4-16
Cabling and Wiring
4–20mA Current Loops
Process control current-loop transducers interface easily with DAS-8 and
DAS-8/LT by adding a suitable shunt resistor across the input. Since the
maximum current is 20mA and the maximum input range of a DAS-8 is
+5V, a 250-ohm precision shunt resistor is required. The DAS-8/
PGA(G2) and DAS-8/AO default range is also ±5V, so that this method
also works here. The resistor should be of low temperature-coefficient
metal film or wirewound construction for stability with time and
temperature.
Using this interface, the 4–20mA working range of the current loop
corresponds to 1638 bits of input, a resolution of about 0.06%.
The Reference (DAS-8 and DAS-8/LT Only)
The DAS-8 and DAS-8/LT provide a +10V A/D voltage reference (Vref).
It may be used for offsetting signals, but should not be heavily loaded.
The maximum available output current is 2mA. Since this reference is
shared by the A/D, any overload or shorting of the reference will affect
the scaling of the A/D.
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