Page 172 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Stability and Tempco Issues

Stability and Tempco Issues 165

similar variation, and its self-heating is likely to be greater due to the higher
currents flowing.
For very little extra cost, we could use high stability metal film resistors with
a tempco of ±50ppm/°C, reducing the gain variation to ±0.25 percent total.
However, this assumes that resistors are available. While the 10W to 1MW resis-
tors used for LED bias and general operational circuitry will surely be avail-
able, the 10MW to 10GW needed for a sensitive transimpedance amplifier may
be harder to obtain. These high value resistors often show much greater tem-
perature variations. However, with the high intensity we don’t have that
problem here. If even greater stability is required for analog circuitry, 10ppm/°C
metal film resistors are widely available, and specialist wire-wound and other
resistors can be obtained to better than 1ppm/°C, at a price greater than most
operational amplifiers.

8.2.2 Capacitors, lumped and parasitic
The analysis of Chaps. 2 and 3 showed that a photoreceiver circuit’s noise and
signal output characteristics as a function of frequency depend not only on the
transimpedance resistive load, but also on other deliberately introduced com-
ponents, and on circuit strays such as the parasitic capacitance of the load resis-
tor. We can use the same considerations to calculate output stability with
temperature variations. For example, the temperature coefficient of any feed-
back capacitor used to limit receiver bandwidth or provide stability will affect
the signal gain, unless the operating frequency is well below the characteristic
break frequencies. Even if no additional capacitance is used in parallel with the
feedback resistor, the remaining stray capacitances of the resistor and circuit
wiring may dominate temperature stability performance. These parameters are
unlikely to be known, well-characterised, or even repeatable from one fabrica-
tion to another, so this is probably a situation to be avoided. Although these
components are generally neglected in considering gain stability, when work-
ing at the limits of a receiver’s bandwidth, even if this is only 1kHz, they can
become dominant factors. Hence if everything has been done right, and the
tempco is still worse than expected, these capacitive components should be
suspected.

8.2.3 Photodiode temperature coefficient
Next we have the photodiode itself. Many photodiodes exhibit a photocurrent
tempco (1/I p dI p /dT) near to +0.25 percent/°C. For example, data-sheets for the
2
well-known BPX65 photodiode (1-mm area, silicon) give a change in photo-
current with temperature of +0.2 percent/°C. This equates to a 10 percent sen-
sitivity change over the above 50°C temperature range, a serious uncertainty
in many instrumentation applications. Several years ago Hamamatsu intro-
duced a range of photodiodes with low tempcos of ±100ppm/°C and less. The
tempco is wavelength-dependent, even changing sign. Nevertheless, these have
greatly improved the precision of many optical measurements.

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