Handheld instruments
buffered serial interface. With nine LED channels and four PD channels, the AFE supports multiassay testing and adequate channels for future testing expansion without hardware upgrades. Programming over SPI or I2C allows the fine tuning of parameters to a given assay, such as integration time, averaging, and dynamic range. A FIFO enables MCUs to remain in sleep mode while measurements are taken, extending battery life in handheld PoC systems. Most importantly, the high performance and low noise nature of the device enable a high sensitivity detection system. Averaging and low 1/f noise allow a dark current noise of just 11 pA rms for the whole optical signal chain with a
Figure 5. A low light detector measurement with the MAX86171. photodiode area of 7.5 mm2
. This allows for
reliable detection of low photodiode currents in the range of 1 pA to 10 pA, typical for low yield fluorescence applications. In addition, excellent PSRR and ambient light rejection ease the system engineer’s burden while designing the power supply unit and mechanical enclosure. To validate the performance of the MAX86171, an LED driven by the MAX86171 was passed through varying levels of neutral density (ND) optical filters and received via a photodiode by the MAX86171 as shown in Figure 5. By increasing the density of the ND filters, the optical attenuation can be varied from 40 dB (ND2) to 140 dB (ND7), simulating reducing fluorescent yields in a PCR or LAMP-
based detection system. Under 140 dB of attenuation, the MAX86171 can reliably detect the increased photodiode current of <10 pA above the dark current floor. This high sensitivity is due to the low dark current noise of 11 pA rms, measured with the photodiode connected to the optical front end.
This level of performance exceeds what is typically required in PoC instruments and allows the full potential of the biosensor or chemistry to be showcased. The internal registers of the MAX86171 enable parameters such as pulse width, pulse intensity, photodiode gain, and photodiode biasing to be programmed via firmware. Signal filtering, averaging, and ambient light rejection options are also available to optimise optical detection. Together, this provides a solution with maximum flexibility to adapt to new assays without the need for hardware rework.
CONCLUSION
Designing a circuit for an IVD system requires careful attention in both the electronic section and system design to ensure that high sensitivity detection is achieved without sacrificing selectivity. Identifying the weak electronic signal is critical when creating a system that can showcase the full potential of the biosensor or chemistry, and ultimately results in a device with accurate diagnostic results. In the rapidly advancing PoC market, flexibility and future-proofing are crucial as receivers and must be adaptable to the growing and changing menu of tests. The MAX86171 integrated optical front end from ADI meets these stringent performance needs while providing software programmability, taking the risk out of the electronic receiver design, and providing a future-proofed solution.
Figure 6. The MAX86171’s performance results. Instrumentation Monthly May 2023 Analog Devices
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