Lake Shore Model 102 User’s Manual
2-4 Installation
2.4.3 Measurement Errors Due To AC Noise
Poorly shielded leads or improperly grounded measurement systems can
introduce AC noise into the sensor leads. In diode sensors, the AC noise
shifts the DC voltage measurement due to the diode non-linear
current/voltage characteristics. When this occurs, measured DC voltage
is too low and the corresponding temperature reading is high. The
measurement error can approach several tenths of a kelvin.
To determine
if this problem exists, perform either procedure below.
1. Place a capacitor across the diode to shunt induced AC currents.
Capacitor size depends on the noise frequency. If noise is related to
power line frequency, use a 10 µF capacitor. If AC-coupled digital noise
is suspected (digital circuits or interfaces), use a 0.1 to 1 µF capacitor. In
either case, if measured DC voltage increases, there is induced noise in
the measurement system.
2. Measure AC voltage across the diode with an AC voltmeter or
oscilloscope. Most voltmeters do not have the frequency response to
measure noise associated with digital circuits or interfaces (which
operate in the MHz range). For a thorough discussion of this potential
problem, and the magnitude of error which may result, request the paper
“Measurement System-Induced Errors In Diode Thermometry,” J.K.
Krause and B.C. Dodrill, Rev. Sci. Instr. 57 (4), 661, April, 1986 from
Lake Shore.
To greatly reduce potential AC noise, connect twisted leads (pairs) between
the measurement instruments and the diode sensors. Use 32 or 36 AWG
Lake Shore Duo-Twist™ Cryogenic Wire, which features phosphor bronze
wire twisted at 3.2 twists per centimeter (8 twists per inch). See the Lake
Shore Product Catalog or contact Lake Shore for further information.
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