Lake Shore Model 321 Autotuning Temperature Controller User’s Manual
D-18
Application Notes
IV. CONCLUDING REMARKS
Noise in any measurement circuit is undesirable and should be eliminated to as great an extent as possible. The first step
is to electrically shield all instrumentation and wiring and use proper grounding techniques.
8
Secondly, the diode
measurement circuit should have a single circuit ground which is generally made at the voltmeter and which then requires
a floating current source. The installation of the diode and its connecting leads should be done carefully to avoid
introducing any unwanted circuit ground connections such as an electrical short to a cryostat.
As a last resort, a "quick fix" can be used to eliminate much of the dc offset voltage with some degradation in the diode
circuit performance. A good quality capacitor (low leakage) can be placed across the diode to shunt the induced ac
currents similar to the test procedure used for identifying a noise problem. This is most easily done by connecting the
capacitor across the input to the voltmeter. The size of the capacitor needed will depend on the frequency of the noise
(generally related to the power line frequency of 60 Hz) and the dynamic impedance of the diode (on the order of a few
thousand ohms at a 10 µA operating current). A capacitor in the range of 10 to 20 µF should reduce most noise effects to
an acceptable level. However, because the capacitor increases the time constant in the circuit, a sluggish response
should be expected. In switching operations, 30 seconds or more may be required for the circuit to stabilize. This "quick
fix" is not meant as a substitute for proper measurement techniques, but in certain circumstances it may be useful.
Note added in proof. The capacitance values given above are for the elimination of the effects of low-frequency noise
such as 60 Hz. If high-frequency noise is a problem, an additional capacitor of lower capacitance value may be needed.
The reason for this is because larger capacitors often have an associated inductance which limits their usefulness as a
high-frequency shunt.
1
A. S. Grove,
Physics and Technology of Semiconductor Devices
(Wiley, New York, 1967), Chap. 6.
2
S. M. Sze,
Physics of Semiconductor Devices
(Wiley Interscience, New York, 1969), Chap. 4.
3
D. A. Fraser,
The Physics of Semiconductor Devices
(Clarendon, Oxford, 1983).
4
R. V. Aldridge, Solid-State Electron.
17
, 617 (1974).
5
V. Chopra and G. Dharmadurai, Cryogenics
20
, 659 (1980).
6
D. A. Kleinman, Bell Syst. Tech. J.
35
, 685 (1956).
7
P. R. Swinehart, L. A. Smith, and J. K. Krause (private communication); values are consistent with numerous other
measurements made at Lake Shore Cryotronics, Inc.
8
R. Morrison, Grounding and Shielding Techniques in Instrumentation (Wiley, New york, 1977), Vol. 2.
