Chapter2 HardwareOverview
NI5911 User Manual 2-2 ni.com
Differential Input
When measuring high dynamic range signals, ground noise is often a
problem. The PGIA of the NI 5911 allows you to make noise-free signal
measurements. The NI 5911 PGIA is a differential amplifier. The PGIA
differentialamplifier efficiently rejects any noise which may be present on
theground signal. Internal to the PGIA, the signal presented at the negative
inputis subtracted from the signal presented at the positive input. As shown
in Figure 2-2, this subtraction removes ground noise from the signal. The
inner conductor of the BNC is V+, the outer shell is V–.
Figure2-2. Noise-FreeMe asurements of Signal
Grounding Considerations
Thepath for the positive signal has been optimized for speed and linearity.
You should always apply signals to the positive input and ground to the
negative input. Reversing the inputs will result in higher distortion and
lower bandwidth.
The negativeinput of the amplifier is grounded to PC ground through a
10 kΩresistor.The PGIA is therefore referenced to ground, so it is not
necessary to makeany external ground connections. If the device you
connect to the NI 5911 is already connected to ground, ground-loop noise
voltagesmay be indu cedinto y our system. Notice that in most of these
situations,the 10 kΩresistancetoPC ground is normally much higher than
thecable impedances you use. As a r esult, most of the noise voltage occurs
at the negativeinput of the PGIA where it is rejected, rather than in the
positiveinput, where it would be amplified.
+
–
V
out
V+
InputSignal
GroundNoise
V– PGIA