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Omega Model CYD201/202 User’s Manual
Installation 2-11
Current Source Dependence (Continued)
If the output from a current source is not precisely 10 µA, the error in
temperature can be calculated by this relationship between the dV/dT and
dV/dI values:
DT = (DI • dV/dI) / (dV/dT)
Note values dV/dI and dV/dT are derived at same temperature (T).
Table 2-2. Typical CY7 dV/dI Values for Selected Temperatures
T (K)
dV/dI (approx.)
300
77
4.2
3000
Ω
1000
Ω
2800
Ω
2.4.5
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 microfarad capacitor. If AC-coupled
digital noise is suspected (digital circuits or interfaces), use a 0.1 to 1
microfarad 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 Omega.
To greatly reduce potential AC noise, connect twisted leads (pairs) between
the measurement instruments and the diode sensors. Use 32 or 36 AWG
Omega Engineering Duo-Twist™ Cryogenic Wire, which features phosphor
bronze wire twisted at 3.15 twists per centimeter (8 twists per inch).
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