Page 73 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Fundamental Noise Basics and Calculations

66 Chapter Three

+15V C f

200
R f
2x2k
+
-
D D
G G AD711
S S
10k
2k
-15V
Figure 3.14 Use of a discrete FET
with an opamp to reduce the voltage
noise density e n. This is especially
useful with large, high-capacitance
photodiodes.

going to be the multiplied voltage noise density of the amplifier. Consequently
it pays to find a device, typically an FET, with the lowest possible voltage noise.
The lowest voltage noise FETs are usually obtained with large dice, for example
National Semiconductor’s Processes 51 (J113) and 92 (J309 and similar) or
the Toshiba meshed gate devices (2SK147 and similar). These can approach
1nV/ Hz , which is significantly better than most opamps. The downside is
high gate-source capacitance. This is not usually a problem since the photode-
tector already has a large capacitance and is the reason for adding this FET.
There are many ways to use FETs with opamps. The usual approach is to use
a pair of matched FETs as a differential amplifier in a “long-tailed-pair” con-
figuration driving an opamp, with overall feedback (Fig. 3.14; Maxwell 1982;
Pease 2001).

3.12 Noise and Signal-to-Noise Measurement

3.12.1 Noise measurement tools
Noise measurement systems are as difficult to design as the low-noise elec-
tronics being tested and for all the same reasons. The test electronics used
must be designed for lowest noise so as not to swamp the noise you are looking
for. Frequency-dependent elements must be understood and characterized to
avoid serious modifications to the noise spectra. Interferences from line volt-
ages, monitor refresh signals, and clock signals must be suppressed to insignif-
icant levels. Despite the difficulties, it is important for almost every project to
have a go and determine the S/N, understand the limiting factors, and dig
further into any unexpected aspects. Several tools can be put into service
for this.

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