Measurement Computing PCM-DAS16x/16 - Manual (Page 28)

6.2.3 Common Mode Voltage < +/-10V/Single-Ended Inputs
This is not a recommended configuration. In fact, the phrase common mode has no
meaning in a single-ended system and this case would be better described as a system
with offset grounds. Anyway, you are welcome to try this configuration, no system
damage should occur and depending on the overall accuracy you require, you may
receive acceptable results.
6.2.4 Common Mode Voltage < +/-10V/Differential Inputs
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 differ-
ential (referred to as the common mode voltage) does not exceed the common mode
range of the A/D board (+/-10V on the PCM-DAS16x/16). The diagram below show
recommended connections in this configuration.
24
+
-
In p ut
A m p
To A /D
A /D B o a rd I/O
C o n n ec to r
L L G N D
C H H ig h
C H L o w


S ig n
al S o
u rce
w i
th C o
m m o
n
M o d
e Vo
ltage
S ig n a l s o u rc e a n d A /D b o a rd
w ith c o m m o n m o d e v o lta g e
c o n n e c te d to a d iffe re n tia l in p u t.
G N D
T he vo ltag e d ifferen tia l
b etw ee n the s e groun d s,
a dde d to the m a x im u m
in p ut s ig na l m ust s ta y
w ithin +/-10 V
S ys tem w ith a Large C om m on M ode Voltage ,
C onne cte d to a S ingle-E nded Input
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 arge com
m on
m
od e v
oltag
e
bet
w ee n
signal
sou rce
& A
/D boar
d
G N D
Is o la tio n
B a rrie r
W h en the voltage difference
betw e en sign al source a nd
A /D boa rd gro un d is large
eno ugh so the A /D bo ard ’s
com m o n m od e ra n ge is
excee de d, isolate d sig nal
cond ition ing m ust be add ed.