The information above also applies to resistance measurement. A common way to measure resistance is to build a voltage
divider as shown in Figure 2-4, where one of the resistors is known and the other is the unknown. If Vin is known and Vout is
measured, the voltage divider equation can be rearranged to solve for the unknown resistance.
2.7.3.7 - Measuring Current (Including 4-20 mA) with a
Resistive Shunt
The best way to handle 4-20 mA signals is with the LJTick-CurrentShunt, which is a two channel active current to voltage converter
module that plugs into the UE9 screw-terminals.
The following figure shows a typical method to measure the current through a load, or to measure the 4-20 mA signal produced by
a 2-wire (loop-powered) current loop sensor. The current shunt shown in the figure is simply a resistor.
Figure 2-6. Current Measurement With Arbitrary Load or 2-Wire 4-20 mA Sensor
When measuring a 4-20 mA signal, a typical value for the shunt would be 240 Ω. This results in a 0.96 to 4.80 volt signal
corresponding to 4-20 mA. The external supply must provide enough voltage for the sensor and the shunt, so if the sensor requires
5 volts the supply must provide at least 9.8 volts.
For applications besides 4-20 mA, the shunt is chosen based on the maximum current and how much voltage drop can be
tolerated across the shunt. For instance, if the maximum current is 1.0 amp, and 2.5 volts of drop is the most that can be tolerated
without affecting the load, a 2.4 Ω resistor could be used. That equates to 2.4 watts, though, which would require a special high
wattage resistor. A better solution would be to use a 0.1 Ω shunt, and then use an amplifier to increase the small voltage produced
by that shunt. If the maximum current to measure is too high (e.g. 100 amps), it will be difficult to find a small enough resistor and a
hall-effect sensor should be considered instead of a shunt.
The following figure shows typical connections for a 3-wire 4-20 mA sensor. A typical value for the shunt would be 240 Ω which
results in 0.96 to 4.80 volts.
Figure 2-7. Current Measurement With 3-Wire 4-20 mA (Sourcing) Sensor
The sensor shown in Figure 2-7 is a sourcing type, where the signal sources the 4-20 mA current which is then sent through the
shunt resistor and sunk into ground. Another type of 3-wire sensor is the sinking type, where the 4-20 mA current is sourced from
the positive supply, sent through the shunt resistor, and then sunk into the signal wire. If sensor ground is connected to UE9 ground,
the sinking type of sensor presents a couple of problems, as the voltage across the shunt resistor is differential (neither side is at
ground) and at least one side of the resistor has a high common mode voltage (equal to the positive sensor supply). If the sensor
and/or UE9 are isolated, a possible solution is to connect the sensor signal or positive sensor supply to UE9 ground (instead of
sensor ground). This requires a good understanding of grounding and isolation in the system. The LJTick-CurrentShunt is often a
simple solution.
Both Figure 2-6 and 2-7 show a 0-100 Ω resistor in series with SGND, which is discussed in general in Section 2.7.3.4. In this
case, if SGND is used (rather than GND), a direct connection (0 Ω) should be good.
The best way to handle 4-20 mA signals is with the LJTick-CurrentShunt, which is a two channel active current to voltage converter
module that plugs into the UE9 screw-terminals.
2.7.3.8 - Floating/Unconnected Inputs
The reading from a floating (no external connection) analog input channel can be tough to predict and is likely to vary with sample
timing and adjacent sampled channels. Keep in mind that a floating channel is not at 0 volts, but rather is at an undefined voltage.
In order to see 0 volts, a 0 volt signal (such as GND) should be connected to the input.
Some data acquisition devices use a resistor, from the input to ground, to bias an unconnected input to read 0. This is often just
for “cosmetic” reasons so that the input reads close to 0 with floating inputs, and a reason not to do that is that this resistor can
degrade the input impedance of the analog input.
13
Summary of Contents for UE9
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