MARKO HEPPMAGNUS MEIER

The goal is to realize the integrationof an optical sensor onto a singleship (SoC) as an optimized on-chip

optical system. The high level of integra-tion yields significant advantages interms of speed, size and system cost.Typical examples that illustrate well thetrends and possibilities include an ab-solute encoder and an LED/laser triangu-lation sensor.

Optical sensors undertake variousphysical measurement tasks, for examplethickness, length, angle, separation anddistance. For example, optical encodershave been used successfully for decadesfor precise determination of rotational orlinear movements. For evaluation of sepa-ration with distance sensors, or even toassure movement within an (illuminated)safe area, it is often sufficient to deter-mine a simple ‘yes/no’ statement. In eachcase a rapid and reliable determination ofposition or separation is required, so thatappropriate decisions can be made by thecontroller. The sensor assembly makes usein this case of a regulated LED/laser light

source [1], while photodiodes, a PSD (po-sition sensitive detector) or even line orarea sensors (CCDs) are utilized to provideconversion into an evaluable electricalsignal. Application-specific amplifiers anddiscrimination electronics have alreadybeen incorporated into standard CMOStechnology many years ago. Newer, sub-micrometer CMOS technologies now pres-ent the possibility of integrating almostunlimited detection elements and appli-cation-specific amplification and evalua-tion circuit, as well as an interface forcontrolling the light source or for externalcontrol, all onto a single chip. The targetfor this optical integration is to find thesweet spot between performance, cost,flexibility and reliability.

New packages for opticalsystem integration

Ceramic housings with glass windowshave long been available for optical sen-sors, although these and their newer vari-ants are often too costly to manufacture.For automated surface mounting involv-ing small dimensions, so-called chip-on-board (COB) variants with transparentplastic encapsulation or with solderedmetal housings have been developed(Figure 1, above right). SMD housings foroptical system integration have to pro-vide various numbers of connecting pinsand being capable of accommodatinglarger chip formats – both critical as-pects, as the chip size is largely deter-mined by the number and size of the pho-toelements to be integrated.

In order to decrease development andmanufacturing costs even further, an ›op-toBGA‹ has been developed (Figure 1,below left), comprising a ball grid arraywith varying numbers of solder points(balls) of a Sn/Cu/Ag alloy. The optical

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Integration of optical sensors MORE ACCURATE AND RAPIDOPTICAL DETERMINATION OFANGLE AND DISTANCEPOSITIONING

Modern sub-micrometer CMOS

technology in combination with

design know how and innovative

optical packaging form the basis

for successful integration of opti-

cal sensors for applications in

industry and medicine.

C O N TA C T

iC-Haus GmbH 55294 Bodenheim, Germany Tel. +49 61 35 92 92-300 www.ichaus.de

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chip is mounted and connected to thesubstrate (PCB or ceramic) using standarddie attach and wire bonding techniques.A special thin glass lid serves as an opti-cal window for the light sensitive detec-tor. An optoBGA with 30 pins and dimen-sions of 7.6x7.1x1.6 mm3 is illustratedon lower left side in Figure 1. On theright is a version with 15 pins (9.4 mm2

and a thickness of 2.29 mm, includingsolder balls) shown. Top left in Figure 1illustrates the latest development of aQFN package into an ›optoQFN‹, providing38 pins, either a glass window or achrome reticle, and with dimensions of7x5x0.9 mm3. The packaging conceptsare vital for an optimized optical systemintegration onto a single silicon chip.

Optical absolute encoder ona single chip

The electrical performance characteristicsof an optical encoder are determinedlargely by the code disc (resolution), aregulated LED light source (stability), the

photodiode array (sensitivity), a pro-grammable signal amplification (accu-racy) with temperature compensation,the sine/cosine interpolation (resolution,accuracy) and the control interface (forexample, SPI/SSI/BiSS). The integrationof all component blocks onto a singlechip (except the LED and the code disc),reduces not only size requirements, butalso improves accuracy and speed of theposition measurement compared to a sys-tem with discrete components.

Figure 2 illustrates an example com-prising a 42 mm disc with 1024 divisionsand an optical absolute single-chip en-coder (iC-LNB) with 18 bit resolution. 26photodiodes are integrated over a rangeof 5 mm on the chip for scanning thecode disc. Figure 2 also shows the geo-metrical formation – one sees that chipsize is largely determined by the numberand locations of the photodiodes. Suit-able packages include both the 38 pinoptoQFN and the slightly wider optoBGAwith 30 pins.

Use of sub-micrometer CMOS technol-ogy means that adequate space is avail-able between the photodiodes (whoselocations are determined by the codedisc) to increase functionality. Thus itwas possible to incorporate adjustment ofthe interpolation factor for the digitalsine wave interpolation. This value is setbetween 1 and 65536 steps per rotationvia the SPI interface. The 16 bit positionvalue is additionally outputted in parallelas a gray code. The latency of the single-

chip encoder is just 500 ns, as the entiresignal processing is performed in parallelon-chip. In an application this meansthat position values are given accuratelyand reliably, even at higher rotationspeeds (up to 12000 rpm at 16 bitresolution and 3000 rpm at 18 bit) [2].

An absolute 18 bit single-turn resolu-tion can be read-out over the SPI inter-face at a clock rate of 10 MHz. An inte-grated current supply ensures a constantlight source for the photodiodes and evenprovides protection against variations in

power supply and temper-ature and against agingprocesses in the LED. Thewavelength of the LED(680 nm) was chosen tobest match the sensitivityof the integrated photo-diodes.

Internal memory ismonitored continuouslyvia a parity bit. Comparedto magnetic integratedabsolute single-chip en-coders [3] with similarresolution, the higheraccuracy of the code disc(100 nm to 1 µm) con-tributes directly to overallsystem accuracy.

Triangulation sensoron a single chip

Triangulation measurements are based onsimple geometry. The light from an LED ora laser reflects off of the object to bemeasured and collected by an optic andpassed on to the optical sensor. In thesimplest case the sensor is a PSD, but aline detector (for example, with 32 to1024 photoelements) can also be used, oreven a special application-specific photo-diode array. Figure 3 illustrates the

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1 A packaging exam-ple for optical systemintegration

2 An optical code disc and an single-chip absolute encoder with 18 bit resolution in a single package

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principle of triangulation with an LEDlight source and specially developed inte-grated single-chip triangulation sensor.

Dependent of the object position ›1‹ or›2‹, the reflected light is incident on P1 orP2 of the sensor, respectively, and is eval-uated. This particular optical sensor in-cludes an array of 129 photodiodes, asshown in on the left in Figure 3 on theChip (iC-LO). For rapid object recognitionthe sensor was divided into three differ-ent fields: a near-field photodiode with anarea of 0.927 mm2, a far-field photodiodewith 0.15 mm2 and for actual measure-ment a line array with 127 photodiodeswith an effective area of von 2.23 mm2.The latter can be tied to either the near-field or far-field sensor. As illustrated inFigure 3, this configuration of photodi-odes determines the necessary chip sizeand appropriate optical package – in this

case an optoBGA with 18 pins and an areaof 0.45 cm2 and a height of only 1.8 mm.

The photocurrents for the near and far-field are processed independently via anautomatically regulated ac amplifier, fol-lowed by sum and difference values andevaluation by programmable comparators.The amplifier response curve adapts dy-namically to the amount of light collected,with a warning being given should thelight level fall too low. The logarithmicresponse curve results in a high dynamicrange, and even copes well with directreflections from the object. An optical fil-ter is incorporated for effective daylightsuppression (up to 100000 lux).

The SPI interface provides simple ac-cess to parameters via an external micro-controller. For large separations, a laserdiode with driver circuit [1] can be uti-lized. In order to use today’s high-bright-

ness LEDs (HBLEDs), the integrated LEDdriver is programmable in pulse width andthe LED currents of up to 1150 mA.

Summary

Even for triangulation sensors, integra-tion of photodiodes and the entire logiccircuitry brings significant advantagesin terms of space requirements and pro-cessing speed. The entire sensor, a +5 Vvoltage supply and I/O interface [4]need only 3 cm2, and the position eval-uation loop requires only 72 µs. As dis-cussed with these two examples, inte-gration approaches for optical sensorswith appropriate packaging results insignificant performance increases, andadditionally offers considerable advan-tages compared to systems made ofdiscrete components.

LITERATURE 1 › Design and test of fast laser diode driver circuits‹,

white paper, iC-Haus

2 ›Fast and simple measurement of position changes‹,

white paper, iC-Haus

3 ›Optisch vs. Magnetisch‹, Elektronik 5/2012

4 ›PNP/NPN/PP oder IO-Link?‹, Elektronik Informa-

tion 03/2011

AUTHORS Dipl.-Ing. (FH) MARKO HEPP began working at iC-Haus

in 1997 and is today responsible for industrial applica-

tions such as drivers for laser diodes, optical sensors

and BiSS interface support.

Dipl.-Ing. MAGNUS MEIER is responsible for encoder

and power management applications, and has been

with iC-Haus since 2007.

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3 An optical triangulation sensor in an optoBGA package

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