Playing with delta sigma converter, poor man's meter.

[joeqsmith]

Having a little fun with the PC and an ADC.   The ADC is a bit sensitive to outside fields and I ended up putting it into the cookie tin with a towel over it.  The reference is not very stable and would need to be in a temperature controlled housing.   The attached shows how the simple ADC compares with the Gossen Ultra and my HP 344401A using the same 1.9Meg Caddock TK 10PPM part I have been using. 

Second graph shows all of the meters I have looked at.  I let the ADC run for a fair amount of time and it does eventually start to calm down.   

[s8548a]

Which software is this? link please? (I looked at the UT181a product page, but that looks different)

Thanks.

[joeqsmith]

Which software is this? link please? (I looked at the UT181a product page, but that looks different)

Thanks.

Its something I put together for the 181A.   If you want to know more about it, just go to my YT channel (link in signature) and it was the last video I made.

[joeqsmith]

To use the ADC as a simple volt meter, I used a Caddock USVD2 for divider network.  These have a tempco of 2 ppm.  Again, the ADC is in the cookie tin with a towel and was allowed to warm up before I collected any data. 

http://www.caddock.com/Online_catalog/Mrktg_Lit/TypeUSVD_HVD.pdf

I added support to my software to support my the HP34401A so the two could be displayed on one graph.  I used the Fluke 731 to do a simple 2 point alignment.   

The attached graph shows the two volt meters attached to my large 1KV power supply, set to roughly 100 volts.   The HP34401A is the Red trace and the deltasigma is White.    I started the test early and you can see the furnace cycling.   As I walk into the room, we can see a fairly large change in the supplies output voltage as well.   

******************
I should mention that the screen shot was luck.  They are tight but not that tight. 

[joeqsmith]

My little lab has stayed open and we can clearly see the two meters continue to track very nicely while the supply continues to drift. 

[joeqsmith]

Had to open the cookie tin to change over to the reference standard.    Short clip sped up showing the initial warmup.  It will take some time to settle again...

https://youtu.be/P49chFp0-58

[joeqsmith]

I let the board's temperature settle out and realigned it to the reference then restarted the test.

The HP is very stable compared to my little experiment which wanders all over the place. 

[joeqsmith]

The Fluke standard was set to 990uV but I had aligned the ADC thinking it was at 1mV.   The HP is close to matching the reference. 

I had left the cookie tin and towel and just changed the output of the reference.   You can see the HP with the Fluke reference are very stable while the ADC test circuit has a poor drift.   I suspect this is all thermal. 

[joeqsmith]

Into the meat packing box.  I use the HP34401A to measure the boxes thermistor but we know the reference is stable and how much the ADC circuit was drifting. 

I plan to run it at 35.  Just enough so I can get the temperature stable.   You can see how badly it drifts as I ramp the temperature. 

[Vgkid]

What is the adc/reference?

[joeqsmith]

Temperature was 35 and let it settle to within +/-0.05 deg.  I then realigned the ADC and started collecting.   The test was ran for 11 hours and the temperature remained within this window for the duration of the test.

Once again, roughly 70uV.   There are no gain stages at this time.   It may be time to try adding a low noise front end to it.   

[joeqsmith]

My old HP34401A still has me by a fair margin but its moving in the right direction. 

Don't ask me why I was posting a uV.  Not enough sleep.  The Fluke 731B is still set to 1mV.   Back to the cookie tin and towel.   Let it run more than double the time (several hours).   

[joeqsmith]

Tracking the drift of my vintage HP power supply while running in current mode.  Switching from 170mA down to 5mA.   Red is my HP34401A bench meter. 

[joeqsmith]

Measuring the voltage and current to my UNI-T UT61E with a dead 9V.    Dip is the backlight and then watching the battery recover.   

http://www.ti.com/lit/ds/symlink/ref5025.pdf

[joeqsmith]

Looking at the UT61E's power draw for 4 hours w/ the same 9V battery.   The wide increases are the backlight.  I am not sure what the spikes are.  The seem fairly periodic. 

[joeqsmith]

No more battery.  Ramping down the UT61E's supply voltage until the meter drops out.

***
Two signals combined with current on graph set to 1000X (mA rather than Amps).   

[joeqsmith]

Project was getting to be a bit of a mess to work with.  Put together a simple insulated and temperature controlled, shielded case.   Shown with the cover being removed and installed a few times.   

[joeqsmith]

The temperature is much better now but the case could use a bit more insulation.   I'll let it run for several hours to get an idea how the lab temperatures are going to effect it.   

[joeqsmith]

It's a big improvement but it's still too sensitive.  I tossed a towel over the whole setup and it causes a fairly large disturbance.   

[joeqsmith]

First graphs shows switching from 10K to 100 to 1K ohms.  The second is zooming into the 1K ohm area.   Still a fair bit of drift. 

[joeqsmith]

Measuring the Fluke 731B and continuing to to make changes.   Also included is my HP 34401A.    Sample rate is 1Hz, or 2.8 hours per run.

[joeqsmith]

With the circuit now somewhat stable, I let it run along with the HP34401A for roughly 7 hours.   Standard deviation is still not as good as the HP. 

Wrong plots.....

[joeqsmith]

For the diode test the voltage and current it can supply is a bit higher than your typical meter.     

Here the Brymen's battery leads are connected.  I select that the meter uses a 9V alk battery.  The voltage is then swept to 9V, held for some time then ramped down.    Both current and voltage profiles are recorded.  The nominal current and eventually the cutoff voltage are measured.   I am thinking it could then provide some figure of expected life.   The cutoff may be different for each meter.  The Brymen will throw up some really strange numbers with a dead battery.  Maybe just detect the first sign of a slope change.   I am thinking the battery database would be common.  The idea would be to automate this measurement to a standard.   

Just playing for now. 

[BFX]

Interesting thread Thank you joeqsmith 
 

[joeqsmith]

Having a meter like this being controlled from a PC means we can automate various tests.  For example, I made a simple menu to walk through a battery life test for the handheld meters.   In the following, the Brymen BM869s is measure twice.  The voltage is set, then the meter is turned on.  The backlight is then turned on.  All the functions of the meter are then selected.   

I then repeat the test for the Fluke 189.   The 189 has two backlight settings.   We can also see the current draw is not flat like the Brymen. 

The software measures the nominal current, the backlight current and the cutoff voltage.  It also calculates the battery life.  For the BM869s which uses a single transistor battery, it estimates 80 hours.  For the 189 with it's 4 X AA, 127 hours.   These are just using some Wiki numbers for alkalines.  All of this data would be saved for later. 

I ran one other meter for fun that also uses a 9V.  There are some major differences.  Of course battery life for something you  use in the lab may not be as high a priority as say measurement performance.