©
National Instruments Corp.
11
NI 9234 Operating Instructions and Specifications
Figure 4.
NI 9234 Input Circuitry for One Channel
The NI 9234
u
ses common-mode bias c
u
rrent to bias the
c
u
rrent-limiting diodes when IEPE c
u
rrent is t
u
rned off. When the
NI 9234 is
u
sing gro
u
nded signal so
u
rces, this c
u
rrent ca
u
ses an
error that is dependent on the AI– lead impedance. This error is
approximately 50 ppm of range and 15 ppm of reading per
Ω
of
AI– impedance. The common-mode bias c
u
rrent ca
u
ses an error
only with gro
u
nded so
u
rces and is not an iss
u
e with floating signal
so
u
rces. For best acc
u
racy,
u
se a floating connection or
u
se
low-impedance leads when connecting gro
u
nded signal so
u
rces.
2 mA IEPE on/off
AI+
AI–
Amplifier
and
Prefilter
ADC
+
–
AC/DC Coupling
NI 9234
Common
Mode
Bias
Current
Current
Limiting
Diodes
50
Ω