ADLINK USB-2401 - Manual (Page 42)

32 Operation
Figure 3-6: Thermocouple Connection
3.3.5 RTD Input Mode
The resistance temperature detector (RTD) measures tempera-
ture by correlating the resistance of the RTD element with temper-
ature. The USB-2401 can generate a steady 0.5 mA excitation
current source to each channel in RTD input mode to measure
cross-voltage on the RTD. The actual input range is ±2.5V with a
formula of voltage to RTD resistance conversion of:
Since the excitation current can only drive cross-voltage up to
1.5V with good linearity, the maximum equivalent value of the RTD
resistor is limited to 3kΩ.
The USB-2401 can support two, three, and four-wire RTD mea-
surement. Adopting three- and four-wire connections rather than
two-wire can eliminate connection lead resistance effects from
measurement. Three-wire connection is sufficient for most pur-
poses and most universal industrial applications. Four-wire con-
nections are used for the most precise application requirements.
ADC
MUX
SC+
SC-
Thermocouple
USB-2401
0.0005
V(volt)
) ( = Ω RTD