Flow Cytometry Systems

In a flow cytometry system, microscopic particles are suspended in a stream of fluid and individual cells are characterized simultaneously for physical and/or chemical properties. Fluorescent chemicals found in cells or attached to cells are analyzed, as they flow through a tube and are passed in front of a light source. The light that is scattered from each cell is measured with a photodiode and by analyzing the changes in brightness at each detector, it is possible to determine various types of information about the physical and chemical structure of each individual cell.

The amplification stage requires wide bandwidth for accurate signal detection, low bias current, and low input capacitance for compatibility with the photodiode detector output, and fast settling time to ensure limited distortion and speed of the system. The AD8616 is ideally suited to the photodiode sensor return loop. It is a 20 MHz, CMOS, rail-to-rail, dual op amp featuring low offset voltage of 65 µV, just 8 nV/√Hz noise, 1 pA input bias current, and 0.002% total harmonic distortion (THD). The AD8067 high gain bandwidth precision FastFET™ op amp is also a good choice with features including 0.6 pA input bias current, stable gains of >8, 54 MHz, –3 dB bandwidth, 640 V/µs slew rate, and 6.6 nV/√Hz/0.6 fA/√Hz noise.

From the photodiode sensor return loop, the data is digitized by the data converter and then processed by a digital signal processor (DSP). In selecting an ADC, high speed and maintenance of low signal-to-noise ratios are important specifications. The AD9248 is a high performance 65 MSPS ADC with dual sample-and-hold amplifiers and an integrated voltage reference that features a SNR = 71.6 dBc to Nyquist. It is available in a 64-lead LQFP. The tasks of the DSP include receiving the FPGA dispersion, conversion to a floating point format, compensating for filter overlap, and comparing to the desired cell type. The ADSP21160 family of floating-point DSPs offer rich, powerful instruction sets, floating-point precision, and high speed execution.

Steer cell (desired or undesired) via high voltage cell sorting is a specialized type of flow cytometry where cells are sorted into different collection bins by using a set of deflection plates. These plates are kept at a high voltage potential. Uncharged cells drop straight down, while charged cells are attracted to a particular plate, effectively steering them into different collection bins. Cells can be charged individually by applying a pulsed voltage to the charging collar in the diagram. The AD5445 is a 12-bit, high bandwidth, multiplying current-output digital-to-analog converter (DAC). It enables the pulse to the optical beam to charge each cell. It can multiply pulses of different widths to accommodate cell velocity (500 kHz to 2 MHz), and has a high speed parallel interface. The AD5445 is available in a 20-lead LFCSP and TSSOP.

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