The first PCB design of the open source voltmeter is ready

After 5 years of researching, developing and testing, I'm ready with the first PCB design:

The main PCB fits in 10x10cm layout and contains 4 modules : Analog front-end with OVP and two voltage ranges, ADC based on LTC2440, Voltage Reference based on LTC6655 and power supply using classic LM317/LM337 LDO. 

The PCB does not includes the controller for reading out and displaying the measurement results from the ADC.

I have to double check availability of the components, schematics and layout for possible errors before ordering all PCBs next week.

Analog front end schematics:

ADC schematics:

Power supply and voltage references schematics:

There are 2 more PCB : 

- one for AC input circuit with over-voltage protection using GDT and varistors (based on IEC 60950-1 schematics)  ̶a̶n̶d̶ ̶p̶r̶o̶t̶e̶c̶t̶i̶o̶n̶ ̶a̶g̶a̶i̶n̶s̶t̶ ̶i̶n̶t̶e̶r̶f̶e̶r̶e̶n̶c̶e̶ ̶v̶o̶l̶t̶a̶g̶e̶ ̶f̶r̶o̶m̶ ̶t̶h̶e̶ ̶m̶a̶i̶n̶s̶. A friend of mine gave me advice, that main filter is not needed when transformer is used, because it acts like low pass filter for external interference. Only if switching power supplies are used, it is worth to place such of filter. A reader of the blog notice that the transformer is not ideal and always a capacitance exists between the primary and secondary side. Which means that any AC signal (noise) can pass through the transformer. The frequency band is limited to the capacity value and the transformer's frequency loss. So far I was not able to find a tests results about this impact. I found an application note from Schaffner where noise filter is placed between a main line and linear power supply. 

- one for AC-DC conversion which includes again over-voltage protection using TVS diodes and CLC ripple filter. 

I paid special attention for input over-voltage protection after 2 times literally burning the op-amp in my LTZ1000 based 10V voltage reference due mains AC over-voltages. 

Here is the AC input PCB:

Here is the AC-DC PCB:

The AC-DC PCB supports 2 sizes of transformers.

Based on LTSpice simulation and LM317/337 ripple rejection vs. output load current chart, I expect to have around 15uV noise in the 0.1-0.3A current load range after LDO outputs.

And one more news : finally I bought an Agilent 3458A DMM :-)

The DMM is 10 years old and I will make later a post about the modifications and testing some DMM parameters, I was able to made. This will include also a INL against Programmable Josephson voltage standard. 

I think to make a voltage calibrator based on AD5791 DAC and with the help of the 3458a linearity I can measure the real INL of the AD5791. Once this is done, I will be able to measure the INL of the open DCM voltmeter using the AD5791 based calibrator. 

Why I started this project

The short answer : because I was not able to find a digital multi-meter (DMM) with accuracy ~ 100 ppm per year for less then 300 USD.

The long answer : I lost somewhere my Kyoritsu DMM 1012 and start looking for a new DMM with a good price for the accuracy. After few days spent on reading datasheets, comparing prices and reading reviews, I was not able to find good enough for the my price limit. So I started to think, why not to make one?

Based on the basic DC accuracy, I found that there are broadly three categories on the market:
- low end, mostly handheld with DC accuracy > 500 ppm.
- middle end, mostly bench top with DC accuracy between 20 ppm and 250 ppm.
- high end, which are used in calibration laboratories with DC accuracy < 10 ppm.

The price starts from few dozens USD and end up with 10000 USD price tag.

The price of several DMM with accuracy ~ 100 ppm was far away from my budget:
Siglent SDM3055 : 150 ppm basic DC accuracy for 449 USD
Rigol DM3058 : 150 ppm basic DC accuracy for 695 USD
Keithley 2110 : 120 ppm basic DC accuracy for  622USD
Keysight U3402A : 120 ppm basic DC accuracy for  759 USD
Keysight 34460A : 75 ppm basic DC accuracy for 945 USD

I made some preliminary calculations which point me that 100 ppm DC only DMM can be made for less than 300 USD. Of course if you have access to calibration grade DMM which in my case was true.

Why open DC Multimeter?
I believe that the openness, transparency and the free, unrestricted access to knowledge and information keeps the technology to move forward.