Page 43 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Amplified Detection Circuitry

36 Chapter Two

5.5
Chip Resistors
4.5
1/4W axials
3.5
Intensity 2.5

1.5

0.5

-0.5
-125 -75 -25 25 75 125
Time (ms)
Figure 2.13b Detailed view of the transient response. The slowest responses are
for a 1/4-W axial resistor, the two fastest for a chip component wired “casually”
and in an optimized compact construction.

varying from 25 to 50ms. Figure 2.13b looks a little more carefully and adds a further
chip device measurement. Here I resoldered the chip as close as I could to opamp pin
2 and generally tidied up the wiring. The result is a further substantial reduction in
rise time, down to as short as 5ms.
All this shows that performance does not require the use of fancy and expensive
amplifiers. Equally large gains can be obtained with passive components, in particu-
lar the transimpedance resistor, and by careful layout and construction. It also sug-
gests that for all but the simplest of applications, small-package opamps and
surface-mount components will give significantly better performance than through-
hole designs.

2.7.3 Split resistors
Another, related component choice that is frequently suggested in articles is the
use of split transimpedances. Figure 2.14 shows a 100-MW, 0.4-W resistor with
its intrinsic 0.2pF parasitic capacitance replaced by two similar 50-MW resis-
tors in series. This will almost double the frequency of the -3dB point. The idea
is that although there is double the total capacitance in the feedback, the com-
bination maintains its resistive nature up to higher frequencies. As the output
voltage is just the product of the photocurrent I p with the combined complex
impedance of the feedback network, we can easily calculate the output as a func-
tion of frequency. With a single resistor the total impedance is:

ZR 1 with R 1 = 100 MW. (2.5)
C1
Z C1 + R 1

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