Page 31 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Amplified Detection Circuitry

24 Chapter Two

low fractional errors the bias currents of the highest performance amplifiers. A
fascinating discussion of such experiments has been published by Pease (1993).
For the highest impedance designs, it is often better to dispense with inte-
grated circuit (IC) sockets and circuit boards altogether. ICs can be directly sol-
dered to cleaned PTFE stand-off terminals that are hand wired, or a compromize
is to use a conventional IC socket with the critical opamp input pins lifted clear
and soldered directly to the photodetector and load resistor. A disadvantage of
this construction technique, apart from its time and expense, is its performance
under acceleration. Vibration will vary the mechanical separation of compo-
nents, modulating parasitic capacitances and causing microphonic pickup.

2.3.3 Cleanliness and flux residues
Some types of solder fluxes seem to be almost conducting! I have done no quan-
titative tests, but the water-soluble fluxes contained in “green” water-washable
solders seem to be the worst offenders. Hence all flux and contamination must
be carefully washed away from PCBs before use. In the past fluorocarbon
solvent vapor cleaning systems were used for this. More environmentally
friendly aqueous cleaners should now be used. These are available for large tank
use or as small aerosols. They must in turn be followed by thorough rinsing
in deionized or low-conductivity water, with the outlet water checked using a
good conductivity meter to be sure that it approaches the limiting water value
(ª18MW· cm).
For hobby or occasional use diligent scrubbing of solder residues with an old
toothbrush and methanol, isopropanol, a specifically designed aqueous cleaner,
or even household detergent, followed by long rinsing under running tap water,
can be almost as effective. In every case the board should be shaken or blown
clean of excess water and allowed to dry thoroughly in warm air before use.
Hair dryers and airing cupboards are key equipment here! Failure to remove
these residues can lead to very strange behavior of detector circuits.

2.4 Big Problem 1: Bandwidth Limitation Due to
Photodiode Capacitance
A significant problem with high-value load resistors is their very sluggish
response to incident optical power changes. As Fig. 2.2 shows, the photodiode
parasitic capacitance (junction + packaging + wire connections), along with the
amplifier input capacitance, appear in parallel with the load resistor. Hence for
the voltage on C p (and therefore on R L) to change, the capacitance must be
charged and discharged. Because the photodiode operates as a current genera-
tor, this voltage change takes time.
For a step change in input power, the output voltage exponentially approaches
-1
its final value, reaching in one R LC p time constant 63 percent (1 - e ) of final
value. If the photodiode is a hardly biased BPW33 with a capacitance of 720pF,
-3
and R L = 1GW, the time constant is 0.7 seconds. Hence 95 percent (1 - e ) of

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