National Instruments NI 5911 - Manual (Page 13)

Chapter 2 Hardware Overview
NI 5911 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 PGIA differential amplifier efficiently rejects any
noise present on the ground signal. Internal to the PGIA, the signal
presented at the negative input is 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–.
Figure 2-2. Noise-Free Measurements of Signal
Grounding Considerations
The path 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 negative input of the amplifier is grounded to PC ground through a
10 k
Ω resistor. The PGIA is therefore referenced to ground, so it is not
necessary to make any external ground connections. If the device you
connect to the NI 5911 is already connected to ground, ground-loop noise
voltages may be induced into your system. Notice that in most of these
situations, the 10 k
Ω resistance to PC ground is normally much higher than
the cable impedances you use. As a result, most of the noise voltage occurs
at the negative input of the PGIA where it is rejected, rather than in the
positive input, where it would be amplified.
+
–
V
out
V+
Input Signal
Ground Noise
V–
PGIA