Chapter 4
Analog Input
X Series User Manual
4-50
ni.com
DC-Coupled
Yo
u
can connect low so
u
rce impedance and high so
u
rce impedance
DC-co
u
pled so
u
rces:
•
Low Source Impedance
—Yo
u
m
u
st reference the so
u
rce to AI GND.
The easiest way to make this reference is to connect the positive side
of the signal to the positive inp
u
t of the instr
u
mentation amplifier and
connect the negative side of the signal to AI GND as well as to the
negative inp
u
t of the instr
u
mentation amplifier, witho
u
t
u
sing resistors.
This connection works well for DC-co
u
pled so
u
rces with low so
u
rce
impedance (less than 100
Ω
).
•
High Source Impedance
—For larger so
u
rce impedances, this
connection leaves the DIFF signal path significantly off balance. Noise
that co
u
ples electrostatically onto the positive line does not co
u
ple
onto the negative line beca
u
se it is connected to gro
u
nd. Hence, this
noise appears as a DIFF-mode signal instead of a common-mode
signal, and the instr
u
mentation amplifier does not reject it. In this case,
instead of directly connecting the negative line to AI GND, connect the
negative line to AI GND thro
u
gh a resistor that is abo
u
t 100 times the
eq
u
ivalent so
u
rce impedance. The resistor p
u
ts the signal path nearly
in balance, so that abo
u
t the same amo
u
nt of noise co
u
ples onto both
connections, yielding better rejection of electrostatically co
u
pled
noise. This config
u
ration does not load down the so
u
rce (other than the
very high inp
u
t impedance of the instr
u
mentation amplifier).
Yo
u
can f
u
lly balance the signal path by connecting another resistor of
the same val
u
e between the positive inp
u
t and AI GND. This f
u
lly
balanced config
u
ration offers slightly better noise rejection b
u
t has the
disadvantage of loading the so
u
rce down with the series combination
(s
u
m) of the two resistors. If, for example, the so
u
rce impedance is
2 k
Ω
and each of the two resistors is 100 k
Ω
, the resistors load down
the so
u
rce with 200 k
Ω
and prod
u
ce a –1% gain error.
AC-Coupled
Both inp
u
ts of the instr
u
mentation amplifier req
u
ire a DC path to gro
u
nd in
order for the instr
u
mentation amplifier to work. If the so
u
rce is AC-co
u
pled
(capacitively co
u
pled), the instr
u
mentation amplifier needs a resistor
between the positive inp
u
t and AI GND. If the so
u
rce has low-impedance,
choose a resistor that is large eno
u
gh not to significantly load the so
u
rce b
u
t
small eno
u
gh not to prod
u
ce significant inp
u
t offset voltage as a res
u
lt of
inp
u
t bias c
u
rrent (typically 100 k
Ω
to 1 M
Ω
). In this case, connect the
negative inp
u
t directly to AI GND. If the so
u
rce has high o
u
tp
u
t
impedance, balance the signal path as previo
u
sly described
u
sing the same
val
u
e resistor on both the positive and negative inp
u
ts; be aware that there
is some gain error from loading down the so
u
rce.
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