Application Note

Calibrating voltage input
signal conditioners
The precision current sourcing
and simulation capabilities of
Fluke loop calibrators make
them ideal for calibrating many
4-20 mA signal conditioners.
However, there are many signal
conditioners that require a pre-
cision voltage source for proper
calibration. Using a simple pre-
cision resistor and standard
connector, Fluke loop calibra-
tors can field calibrate many
standard and non-standard
voltage input signal condition-
ers. This approach works well
for the 705, 707, 787 and 789
loop calibration tools (this is
not necessary with the 715
loop calibrator as it has a preci-
sion direct voltage output).
Voltage input signal condi-
tioners come in many varieties.
The most common are 0-10 V,
0-5 V and 1-5 V input levels.
Typical outputs of these devices
are an isolated or non-isolated
0-10 V or 4-20 mA. Resistors
with values of 250 Ω to 500 Ω
are common loop load resistors
and provide voltage input lev-
els as a function of the loop
current (see Figure 4).
Using Fluke loop calibrators
as a voltage source
A precision shunt resistor may
be used to derive voltages for
calibration using the calibra-
tor’s current source mode.
Using this system, Fluke loop
calibrators are capable of gen-
erating voltages for dev
ices
with input spans as low as 10
mV to as high as 24 V. Table 1
gives values of precision resis-
tors to accommodate a variety
of voltage calibrations and the
Fluke calibrator ideal for each
application.
Constructing a precision
load resistor assembly
A simple precision current
shunt can be constructed using
a precision RN60, 1 W resistor,
(see table for the correct value),
a dual banana jack connector
and some test leads with alli-
gator clips (see Fig
ure 5). The
RN60 class resistor is available
from many commercial sources.
Construct the assembly as
shown in Figure 5. This preci-
sion resistor assembly, coupled
with the precision current
sourcing capabilities of Fluke
loop calibrators, generates pre-
cision voltages to cover 1-5 V
or 2-10 V applications. The
1,000 Ω assembly has an
advantage in that it allows a
direct one-to-one display cor-
relation to voltage when
sourcing current from the Fluke
loop calibrator during calibra-
tion (1 mA = 1 V). The following
example will utilize a 250 Ω to
take advantage of the 0-100 %
and 25 % buttons on the Fluke
707 (25 % or 4 mA = 1 V).
4 Fluke Corporation L
oop calibration and maintenanc
e
Loop
supply
Fig
ure 4
Resistor Sourced Generated Fluke loop
value application current value voltage Calibrator
10 Ω mV 0-20.000 mA 0-200.00 mV 707, 705, 787, 789
transduc
er
250
Ω 0-5 V Source 0-20.000 mA 0-5.000 V 705, 707
1
000 Ω 0-10 V Source 0-10.000 mA 0-10.000 V 705, 707, 787, 789
N
ote: Input impedance of the device under test should be ≥ 1 MΩ
T
able 1. Values of precision resistors to accommodate a variety of voltage calibrations
1 kΩ resistor
Figure 5
Constructing a pr
ecision curr
ent shunt
Parts required:
1. Resistor: 1000 Ω
Type: R
N6
0 (50 ppm @ 70 °C) 1 W
2. 36˝ test leads with alligator clips
3.
Connector: Dual banana plug 2/screw terminals