Sensoray 417 - Manual (Page 32)

31 Sensoray Model 417
Chapter 5 Sensor Connections
4-to-20 mA Current Loops
A 250
Ω
, 0.01% resistor must be installed as shown below. These resistors are available as an
option for the Model 417; order Sensoray part number 7408R. Make sure the resistor is at the
grounded end of the loop so that the common-mode voltage does not exceed 5 Volts.
Solving Noise Problems
Electrical noise can sometimes interfere with measurements. Fortunately, there are a handful of
simple procedures that will solve most noise problems:
1. Examine the input signal with a DVM and/or oscilloscope to see if you really have the sig-
nal you think you have. Measurement “noise” may actually be the result of incorrect wir-
ing or a fault at the signal source.
2. Make sure the input is referenced to ISAbus ground. Inputs that are not ground-referenced
are, in effect, high-impedance “antennas” that pick up ambient noise from the environ-
ment. Such noise can elevate the common-mode voltage amplitude beyond the Model
417’s allowable common-mode input limits.
Thermocouples—try grounding the “hot” end of the TC.
DC Voltage inputs—if the input is isolated, try connecting one of the input terminals to
ISAbus ground, either directly or by means of a resistor (i.e., 10K
Ω
).
Current Loops—make sure one end of the 7408R resistor is connected to ISAbus ground.
3. Shield the input wires. Both input wires should be surrounded by the shield conductor, if
present. Terminate the shield conductor only at the 417 end of the sensor cable assembly.
Make sure the remote end of the shield conductor is electrically insulated from all other
electrical conductors, otherwise you may end up with a ground-loop error.
4. Invoke the channel software filter. This should be tried as a last resort if everything else
fails. If it becomes necessary to do this, it probably indicates that (a) the noise is funda-
mentally differential, and (b) the noise is produced at the signal source and is not caused
by the interface between source and measurement system. In such a case, it may be worth-
while to evaluate the signal source itself for potential noise reduction solutions.
+
S
Sensoray P/N 7408R
(+)
(–) –
4-20mA