LTC6655 based voltage reference stability after 1 year

Last March 2016, I made my first voltage reference based on LTC6655 BHMS8 and recorded the first stability results. I haven't the time to make PCB and breadboard with low noise LDO and several decoupling capacitors was used. For the first 8 days, using Keithley 2002 DMM, I measured voltage fluctuations in the range of 124 ppm (min 2.500100V, max 2.500410V). 

The result was not so great, but expected because any voltage reference needs hours for stabilization in the long term. The value is specified in the datasheet under long term drift name. In my case, expected long term drift was 60 ppm after the first 1000h. Usually this characteristic has logarithmic nature and with the time is decreasing.

Next measurements were made at September 2016 with better results. After approximately 4000 hours, during period of 2 weeks measurement, I got 52 ppm stability : min 2.500320V, max 2.500450V.

The last measurements, made in April 2017 were much better. The result was 5.2 ppm stability: min 2.500255V, max 2.500268V. This result was more with regard of the room's temperature changes. According the datasheet, the expected temperature coefficient has typical value of 1 ppm/°C and maximum value of 2 ppm/°C. The recorded temperature difference in the room was 7°C (17°C-24°C). This is less then 1 ppm/°C. All measurements were performed automatically for about 70 hours during period of 9 days. If I compare them with the last September results, they are within 25 and 70 ppm range for about 5000 hours period.

All measurements for the last year were made with the same DMM. It was calibrated in March 2016 and March 2017.  

I will keep running the voltage reference for the next 12 mounts. I want to collect voltage drift data after stabilization to figure out the expected DMM accuracy using this low priced voltage reference.

Project status update, March 2017

I will use this post to give some updates for the status of openDCM project for the last 4 months. 

During this period I tried to collect information about calibration process and had contact with two National Measurement Institutes about possibility to calibrate direct DC voltage reference standard. The prices were reasonable for calibration against Josephson voltage standard (around 500 Euro for ±0.02 ppm uncertainty) and against Fluke 7010N/T (around 100 Euro for ±0.8 ppm uncertainty).

Voltage reference standards on the market costs several thousand USD (even for the second hand) and they are not affordable for me. Voltmeter calibrators costs even more. So I started digging into this area and found the excellent thread about LTZ1000 based voltage reference. Even with the most expensive Vishay HZ series resistors, stable 7 to 10V amplifying, gold plated tellurium copper banana jacks, and low EMF cables it will cost less then 500 EUR. The cost here is very important, because for the good calibration practice I have to make 3 or 4 voltage references. For the voltmeter calibration and linearity testing, I'm thinking to use a PWM voltage divider and the LTZ1000 based voltage reference.

One more thing, which I'm thinking to build is a null voltmeter detector. It is used to compare two voltage sources if they are equal. Using this tool I can transfer the hot calibrated voltage reference to other voltage references and to check drift between them.

My plan for 2017 year is to finish the low accuracy openDCM prototype and to start working on the calibration process (voltage standard, null meter, PWM voltage divider) and the high accuracy prototype based on the LTZ1000 reference and multi-slope architecture.  

The reason for starting this project was found in a shoebox :-)

Finally, I found my lost Kyoritsu DMM KEW1012 in a old shoe box.

Now after almost one year of research and all the knowledge, which I collected about the measurements, I have feeling that this DMM is like a toy. His basic DC accuracy is ±0.5% ±2 digits and the input impedance is 10MOhms for the useful ranges.

Even if my calculations for the first voltmeter prototype are 10 times worst, it will be still 50 times better then the specification of the KEW 1012 DMM. 

Nevertheless, I can still use this DMM for the high voltage ranges and for the frequency and duty cycle measurements. So, I will not throws it in the bin for sure :-).

Initial schematics for low accuracy voltmeter prototype PCB is ready

The initial schematics for the first PCB prototype is done. 

It can be found in the git opendcm repository:

https://github.com/opendcm/opendcm/tree/master/prototype/pcb/low_accuracy/kicad

The changes from breadboard prototype are related mostly to increased over-voltage protection. The voltage supply of the input buffer LTC2057HV was increased to ± 30V, which will protect up to ± 40V with 1K resistor in series. Because the automatic voltage range switch ADG5419 is rated maximum to ± 22V, additional 1K resistor in series was added to the 20V range op-amp buffer LTC2057HV powered by ± 22V. By this way if input voltage is increased to ± 40V, the input voltage of the ADG5419 switch will be limited to ±22V by the power supply of the LTC2057HV buffer on the pin 8.

There are no changes in the ADC section at all:

 The power supply and reference section:

The power supply will be more complicated as I though in the beginning. It should provide 7 positive and negative voltages. I'm not sure now if an AC with isolated transformer or step-up dc-dc converter from battery will be used as main power supply. The current consumption which I measured only on analog part, except the uController, was ~15mA, which is suitable for battery powered supply.

The next thing which I have to experiment is the connection between the ohm-meter and the voltmeter input.

The final multi-meter prototype will contain 3 PCB - one for the voltmeter and the ohm-meter, second for the current meter and the third for the power supply.  

Preliminary accuracy estimation for the voltmeter

It is time to make preliminary accuracy calculation before making the first PCB prototype.

Initially I had target of 100ppm year accuracy for the low accuracy version with off-the-shelf ADC and 20ppm for multi-slope ADC version. Lets see if selected components will fit into the sub 100ppm target.

The relative accuracy of the voltmeter is given as +/- percent of reading plus percent of the range. The reading corresponds to the gain error of the voltmeter and the range is related to the offset error. Because the voltmeter contain several components, the worst case error is the sum of all errors from each component, but if errors are uncorrelated, root sum square method can give more accurate estimation.

I will try to estimate the accuracy for one year period and for 10 °C temperature range between 18 and 28 °C.

The picture below shows all components contributing to the accuracy errors of the voltmeter:

The accuracy estimation is shown in the table below. I used the worst maximum values extracted from the datasheets of the components.
The accuracy of the LTC2057 op-amp is calculated twice : as input buffer and as buffer in front of the SPDT switch. The accuracy of the voltage divider resistors LT5400 is only calculated for the 20V range, because it is used only for this range. The gain error of the op-amps are calculated based on the open loop gain value. The gain error of the ADC is calculated when the offset is subtracted from the Full Scale Error (FSE).

		2V Range		20V Range
		Gain error, ppm	Offset error, ppm	Gain error, ppm	Offset error, ppm
LTC2057
	Offset		2.50		0.25
	Offset drift temp.		0.13		0.01
	Gain	0.18		0.18
LTC2057
	Offset		2.50		0.25
	Offset drift temp.		0.13		0.01
	Gain	0.18		0.18
LT5400
	Temp drift	0.00		10.00
	Long term year	0.00		2.00
LT1001A
	Offset		7.50		0.75
	Offset drift year		6.00		0.60
	Offset drift temp.		3.00		0.30
	Gain	3.33		3.33
LTC2440
	INL		5.00		5.00
	Offset		2.50		0.25
	Offset drift temp.		0.10		0.01
	Gain (FSE-Offset)	8.00		8.00
	Gain drift temp.	1.92		1.92
LTC6655
	Temp drift	20.00		20.00
	Long term year	20.00		20.00
		2V Range		20V Range
Accuracy		Reading, ppm	Range, ppm	Reading, ppm	Range, ppm
Using worst case method		53.61	29.35	65.61	7.44
Using root sum square method		29.65	12.04	31.35	5.12

Using the worst case method, I got ~83ppm for full-scale measurement for the 2V range and ~73ppm for the 20V range.

Using the root sum square method, I got ~42ppm for full-scale measurement for the 2V range and ~36ppm for the 20V range.

The actual accuracy can be reduced even more with the software calibration for the gain and the offset and only temperature drift and long term gain and offset errors will remain. 

Looking into the table data, the voltage reference will contribute the most for the gain error and the ADC buffer for the offset error, except for the 20V range, where the ADC INL is dominant.

Of course, the final accuracy shall be measured after one year usage of the voltmeter using a laboratory grade voltmeter or with voltage standard.