Page 29 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Amplified Detection Circuitry

22 Chapter Two

OPA128 (±75fA), and National Semiconductor LMC6001A (±25fA) reduce bias
errors even further. These would allow the use of 100GW resistors with only
2.5-mV DC offset. Note, however, that these FET bias currents exhibit a rapid
increase with temperature, which for high temperature operation may even
exceed the current of some bipolar devices. This must be considered in the full
design of a receiver subjected to high temperatures.

2.3.2 Guarding
At these high impedance levels, other currents can play a big role. Leakage
current flowing along the surface of printed circuit boards (PCBs) is often sig-
nificant and is greatly increased by humidity films and surface contamination.
It can be reduced through the use of guard rings. These are electrodes that
surround the sensitive input pins and are either grounded or driven at low
impedance at the same voltage as the input pins. If the voltages of pin and inter-
fering electrode are identical, by definition no current can flow. A voltage-
follower can be used to drive the guard electrode to the input voltage, to an
accuracy limited by the amplifier voltage offset errors. To understand how guard
rings work, it is really necessary to “think in 3-D” (Fig. 2.3a). The pin shown
passing through the PCB is the sensitive point, for example connecting a low-
current photodiode to the front end of a sensitive receiver. There is also an inter-
fering electrode close by, at a source potential V source . Imagine that the PCB is
coated in a 1-mm-thick layer of weakly conducting material, an electrolyte that
couples the interfering electrode and sensitive pin. To work out where the cur-
rents flow, we really need to perform a 3-D finite element analysis, when it
would be seen that volume currents flow in the full thickness of the conduct-
ing layer, part of that current flowing to the sensitive input pin. If now we add
a guard electrode which surrounds the sensitive pin, preferably on both sides
of the PCB, some of the volume currents will flow to the guard electrode,
depending on its potential, size and geometry in relation to the other electrodes.
If the area of the guard electrode is too small, stray currents will bypass the
guard and make it to the input pin. The greater the area of the guard electrode,
the more stray current will be mopped up. Conversely, the thicker the con-
ducting layer, the more stray leakage current will pass by the guard electrode
to the sensitive pin. Hence we cannot say how big the guard electrode should
be without having an idea of the thickness of the conducting layer. Common
sense suggests that the guard ring width should be at least as great as the con-
ducting film thickness.
As shown, current leakage through the bulk of the PCB material can also be
a problem, which is hardly reduced by surface-printed guard rings. Special cop-
perclad boards made from PTFE polymers or ceramics are available which
reduce this leakage contribution, but these are rarely seen in one-off designs.
For test lash-ups it is not hard to make custom boards using sheet PTFE, sub-
sequently carefully cleaned, PTFE integrated circuit sockets, PTFE insulated
terminals or just air! In principle, volume guard rings could also be fabricated,

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