Signal processing for optical sensors

Signal processing for optical sensors

Feature articles |
By eeNews Europe

For the machine builder, the challenge is that these kinds of sensors have a very low signal level to process and with poor illumination, this can often be interpreted as background noise. The integrated transimpedance amplifier and analysis circuitry developed by MAZeT are able to boost even low input signals to a signal voltage in a measurable area for electronic engineers and developers.

Multi-channel transimpedance amplifiers

The output signal of photodiodes is an electric current and depending on the application these currents can vary from nano-ampere to micro-ampere. The transfer functions of transimpedance amplifiers are described by Ohm’s law. Other than usual resistors, which are the most basic current-to-voltage converters, an active transimpedance amplifier can regulate the voltage of a photodiode to a constant value (usually the analog mass). Therefore the photodiode is processing via short-circuit mode and is used as pure current generator. Additionally the capacity of the photodiode is compensated, which results in increasing the signal bandwidth. 

Figure 1: Block diagram of the MTI04E.

The multi-channel integration of these amplifiers allows significant space-saving and offers very good parallel feed characteristics of the single channels. What’s more, amplification can be compensated for temperature variations. The MTI04E and the MTI08 from MAZeT are two examples of application-specific transimpedance amplifiers. In contrast to the MTI04CS/CQ the MTI04E uses channel 1 as reference channel for the temperature compensation. Therefore it is a perfect fit for analyzing 3-sector color sensors, but it can also be used as multi-channel current-to-voltage converter for sensors with voltage output.

Figure 2 shows a circuit targeting a low level temperature coefficient. The current VTK is used as compensation of the temperature-dependent signal current of the other three amplifier channels.

Figure 2: Circuitry of the MTI04E for temperature compensation.

The transimpedance amplifier MTI08 – see figure 3 – has eight programmable option channels. In contrast to the MTI04 every channel can be individually controlled via 8 amplification levels between 200 kOhm and 25.6 MOhm. Programming is applied via an SPI interface. The lowest level of photoelectric current is 20nA with a 19kHz signal frequency. The MTI08 uses a sample & hold circuitry as output as well as a multiplexer and operates from 3 to 5V.

Figure 3: Block diagram of the 8-fold transimpedance amplifier MTI08.

It can use an amplifier channel while providing a temperature dependent voltage, to compensate the temperature coefficient of the other amplifier channels. While transimpedance resistors have an uncompensated temperature coefficient (TK) of -3300 ppm/K, they change after compensation to a ‘TK’ that is mainly defined by the reference voltage source ‘VREF’ and the ‘TK’ of the external resistor: optionally the output sample & hold circuitry can be used simultaneous to save all measured data. The multiplexer allows a serial readout of this data.

Current-to-digital converters

Laid-out with 4 channels, the current-to-digital converters MCD03AQ and MDDC04AQ from MAZeT are signal processing ASICs specifically designed for opto-electronic sensors. Both converters feature great conversion accuracy as well as great temperature stability. The MCDC03AQ – see figure 4 – uses an internal channel for the analysis of the dummy structure that is used for temperature and error compensation.

Figure 4: Block diagram of the MCDC03AQ.

When photodiodes are connected to the other three remaining signal inputs, the MCDC03AQ operates as a light-to-digital converter, which converts the irradiated light power to linear digital information. The AD conversion is performed based on the principle of charge balancing. The three signal channels were designed to match MAZeT’s 3-range colour sensors. The output signals of every AD converter provide a pulse-density modulated digital signal, which is covered during the integration period by a 16-bit counter, each one is saved in a 16-bit registry. Readout of the measurement values is performed by an I²C serial interface. The MCDC03AQ’s programmable conversion unit allows a dynamic work range of up to 1:106 . It is well suited for applications where low level currents need to be measured and is delivered in a compact SSOP20 package.

The MDDC04AQ from figure 5 is a 4-channel current-to-voltage converter with adjustable transimpedance and subsequent 12-bit AD-conversion. The current-to-voltage conversion will be processed via a usual transimpedance amplifier with a feedback resistor. Therefore the MDDC04AQ is well suited for application with high level signal frequencies. The full scale area can be adjusted at 17 different levels from 11.7nA to 12µA (dynamic area of 30dB), separately for each channel, via a reference current. Additionally the transimpedance of every channel can be adjusted at 16 levels, from 0.1MOhm to 102.4MOhm. In this chip, the measured values on all 4 channels are covered simultaneously and are saved via four track & hold circuits. Then each channel is converted separately. The temperature coefficient of the transimpedance resistors (uncompensated -4300 ppm/K) will be compensated to 150 ppm/k using an integrated correction algorithm. The remaining TK now only depends on the TK of the reference power source. This same procedure is used to monitor and correct the overall gain of the AD converter.

Figure 5: Block diagram of the MDDC04AQ.

As well as the charge balancing AD converter used in the MCDC03AQ and the 12-bit SAR-AD converter used in the MDDC04AQ, MAZeT has developed a sigma-to-delta converter and a 14-bit or 16-bit pipelined AD converter. The pipelined AD converter features a differential input and is a perfect match for applications with high sampling rates up to 10 MS/s. Depending on users‘ requirements, they can be adapted to custom resolution. Optionally, MaZet can integrate EEPROM in these products to save calibration data.

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