Hacking the Siglent SDM3055 Bench DMM

[alexvg]

If the big error is for high value electrolytic caps it is fairly normal for a multimeter ... even two different Fluke don't display the same value and could be easily 10% off . For the simple method  of charging/discharging the cap the time is dependent on losses and current used ...

The error is spread over all the measures (but above the Siglent specs).
The current used is specified into the SDM3055 datasheet (200nA to 1mA).
I don't understand why Siglent doesn't incorporate a little frequency generator to do a more precise and FAST capacitor (and inductance) measurement using the 2 or 4 wires

[alexvg]

I've done DCI and ACI. It was very funny because DCI 200uA range have an internal inverted polarity (-11uA measure and +11uA display).

[alexvg]

Here is a list of reverse engineering features done :
- DCV (all range, filter mode and 10M/10G for 200mV and 2V)
- DCI (all range and filter)
- ACV (all range)
- ACI (all range)
- Resistance 2-wire (all range including 100M !!!)
- Resistance 4-wire (all range including 100M !!!)
- Frequency (all ACV range)
- Capacitance (all range)

Remaining functions :
- Diode
- Continuity
- Temperature

Edit: All functions done !

[tv84]

Edit: All functions done !

  Where there's a will there's a way!

[Kleinstein]

I've done DCI and ACI. It was very funny because DCI 200uA range have an internal inverted polarity (-11uA measure and +11uA display).

An inverted polarity could happen if they use a trans-impedance amplifier for the low range. In this case I would expect to have at least 2 such ranges, possibly the option to add a 20 µA range.

[alexvg]

It's possible to have more range, but I need to do a noise measurement (FFT analysis of the ADC running at 5KHz).
It's also possible to easily replace the used ADC with a better one (like the AD7175-2 used in SDM3065X with some additionnal electronic).
The SDM3055 board is more interresting than the SDM3045 and SDM3065, modifications are easy but available space inside is very limited.

[alexvg]

This is just a first test for the new interface

[tautech]

Puke green ?   

Otherwise nice improvements. 

[e0ne199]

dang you should use your ability to hack 121GW instead of this DMM...

[alexvg]

I'm currently writing the software using a Windows emulation to have an easier development.

I'm thinking about 2 hardware mods :
1. pre-regulated power supply (I already have components but haven't got time to assemble them)
2. remove ADC and ref and add an additionnal board with a better reference (LTZ1000A) and a better converter (AD7175-2 or AD7177-2)
But i don't know if the pcb and current components provide a noise floor enough lower to enable a good enhancement.

I haven't got the tools to measure the 0.1-10Hz noise at very low level.

[Kleinstein]

Measuring the noise if the DMM is not that difficult. The main tool it the DMM itself.  The first test is measuring the noise with a short at the input and than calculate the noise from the data. The second test needs a low noise external reference to measure a voltage at a significant part of the full scale. This gives the second noise case that includes the noise of the DMM internal reference and maybe some extra ADC parts.

The 0.1 to 10 Hz noise would be approximated by getting data with 20 SPS and calculate the RMS value over some 10 seconds. Chances are one could use a slightly different sampling rate (e.g. 25 SPS) and thus a slightly different frequency range throughout the tests.

For the reference a 7 V reference needs an extra stage to divide down the reference to the ADC level (e.g. 2.5 V maybe 3.5 V). This can add quite some uncertainty / drift. So it is questionable if it is worth using an highly stable LTZ1000 with a divider that adds quite some drift. The 20 V range would still use the front end divider that adds quite some drift and also some noise. For a 2.5 V reference there may be a simpler replacement with another 2.5 V ref chip.

For the ADC, lower noise ADCs may need a lower impedance input signal. As far as I understood it, they use the internal gain for the low ranges (200 mV, current). So the new ADC also needs this option.  So the change is not that simple.  One may have to build a small separate PCB for the Reference and ADC - so first step towards building your own DMM.
The input amplifier (buffer) may be limiting the noise performance. I would expect some AZ OP as a buffer -  a compromise between noise and input bias. At least an AZ buffer should be sufficiently linear.

[alexvg]

Some news :
I'm working on an interface using a HTML/CSS rendering for a more scalable and flexible possibilities/features.
It takes a very long time because the power of the main CPU is very limited: I need to do a high level of optimizations and remove some CSS features (who need too much process time/power).
I don't know when the first alpha/beta version will be release, "probably" in 2-3 months.

Hope you'll enjoy my work.

[supperman]

Quick Questions.. As I'm considering a Siglent DMM:

Why don't they integrate current flow over time to get Amp/H. Seems super easy.. to get them to measure total charge on a battery for instance. (all the data is there). Seems that I would need to log all the readings and then run it in Excel on a PC? Does the Siglent log both Voltage AND current over time? (or only one or the other using the PC software).

@alexvg any thoughts on expanding the math to do integration math? Also, given the time you are spending on this.. why not just design a DMM? Siglent will never let you sell software for their device..

[BillB]

Quick Questions.. As I'm considering a Siglent DMM:

Why don't they integrate current flow over time to get Amp/H. Seems super easy.. to get them to measure total charge on a battery for instance. (all the data is there). Seems that I would need to log all the readings and then run it in Excel on a PC? Does the Siglent log both Voltage AND current over time? (or only one or the other using the PC software).

Good question regarding integrating current.  I don't believe the EasyDMM software supports dual measurement mode at all.  For meters without a channel card option, it only performs a single measurement function.

[alexvg]

Hi, sorry for the delay...
This Siglent DMM only do a single a measurement at a time.
Current/Voltage uses time slicing with a relay.

I'm working on a solution to provide multiple length and simultaneous math integrations.

[klausES]

Hi there,

I think I read somewhere that the temperature (at which the factory was calibrated) was 23.5 degrees Celsius?!?
(I'm not sure anymore, can't find the text anymore).

If I remember correctly, it would be the temperature at which the voltage reference and the relevant part of the board later
most closely correspond to the factory calibration in measurement mode.

Are there measurements or empirical values by how much these values in practice, e.g. 30 or 35 degrees C device (inside) temperature could differ
(to what extent the temperature compensation corrects this or not)?

The question has a background (a thought), but I'd rather wait and see if someone answers.

[Kleinstein]

How much the readings can change with temperature is sometimes noted in the instructions. Usually some +-5 K is included in the base accuracy  for more I would expect something on the order of 2-15 ppm/K as an upper limit. The lower ranges (200 mV and 2 V) that don't use the input divider can be better, as the reference used and the ADC have a relatively low TC. The input divider for the higher ranges (20 V, 200 V,...) is expected to have lower accuracy, due to the TC of the divider that is more in the 5-15 ppm/K range.

I would not expect much of an extra correction in software, as this would need an extra step at the calibration.

[klausES]

Thank you for your answer.

Had the following considerations.

It is clear that if constant values ​​are to be compared, it would be advantageous to keep the instrument at the same temperature at all times.
If it is almost all about constant (much more than the best absolute values),
it would not be advantageous to maintain a constant but higher temperature than the constantly fluctuating ambient temperature.

A precisely controlled temperature that is always x degrees higher than the ambient temperature
is much easier to implement inside the device than a possible cooling to keep it the same.
In winter my room has e.g. 18 degrees, but also 30 degrees in midsummer (or sometimes just> 30 degrees, unfortunately no climate).

If the device were now "always from the start" kept at, for example, exactly 30 degrees
(controlled by a precisely temperature-controlled heating and in a control loop proportional fan speeds),
fluctuating room temperatures in the range of 18 to approx. ... 29 degrees would hardly have any changing effects on the temperature in the device.

hence the question:
How much would the absolute values ​​at a constant 30 degrees differ from those of the factory calibration at 23.5?

Edit:
corrected a text error that led to confusion (see above). 

[Kleinstein]

The TC specs for the ADC and reference are at some 1 ppm/K. The actual performance may be a little better than this at moderate / common temperature. So the 6.5 K higher temperature my change the reading by some 5-10 ppm, so hardly detectable with a 5 digit meter.
The ranges with input divider and current ranges are likely less accurate so more like 10 times the change.

For the 5 digit resolution there is not much need for a constant temperature. This would be something for a 7 digit meter. At that level a oven for the reference (e.g. LM399 or LTZ1000) is the normal solution.

[iMo]

hence the question:
How much would the absolute values ​​at a constant 30 degrees differ from those of the factory calibration at 23.5?

In case you have a temperature sensor mounted in the box, you may simply compensate the readings based on that internal temperature. I did it with my meter at 10V range (34401A, I had to mount the sensor).
The absolute accuracy of the temperature sensor is not important here. A resolution of 0.1C would be enough for your meter, imho.
Then you will calibrate against that internal temperature. The DMM starts cold, and you will be logging the external ref voltage readings against the internal temperature - as the internal temperature of the DMM rises it will sweep up the temperature for you. You will get a "Vref" voltage vs. internal temperature dependency.
And the rest is math like y=ax+b (or whatever your dependency is like) within the internal temperature range of interest.
The internal temperature of your meter (after it stabilizes, it may take hours) is always Tdmm = Tx + Tambient, where Tx is a constant (an assumption here is your meter is not internally thermostated).

[klausES]

Thanks for the explanation.

I will take a closer look at the behavior and test the suggestions when the device arrives (hope tomorrow).

At first I only came up with the idea because the fan will get a temperature-controlled control right from the start (possibly also a quieter fan type).
Then a regulated "very specific selectable and then constant" higher temperature than the fluctuating environment would have offered itself.

[Kleinstein]

A temperature regulated fan is a two sided thing. With changing air speed temperature gradients can change - this can have more effect than the overall temperature change.  It really depends on the circuit if a temperature controlled fan helps of makes things worse.

[klausES]

Thanks for the information. I can understand your objections regarding fluctuations.
I'll pay attention to that.

In any case, a linear series regulator is planned here (neither pwm on the power side,
nor something clocked in the measurement and control).

There will also be no "fluctuations" by simply switching on / off,
Instead, an adjustable minimum speed (always active as a base load), an adjustable incline, a delta t> 1 kelvin
and a limitation of the maximum speed (moderate, far below the possible full speed of the fan).

[killingtime]

Really like the new interface. It's an improvement on the standard.

Putting the measurement units after the measured value (e.g. mV) in larger typface like Keithly have done makes it easier to read.

Making the measured value digits larger would be an idea - if there is space on the display. Means you can read the meter from further away.

Well done.

[alexvg]

Hi,

I'm currently working hard on the HTML/CSS rendering engine.
I don't know when I could release a test version.


It's a long way...

Edit: You could try these mods :
6.5 digits : https://www.eevblog.com/forum/testgear/hacking-the-siglent-sdm3055/msg2614659/#msg2614659
7.5 digits : https://www.eevblog.com/forum/testgear/hacking-the-siglent-sdm3055/msg2612406/#msg2612406