Author Topic: EEVblog 121GW Multimeter Issues  (Read 314975 times)

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Offline dcac

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Re: EEVblog 121GW Multimeter Issues
« Reply #1200 on: May 27, 2020, 09:15:34 pm »
I did some test with an hacked routine for the battery display. First optimizing the code considerably, so it's about 40 times faster now which allowed me to increase the oversampling and resolution - which now also is displayed. Pictures show the meter being powered from bench PSU, the Iso-Tech meter showing voltage at battery terminals, or close by as I was powering 121gw through the STlink - this is also why I didn't want to test above 5.5V. It is without any linearity correction at all, just a fix constant to calibrate for the divider error.

I mostly did this hack as I thought it would be cool to monitor the battery voltage more precisely.

@3.50V
 

Offline dcac

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Re: EEVblog 121GW Multimeter Issues
« Reply #1201 on: May 27, 2020, 09:17:25 pm »
@4.00V
 

Offline dcac

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Re: EEVblog 121GW Multimeter Issues
« Reply #1202 on: May 27, 2020, 09:18:04 pm »
@4.50V
 

Offline dcac

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Re: EEVblog 121GW Multimeter Issues
« Reply #1203 on: May 27, 2020, 09:18:44 pm »
@5.00V
 

Offline dcac

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Re: EEVblog 121GW Multimeter Issues
« Reply #1204 on: May 27, 2020, 09:20:09 pm »
@5.50V



Sorry for the picture spamming couldn't get the multi attachment to work...

 

Offline dcac

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Re: EEVblog 121GW Multimeter Issues
« Reply #1205 on: May 28, 2020, 12:57:52 pm »
I’ve also been looking at why 121gw is so ridiculously sensitive to 50/60Hz hum, at least 50Hz seems very poorly rejected, 60Hz is slightly better (less bad) though.

If you look at the hy3131 datasheet the chipset is actually rather capable of rejecting hum, still I’ve seen other ADC’s with better figures but it all also depend on what sample rate (sps) you need and the amount of oversampling (filtering) this equates to, and if you i.e. have the ADC chopper enabled - which helps with both drift and hum rejection but it also cuts the sample rate in half.



This seem to specify 75dB@10sps and 120dB@5sps at 49-51Hz and 59-61Hz but unfortunately it does not specify the filter settings used.
 

Offline dcac

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Re: EEVblog 121GW Multimeter Issues
« Reply #1206 on: May 28, 2020, 01:02:09 pm »
I’m assuming the filter is a sinc^2 as there’s an ^2 indicated in the AD1 diagram, and this also seem to correspond to practical measurements where I’ve used a hacked FW where I can set the filter coefficient manually and have the additional decimation and running average filter bypassed in the 121gw.

But simulating the AD1 filter behavior there seem to be no way to reach 120dB rejection, about 69dB@5sps 49-51Hz is the best it can do at max oversampling. It gets slightly better (about 79dB) with the chopper on but now it is only outputting 2.5sps:


 

Offline dcac

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Re: EEVblog 121GW Multimeter Issues
« Reply #1207 on: May 28, 2020, 01:10:51 pm »
Now in the 121gw the hy3131 is set for 40-80sps in most modes/ranges to give a fast update rate for the bargraph. This is then decimated by 8 or 16 to give 2.5 to 5sps for the number display. Should look something like this:



Notice how poor the rejection is at 49-51Hz only about 33dB, this is probably mostly due to the decimation by 8, which is done by the MCU, only represent a sinc^1 filter response.



(edit: corrected the screenshot for decimated by 16)
(edit2: update traces after bug correction)
« Last Edit: May 28, 2020, 06:12:14 pm by dcac »
 

Offline dcac

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Re: EEVblog 121GW Multimeter Issues
« Reply #1208 on: May 28, 2020, 01:30:57 pm »
Unfortunately decimation by 8 is used for Ohms mode but they selected different hy3131 filter settings for the ranges in an attempt to speed things up, so some ranges have better rejection than others and some worse. But I’m not going to fill this thread with screenshots but this should give some idea what the hum sensitivity problem might be.

The hy3131 documentation is not very helpful, but there's a good application note about the subject here:

https://www.analog.com/media/en/technical-documentation/application-notes/AN-611.pdf

Keep in mind though it mainly describes a sinc^3 filter where hy3131 only uses sinc^2. And I still wonder how it supposedly could achieve 120dB rejection like mentioned in the 3131 datasheet.
 

Offline dcac

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Re: EEVblog 121GW Multimeter Issues
« Reply #1209 on: May 28, 2020, 04:12:46 pm »
This must be a configuration bug - if you look at the Red trace for the 500mV DC range, it is configured for 320sps / 16 = 20sps for the number display. There’s very little 50Hz rejection as the notches falls on either side of this frequency. Also the firmware is much too slow to handle something like 320sps throughput.

Also if you look at the 50mV Blue trace it seems to have little rejection at 60Hz, I’m not sure simulation can be 100% correct though as the filtering is divided over 2 or actually 3 stages if you also count the chopper.

I think the 500mV configuration is a bug for sure though - it's strange it’s been like this in all FW versions.

EDIT: This is not a correct simulation - see  next post


« Last Edit: May 28, 2020, 05:55:20 pm by dcac »
 

Offline dcac

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Re: EEVblog 121GW Multimeter Issues
« Reply #1210 on: May 28, 2020, 05:54:04 pm »
Found a bug in my own simulation code but it mostly affected the 50mV trace.

Anyway this is probably a more correct representation, as you can see 60Hz is much better rejected now. But 50Hz still very bad in the 500mV range.

 

Offline dcac

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Re: EEVblog 121GW Multimeter Issues
« Reply #1211 on: May 28, 2020, 07:18:59 pm »
And this is what the 500mV configuration could look like, blue trace, it's now only 2.5sps but rejection is better - though certainly not great.



121gw also has a running average filter implemented but I've disregarded this because if the interfering hum is above the max delta for this filter - which it most likely is - it will not help at all. Increasing max delta could help but then you get more over/under shoot and longer time for the value to stabilize.

So anyway the 121gw hum rejection could likely be improved by carefully selecting filter parameters, which of course UEi to some extent already done, but I think it can be improved further, and I also think UEi mostly looked at 60Hz rejection. But 50Hz can be improved - perhaps even without needing recalibration.

And actually you could probably improve it allot in some modes/ranges if you give up the fast bargraph and run hy3131 with max filter, but then I'm not sure calibration will hold and the bargraph update rate will be the same as the number display rate about 2.5 to perhaps 10sps.

 
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