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I've started with development of DC voltage reference used for calibration. At first time it seems to be an easy task. There are two solutions. LTZ1000/ADR100 and LM399/ ADR1399. Circuits are well known, there are plenty of reference designs using those components. Depending on what are the priority, you choose certain design. e.g. low drift, low noise, high long term stability. The problem with all precision analog designs is that they are not production ready. You cannot simply follow certain reference design procedures and expect the same performance across all boards. At the end you have to measure and qualify the precision analog design. I have learned from previous project that the devils in precision analog design are in details. I started looking for ways to measure and qualify the high performance DC voltage references. Linear technology provides a lot of useful information. See links below. I'm interested to measure mainly two parameters
- low frequency noise
- Vref stability - find relation between stability and external disturbance factors.
Most of the time you doit need to reinvent the wheel. With this in mind I started looking for existing solutions. I was surprised that most of them just re-iterated around Linear technology ideas from begin 90's.
- cookie box - check
- battery powered - check
- environmentally controlled environment - check
- old fashion scope - I have problem with this component. More on this later
- band limited 0,1...10Hz low noise amplifier. - this component seems to be not up to the modern Vref requirements. We need something 100..1000 better f 0,1mHz...10HZ
- The performance of voltage references has been increased dramatically.
- Initial accuracy of 0.1%..0.5 are common.
- noise performance is improved too - 1/f corner frequency is moved at least one decade low. Sampling windows of 100sec intime domain is not enough .As result even the 0,01Hz low freq range is not low enough.
- modern semiconductors processes allow to produse components with tight tolerances - or at least calibrate device during the production.
- The modern sigma-delta converters allows to measure extreme low voltage signal from sensors. Typical application is sensor sends differential signal with huge common mode component. The signal is differentially amplified and further converted to digital. Application like seismic analysis, torque sensors or weight scale measure differential signals in nanovolts range.
- Scope has never been an precision measurement instrument - I have learned at school that scope is not designed to perform precision measurements. It was long time ago-almost 30years ago, but even the ultra modern 16 bits scope are not even close to the entry level acquisition system.
- Digital signal processing and control theory made huge progress last decays.
- standalone scope is not more the most preferred EE tool - Not only the theory made giant leaps but the engineering tools made huge progression in term of features reliability and easy to use.Traditional scope manufacture struggle to keep pace with the tools development. As result - MATLAB and Python becomes the preferred engineering tool. In most cases the standalone scope is used as acquisition card
- Take the good and well proven designs from the past - cookie box and battery powered design.
- Use differential measurements as much as possible. Modern Sigma delta ADC are capable to measure them.
- Use modern software and hardware equiupment and tools.
- Take care on budget - It is hobby and not sponsored project.
- Project Keywords
- software driven design
- signal processing
- machine learning
- statistical models
- observer
- feed forward from self learning model
- big data science
- Output result to be an Vref model
- lean hardware up to the modern hardware development
- Design guidance
- use signal differences and avoid absolute values.
- absolute values are measured less frequently with high accuracy equipment
- create model of particular Voltage reference sample - parameters like temperature coefficient, long term drift, temperature hysteresis, power up-down hysteresis are calculated based of long term data processing.
- detect pop-corn and 1/f noise in multichannel configuration
- achieve much better performance compared to the classical analog approach.
– The signals are sampled differential.
- The assumption is that samples are statistical independent the noise power at a specific frequency is simply the addition of the noise power of the signal pair.
- The number of differential channels N is calculated N = n*(n-1)/2
- The block diagram below uses only 3 input signals for simplicity. N = 3*2/2 = 3 A = X1-X2 B = X1-X3 C = X2-X3
- for 4 input signals = N=4*3/2=6 ; X1-X2, X1-X3, X1-X4, X2-X3, X2-X4, X3-X4
- The fist input signal noise is calculated with the following equation. X1=(1/3)(((X1+X2)+(X1+X3)+(X1+X4))-(1/2)((X2+X3)+(X2+X4)+(X3+X4)))
- The general rule for noise calculation is: Xi=1/(n-1)*(SumWith(Xi)-1/(n-2)*SumWithout(Xi))
- The first stage LPF filters the broadband noise. Cutoff frequency of 50Hz is chosen. LPF filters has reset input to flush/ or refill the history pipeline.
- pop corn noise detector – not only detect by calculate the amplitude of each Vref channel
- TC estimation
- Long term drift - polynomic calculation
- Humidity Vref model
- Power cycle behavior/model