Omega CYD200 Series, User Manual

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).