Handheld instruments
Figure 4. A block diagram of the MAX86171.
To achieve such performance, a combination of electronic and system design considerations is necessary. The design of the analog front end (AFE) for the PD is particularly important. As the PD current is often very weak relative to the noise floor, the TIA needs to have a high gain and low input bias current. Additional important parameters are low TIA input offset voltage as well as a minimum distance between the PD and TIA. The system design is also very important for achieving high sensitivity detection. Fluorescence detection must be synchronised with the LED excitation, hence the need for a controller that ensures this synchronicity. Averaging of multiple fluorescence readings is often needed for discerning the weak PD current signal from the noise floor. This averaging technique is an important function of the system controller. Ambient light and drift in LED lighting can contribute to a system error. A controller that allows the rejection of ambient light and that accounts for the impact of drift in LED lighting can enable an overall system performance advantage.
THE BENEFITS OF AN INTEGRATED OPTICAL FRONT-END RECEIVER When designing the electronic receiver chain for a PoC reader, there are two distinct architecture
56
choices: a fully discrete solution as shown in Figure 2 or the use of an integrated optical front end, as shown in Figure 3.
The first clear benefit of an integrated solution is the simplification in system design that it provides. The challenge of synchronising the fluorescence detection with the LED excitation is removed as this is handled internally by the optical front end. Integrated optical front ends also provide a more compact solution with fewer electronic components. This reduces BOM and supply management complexity while enabling a smaller end device. The most critical benefit of an integrated optical front end is, however, the ability to adjust key configuration parameters, such as photodiode, LED driver, and signal filtering configuration, via firmware. Programmability is not available with a discrete solution without new hardware being developed. This type of configurability is critical when trying to adapt a platform over time to operate with new or modified assays. As new variants and diseases are frequently added to testing menus, creating a receiver platform that can be modified to accommodate new assays, without the need for hardware modifications, is highly advantageous. Integrated optical front ends have clear advantages, however, determining the performance of an optical front end in low light
fluorescent applications is not a trivial task. Comparing signal-to-noise ratio (SNR) figures between integrated optical front ends does not give a true sense of the real-life performance of an optical receiver. As the levels of light are typically low, the absolute noise floor of the optical front ends is the critical parameter, rather than the SNR. With the timescale associated with fluorescent measurements, averaging can be employed to reduce this noise floor, though the 1/f noise component places a practical limit on what improvements can be achieved through averaging. Therefore, absolute dark current noise, particularly flicker noise, is the dominant factor. The dark current noise of a full system including PD is not characterised in the data sheet of many integrated optical AFEs and must be measured separately.
INTEGRATED FRONT ENDS FROM ANALOG DEVICES
ADI’s integrated optical front ends such as the MAX86171 are ideally suited for PoC fluorescence applications. The integration of the analogue signal chain alongside the digital controller enables a single IC solution for implementing an optical receiver. The MAX86171 contains signal-conditioned photodiode inputs, a 19-bit charge integrating ADC, low noise LED drivers, and a FIFO-
May 2023 Instrumentation Monthly
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82
