably be more accurate to AC couple the function generator and use a separate DC voltage source to add the offset. But that would have been more labor intensive.