Blood Analysis Systems Require Wide Bandwidths and Fast Sample Processing Cycles and Analysis
Blood analysis can be performed in several ways, using technologies that include impedance measurement and flow cytometry. Although each type of design has unique requirements and challenges, both depend on accurate, reliable, repeatable data acquisition and analysis. In addition, high speed and wide bandwidth are critical. Both flow cytometry and impedance measurement system designs typically consist of a stimulus generator, a data acquisition system, an application processor, and a wide range of products that address the needs of each of these elements.
Impedance Measurement Systems
An impedance measurement system provides accurate and fast analysis of
constituent components of liquids and samples. A user-selectable requency
stimulus signal is applied through the sample and the impedance changes,
based on changes induced in the sample. Examination of the resulting
impedance signature is indicative of what is being analyzed. For example,
when a known strain of a virus is added to a blood sample, an antigen response takes place, producing a chemical reaction that can alter the impedance of the blood. By characterizing this effect across different frequencies, it is possible to detect a specific strain of virus.
The AD5933 is a flexible single chip impedance converter capable of generating different excitation frequencies and examining the resulting impedance spectrum. It features a 100 Ω to 10 Ma impedance measurement range, programmable excitation up to 100 kHz with 0.1 Hz resolution, and data output as real and imaginary allowing impedance phase and magnitude to be calculated at each frequency point. The AD5934 offers the same features as the AD5933, but has a lower mpedance measurement range of 0.1 Ω to 500 kΩ and a frequency sweep range up to 200 kHz. Both the AD5933 and AD5934 are available in a 16-lead SSOP.
The ADuC7021 precision analog microcontroller is a fully integrated data acquisition system-on-a-chip, featuring an ARM7TDMI® MCU core and 12-bit, 1 MSPS ADC. It features precision analog-to-digital and digital-to-analog conversion and a flash microcontroller. The ADuC7021 is available in a 40-lead LFCSP.
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 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 ADSP-21160 family of floating-point DSPs offers 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 20-lead LFCSP and TSSOP.
