Posts Tagged precision

Testing Optical Aspheres

Improvements in machining precision, testing and simulation make the use of aspheres available to improve optical system performance.

Most lenses are spherical, in that each curved surface is some part of a sphere (usually a big radius compared to the lens glass diameter). Lately we’ve been working on some systems that require the use of lenses that have an ‘aspheric’ curve. These are more unusual, but if you can solve a problem that is otherwise unsolvable, ‘unusual’ is a good answer. Ok, maybe since I’m the electronics guy, I’m impressed with the precision of these optics and their measurement – I think you’ll be too, when you look into it.

We’ve found some references about designing and testing these asphere elements. Start with the article by Jay Kumler, and  then read the other two about some fancy gear to test these aspheres.

Jay Kumler, Designing and Specifying Aspheres for Manufacturability, by Jay Kumler of Jenoptik-Inc

Interferometric Measurement of Rotationally Symmetric Aspheric Surfaces, by Michael Kuechel of Zygo

Subaperture stitching interferometry of high-departure aspheres by incorporating configurable null optics, by Andrew Kulawiec, Markus Bauer, Gary DeVries, Jon Fleig, Greg Forbes,
Dragisha Miladinovic, Paul Murphy of QED Technologies.

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Low noise, low drift techniques

This presentation has a set of clear, well thought out images describing how chopper techniques can reduce 1/f noise, reduce drift, and even how to cancel the nasty charge injection of FET switches. It shows how modulation can reduce noise in a sensor amplifier system.

link to pdf of Kofi Makinwa\’s presentation, Dynamic Offset Cancellation Techniques

I first learned of Kofi Makinwa’s excellent work through the recent IEEE Solid State Circuits magazine, Winter 2010, Vol. 2, No. 1. He demonstrates a clever accelerometer that uses a small air volume as the ‘proof mass’. The Wheatstone bridge has been around a long time, but it’s clear it can be taught some new tricks. This is the first I’ve heard of a ‘nested chopper’ architecture. Great stuff. Check out Makinwa’s other publications at the IEEE.

I’ve spent some time trying to squeeze good data from MEMS sensors, and I know how difficult it can be. These articles show why adding some switches and circuit complexity can really pay off. And it’s only CMOS and FETs, so we get ‘em for free from Moore’s law, right?

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Radiometry explained by NIST

The measurement of light is complicated by a variety of units and concepts that are not used in other fields. For example, the ‘light level’ could be measured in units appropriate to the sensitivity of our eyes (lux), or by the power level (Watts) – but that’s confounded by the wavelength (nano-meters, but sometimes Angstroms) and you need to think in steradians, etendue must be conserved … you get the idea.

We’ve written about some of these issues in earlier posts, but this is one big, complete reference manual – a kind of ‘everything you wanted to know about light, but were afraid to ask’ – and it’s from NIST. They call it a ‘Self-Study Manual’ and it’s a clearly written tutorial on optical radiometry.

And it’s a free download. Enjoy. The test is Tuesday.

link to NIST Self Study Manual on Optical Radiation

The official title is The Self-Study Manual on Optical Radiation Measurements, edited by Fred Nicodemus

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Quantization noise

We notice the assertion that A/D converter quantization noise is equal to ADU/SQRT(12), where ADU is the quantization unit or LSB. We saw this in Hobbs’ excellent book Building Electro-Optical Systems, Making It All Work.

So, we decided to derive this. Took us a while to get the ‘trick’, and to remember how to perform calculus, to get that pesky root-mean-squared function.

Think of the quatization error as a sawtooth function that repeats. Then work out the RMS noise of that sawtooth wave (it happens to be the same as a triangle wave). And, yes, it does work out to that value.

Now the next part is Hobbs’ assertion that this quantization noise is not a Gaussian distribution. Get to work.

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Precision Resistors, low tempco

Here’s a couple sources for precision resistors.

One source seems to provide a great price/performance trade, the other is really the best quality resistor you’ll be able to buy. BUT precision low drift resistor arrays require careful inspection of the data sheet. Here’s a couple examples.

Recently I noticed that Maxim IC, (famous makers of 5V powered RS232 interface chips), have begin selling a pair of precise resistors in a SOT23 package, at an attractive price.

This family of parts, the MAX5491, claims to have 2ppm/degC resistor to resistor drift of the pair in the same package. Take care – sometimes the large print giveth, and the small print taketh away – the absolute tempco is spec’d as 35ppm/degC, so if you compare two different individual SOT23 units with each other, they may drift more that that nice small 2ppm value at the top of the spec sheet!

link to the Maxim-IC precision resistor array MAX5491 data sheet

For better matching, the VFCD1505 parts made by Vishay, also available at DigiKey, are spec’d for 0.2ppm/degC drift, or 10X better than the Maxim parts (and, well, about 5.7X the price, at $20/qnty 1, vs the $3.50 qnty 1 at DigiKey).

link to the Vishay VFCD1505 precision resistor data sheets

I like that Vishay also details subtle values such as ‘voltage coefficient’ and the current noise (or sometimes called ‘excess noise’, noise that is beyond the thermal noise of the resistor value).

The specifications of the Vishay parts are about where the state of the art is for actual resistors you can buy. Better resistors can be found only at NIST.

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Low noise NMR design

Careful consideration of all the elements of a system’s design can lead you to some very improved performance. Imagine improving a benchtop NMR system by making it 60 times lighter (120kg to 2kg), 40 times smaller, and yet 60 times more sensitive!

This article, from the IEEE Journal of Solid State Circuits (Vol. 44, No. 5, May 2009), shows an excellent example of how this occurs.
link to IEEE abstract of  ‘CMOS RF Biosensor Utilizing Nuclear Magnetic Resonance’ by Sun, Liu, Lee, Weissleder, and Ham

I recommend reading the article – it’s very well written, it describes how NMR works, and it details their systems approach to their improved design. Much can be learned here. The use of a resonant circuit for gain (they call it ‘passive amplification’) is detailed in Figure 8 of the article. (It reminded me of the old ‘regenerative’ type radio receivers, back when a vacuum tube had a power gain of about 12).

Put another way, this article shows that the ‘building block’ approach, when off-the-shelf 50 Ohm compatible RF modules are used, makes it easy to build a system that works – but that it leaves out some great performance improvements that are only possible when you analyze the basic system operation and theory. The design improves when you ask questions like ‘why 50 Ohms’ or ‘where does that noise originate and how can I maximize the signal’ and ‘how can I make this work with a much smaller and lighter magnet’? The article also answers ‘now that I can use a small magnet, can I make a custom CMOS IC that performs the RF detection, and seriously reduce system cost and size’?

Buying as much stuff off the shelf is not bad – it’s a great way to get a proof of principle working FAST, and it demonstrates that an idea or technique can work. Nothing says ‘success’ like working hardware – it allows the investors, managers and engineers to breathe easier.

But that extra performance gain from really digging into the details of how things work can pay off – in this case, it changes a benchtop lab instrument into a battery operated portable clinical test platform – this opens new opportunities and situations where this NMR system can be utilized.

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