Signal Connection And Pin Assignment; 2-Wire Rtd Input Signal Connection - Advantech iDAQ-815 User Manual

2.3

Signal Connection and Pin Assignment

2.3.1

2-Wire RTD Input Signal Connection

Figure 2.2 shows 2-wire RTD input signal connection. The two leads of the RTD are
connected to RTD+ and RTD- terminals. In addition, use short wires to connect
EXC+ to RTD+ and RTD− to EXC− in order to form an excitation current loop.
The excitation current source (I
RTD lead 1, RTD resistor (R
by the red line in Figure 2.2. Assuming both lead-wire resistances are equal (R
the measurement result can be calculated as follows.
where
CODE
FSR
ADC
is the voltage measured by the ADC, and
V
IN
V
REF
As can be seen in the equation, the lead-wire resistances cannot be separated from
the RTD and result in measurement errors. To minimize the errors, use lead-wire with
length as short as possible.
2.3.2 3-Wire RTD Input Signal Connection
Figure 2.3 shows 3-wire RTD input signal connection. Connect single-lead end of the
RTD to RTD+ terminal and the other two leads to RTD− and EXC− terminals. In addi-
Figure 2.2 2-wire RTD input signal connection.
) flows through internal reference resistor (R
EXC
), RTD lead 2, and to the internal ground as shown
RTD
is the output code of the ADC,
ADC
is the full-scale range of ADC (for a 24-bit ADC, it is equal to 2
is the reference voltage of the ADC.
7 iDAQ-815 User Manual
),
REF
),
LEAD
24
),