Measurement Computing CIO-DAS6402/12 - Manual (Page 16)

Systems with varying ground potentials should always be monitored in the differential mode. Care is required to assure that the sum
of the input signal and the ground differential (referred to as the common mode voltage) does not exceed the common mode range
of the A/D board (+/-10V on the CIO-DAS6402). Figure 4-6 below show recommended connections in this configuration.
Figure 4-6. Common Mode Voltage < +/-10V / Differential Inputs
4.2.5 COMMON MODE VOLTAGE > +/-10V
The CIO-DAS6402 will not directly monitor signals with common mode voltages greater than +/-10V. You will either need to alter
the system ground configuration to reduce the overall common mode voltage, or add isolated signal conditioning between the
source and your board. See Figure 4-7 and 4-8 below.
Figure 4-7. Common Mode Voltage > +/-10V. Single-Ended Input
Figure 4-8. Common Mode Voltage > +/-10V. Differential Input
4.2.6 ISOLATED GROUNDS / SINGLE-ENDED INPUTS
Single-ended inputs can be used to monitor isolated inputs, though the use of the differential mode will increase you system’s noise
immunity. Figure 4-9 below shows the recommended connections is this configuration.
12
+
-
In p u t
A m p
To A /D
A /D B o a rd I/O
C o n n e c to r
LL G N D
C H H igh
C H L ow


S ign a
l S o u
rce
w ith
C o m
m o n
M
o de V
oltag e
S ign a l so urce a n d A /D b o a rd
w ith co m m on m o d e vo lta g e
c on n ec te d to a d iffe ren tia l in p u t.
G N D
T he v olta ge d iffe ren tia l
b e tw ee n th e se gro un ds ,
a d de d to th e m a xim u m
in p ut sig n a l m ust sta y
w ithin + /-10V
S ys tem w ith a L arg e C o m m o n M od e Voltag e ,
C on ne cte d to a S ing le -E nd ed In pu t
I/O
C o n n e c to r
+
-
In p u t
A m p
To A /D
LL G N D
C H IN
A /D B o a rd


L arg e com
m on
m
od e vol
tag e
betw een si
gn al
sour
c e & A /D boar
d
G N D
Iso la tio n
B a rrie r
W h en the voltage difference
betw een sign al source a nd
A /D boa rd gro und is large
eno ugh so the A/D board ’s
com m o n m od e ra n ge is
exceede d, isolate d sig nal
conditioning m ust be add ed.
S ys te m w ith a L a rg e C o m m o n M o d e Vo lta g e ,
C o n n e cte d to a D iffe re n tia l In p u t
 
L arge c
om m on
m od e v
oltag e
bet
w een s
igna l
s our ce & A
/D bo a rd
G N D
Iso lation
B arrie r
W h en the voltage differenc e
betw een s ig nal source a nd
A /D boa rd g ro und is large
eno ugh so the A /D boa rd ’s
com m on m od e range is
exceede d, is olated sig nal
conditioning m ust be add ed.
+
-
In p u t
A m p
To A /D
A /D B o a rd I/O
C o n n e c to r
LL G N D
C H H igh
C H L ow
1 0 K
1 0K is a re c o m m e n d ed v a lu e. Yo u m ay s h o rt L L G N D to C H L o w
in s te a d , b u t this w ill re du c e y o u r s y s te m ’s n o is e im m u n ity.