National Instruments SCXI-1320 User Manual page 43

Chapter 2 Configuration and Installation
SCXI-1121 User Manual
The value of all the nulling resistors on your terminal block is 39 kΩ. Notice
that these resistors are socketed for easy replacement. These sockets best fit
a 1/4 W resistor lead size.
To determine your nulling range, use the following formula (refer to
Figure 2-9 for visual help):
V
nullingrange
where
R is the nominal strain-gauge resistance value.
g
R is either a completion resistor or a second strain-gauge nominal
d
resistance.
R is the nulling resistor value.
null
V is the excitation voltage (3.333 or 10 V).
exc
For example, assuming:
V = 3.333 V
exc
= 120 Ω
R
g
= 120 Ω
R
d
R = 39 kΩ
null
V = ±2.56 mV
nulling
Assuming a strain-gauge range with a gauge factor of GF = 2 and a
quarter-bridge configuration, this range corresponds to ±1,498 µε as given
by the strain formula for a quarter-bridge strain-gauge configuration:
where
strained voltage static unstrained voltage
V
= ------------------------------------------------------------------------------------------------------- -
r
V V
exc exc
±
= --------- - – ---------------------------------------------------------- -
2 R R
null
–4V
r
ε
= -------------------------------
(
GF 1 2V
+
–
V
exc
2-26
( )
R R R
+
d null g
( )
R R R
+ +
g d null g
)
r
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