Biomedical Engineering Reference
In-Depth Information
electronics. Consider that the detector used in the image-acquisition component of a microarray
system is commonly either a solid-state charge-coupled-device (CCD) or a glass and vacuum
photomultiplier tube (PMT). The choice of one device over the other involves a tradeoff between cost,
sensitivity, complexity, and dynamic range. A PMT is larger and much more fragile than a solid-state
CCD and requires a more complex power supply because of the PMT's much higher operating voltage.
In addition, a PMT is also more easily damaged than a CCD. However, a PMT provides superior
sensitivity and dynamic range compared to a CCD.
Both CCD and PMT components exhibit non-linearities outside of their optimal operating ranges. For
example, both devices saturate at some input level, so that increases in signal strength aren't
matched with corresponding increases in output, as illustrated in
Figure 6-13
. In general, there is a
tradeoff between the amplification possible and the extent of the linear region. For example,
operating a PMT at the highest voltage and gain that the device will tolerate may produce
phenomenal signal gain, but at the expense of a severely compressed linear operating region. This
non-linearity has the effect of compressing the dynamic range of the device.
Figure 6-13. Detector Operating Curve. Although only one curve is shown
here, most signal detectors are associated with a family of operating
curves.
